School Science Lessons
2024-06-19
Crystals, Water of crystallization
(topic09)
Contents
3.0.0 Prepare crystals
4.0.0 Burning tests for fabrics
3.4.0 Glass transition temperature
3.2.0 Prepare invisible ink
3.6.0 Water of crystallization
3.2.16 Watermarks
3.0.0 Prepare crystals
3.0.0 Crystals
See diagram35.8.3 Crystal shapes, crystal habit (GIF)
Many solids are crystalline.
Their particles exist in an ordered arrangement.
Crystallization involves the formation of the pure solid from its solution.
Many crystals contain water of crystallization, e.g. copper (II) sulfate crystals, CuSO4.5H2O.
However, anhydrous copper (II) sulfate, CuSO4, contains no water of crystallization.
Experiments
3.1.1 Prepare aspirin crystals
3.1.19 Prepare candy with sucrose
3.1.2 Prepare crystal blossoms
3.1.3 Prepare crystal clusters
3.1.4 Prepare crystal crowns
3.1.5 Prepare crystal gardens
3.1.20 Prepare crystal rope
3.1.21 Prepare crystals, (Primary)
3.1.6 Prepare crystals from a melt
3.1.7 Prepare crystals from a mixture of salts
3.1.8 Prepare crystals from solutions
3.1.9 Prepare crystals in an unglazed clay dish
3.1.10 Prepare crystals with different shapes
3.1.11 Prepare double salt crystals
3.1.12 Prepare invert sugar, C12H24O12, HFCS
3.1.13 Prepare large and small crystals
3.1.14 Prepare seawater crystals
3.1.15 Prepare split crystals
3.1.16 Prepare stalactite crystals
3.1.17 Prepare sugar crystals from brown sugar
3.1.18 Prepare sugar crystals from sugar cane juice
3.1.22 Prepare sodium polyacrylate gels, (ghost crystals)
3.6.0 Water of crystallization
Experiments
3.6.1 Heat different crystals, water of crystallization
3.6.2 Heat iron (II) sulfate-7-water crystals
3.6.3 Heat magnesium sulfate-7-water crystals
3.6.4 Make objects and moulds with plaster of Paris
3.6.5 Prepare fibrous plaster board with plaster of Paris
3.2.0 Prepare invisible ink
Invisible ink, disappearing ink, secret writing ink, sympathetic ink.
"Disappearing Ink", acid / base indicator, thymolphthalein, (sold by chemical suppliers)
3.2.1 Ammonium iron (II) sulfate or ammonium chloride
3.2.2 Cane sugar
3.2.3 Cobalt (II) chloride solution
3.2.4 Gold in aqua regia
3.2.5 Iron (II) sulfate
3.2.6 Lemon juice
3.2.7 Milk, invisible writing ink
3.3.7 Phenolphthalein (uncoated laxative tablets)
3.2.9 Prepare red invisible ink with phenolphthalein
3.2.10 Plant gall ink
3.2.11 Prepare black ink from iron (II) sulfate and oak galls
3.2.12 Secret writing inks, alum solution
3.2.13 Sodium chloride, invisible writing ink
3.2.14 Starch, cornstarch suspension, invisible writing ink with iodine
3.2.15 Vegetable juice, onion juice, red cabbage juice, invisible writing ink
3.2.16 Watermarks
3.2.17 Electric writing, Sodium chloride with litmus paper
5.6.3.3 Prepare goldenrod indicator, electric writing
3.2.18 Prepare yellow invisible ink
3.2.19 Prepare invisible ink, (Primary)
3.4.0 Glass transition temperature, Tg
3.4.01 Glass transition temperature, (Tg)
3.4.02 Chewing gum, (Tg) (Experiment)
3.4.03 Cotton, (Tg)
4.0.0 Burning tests for fabrics
(This experiment is based on: Selinger, Ben, 1991, "Chemistry in the market place", Harcourt Brace Jovanovich Publishers,
ISBN 0 7295 0334 8, Experiment 13.13.)
Experiments
4.1.0 Tests for plastics, burning tests
4.2.0 Tests for natural fabrics, burning tests
4.3.0 Tests for synthetic fibres, burning tests
4.5.0 Tests for plastics, flotation tests
4.6.0 Tests for plastics, thermal behaviour
4.4.0 Tests for plastics, transparency and feel
3.1.1 Prepare aspirin crystals
See diagram 16.3.4.11: Aspirin.
Dissolve an aspirin table in methylated spirit.
BE CAREFUL! Filter if necessary.
Heat the solution.
3.1.2 Prepare crystal blossoms.
Soak pieces of charcoal, brick or unglazed porcelain in a saturated solution of sodium chloride.
Keep the pieces covered by adding more saturated sodium chloride solution over a period of two weeks.
At this point mix some Prussian blue dye or ink with the sodium chloride and add this to the pieces of charcoal.
Then leave to evaporate to dryness.
Blossoms of crystals will form.
A variety of colours may be produced by adding different dye compounds.
3.1.3 Prepare crystal clusters.
1. Soak pieces of charcoal or brick or unglazed porcelain in a saturated solution of sodium chloride for two weeks.
Add ink or dyes then heat the solution to evaporate to dryness.
"Blossoms" of crystals form.
2. Prepare a saturated solution of potassium dichromate.
Immerse cardboard shapes, e.g. a crown, and let the solvent water evaporate.
Red crystals form on the shapes.
3. Tie a light weight to a piece of cotton thread and hang the thread in a saturated solution of sodium chloride.
Remove the thread and hang it up to dry.
Light the hanging thread with a match.
The thread burns, but the sodium chloride is left as an ash strong enough to support the light weight.
4. Grow crystal gardens.
Drop crystals of metallic salts in a jar of water.
Leave the jar undisturbed for weeks and note the growth of "trees" of crystals.
Use aluminium potassium sulfate Al2(SO4)3.K2 (SO4).24H2O, chromium (III) potassium sulfate (VI)-12-water, copper (II) sulfate-5-water and iron (II) sulfate-7-water.
3.1.4 Prepare crystal crowns.
See diagram 3.2.59: Crystal crown.
Cut a crown from a piece of tin taken from a fruit can.
Fasten it with a piece of wire as shown in the diagram.
Wrap the crown with strips of cotton cloth.
Dip the whole crown into a solution of potassium dichromate and then leave to dry.
Seed crystals will form on the cloth.
Prepare a saturated solution of potassium dichromate at 80oC and immerse the crown for a day in this saturated solution.
Red crystals should form and make a beautiful display on the crown.
3.1.5 Prepare crystal gardens.
1. Drop crystals of metallic salts in a jar of water.
Leave the jar undisturbed for weeks and note the growth of "trees" of crystals.
Use aluminium potassium sulfate Al2(SO4)3.K2(SO4).24H2O, chromium (III) potassium sulfate (VI)-12-water, copper (II) sulfate-5-water and iron (II) sulfate-7-water.
2. Add 2 cm of sodium silicate solution to a jar.
Add hot deionized water, while stirring, and to bring depth in the jar to 10 cm.
Keep stirring until all the sodium silicate is mixed with the water.
Leave the solution to stand in a place where it will not be disturbed.
Drop into separate places in the solution 2-3 crystals of cobalt nitrate, nickel nitrate, iron (III) nitrate, and magnesium nitrate.
Draw a map to show exactly where the crystals are place in the bottom of the solution.
Leave the solution overnight and the next day note the colours where the crystals were placed.
The colours are characteristic of the metal ions put into the sodium silicate solution.
3.1.6 Prepare crystals from a melt.
1. Put crystals of naphthalene on a microscope slide.
Hold the slide over a flame until the crystals melt.
Put a coverslip over the liquid and leave to cool.
Watch the crystals grow using a magnifying glass or a microscope.
Note whether crystals will grow from several points simultaneously to make boundaries where they meet.
Draw the shape of the boundary between the forming crystals and the melt.
View the crystals through Polaroid sunglasses.
2. Repeat the experiment with stearic acid (octadecanoic acid).
3.1.7 Prepare crystals from a mixture of salts.
Dissolve sodium hydrogen sulfate, ammonium iron (II) sulfate, magnesium sulfate and cobalt (II) chloride in just enough water so that the salts dissolve completely only after heating.
Let the solution stand undisturbed for days.
Examine the separate crystals that grow from the mixture.
3.1.8 Prepare crystals from solutions.
See diagram 3.2.49: Crystal in supersaturated solution.
Seed crystals are very important in the sugar industry.
Only experts know how to reduce the crushed sugar cane mixture to a syrup solution then add about 100 g of seed crystals to produce tonnes of sugar crystals.
1. Add sodium thiosulfate crystals, Na2S2O3.5H2O, to 2 cm of water in a test-tube.
Heat the test-tube and keep adding sodium thiosulfate crystals until no more dissolves.
This solution is now a supersaturated solution.
Let the test-tube cool and note whether crystals form.
If no crystals form, add one seed crystal to help crystallization.
Use a magnifying glass to note whether the crystals in the test-tube have the same shape and size as the crystals added to the water in the test-tube.
2. Tie a cotton thread to a paper clip and suspend the paper clip in the solution.
Leave the solution in a warm place.
As water evaporates from the solution during the next few days, crystals form, first on the rough edges of the paper clip.
Repeat the experiment by gently heating dry crystals in a test-tube.
The crystals dissolve in their own water of hydration to form a saturated solution.
Add a seed crystal for recrystallization.
Repeat the experiment with crystals of aluminium potassium sulfate Al2(SO4)3.K2(SO4).24H2O, ammonium chloride (NH4Cl), ammonium iron (II) sulfate [ferrous ammonium sulfate (NH4)2SO4.FeSO4.6H2O, glucose C6H12O6, iron (II) sulfate, FeSO4.7H2O, add drops of sulfuric acid to prevent the solution turning brown), magnesium sulfate (MgSO4.7H2O), sodium carbonate Na2CO3.10H2O, sodium sulfate Na2SO4.10H2O, sucrose C12H22O11.
3.1.9 Prepare crystals in an unglazed clay dish.
Put a solution of salt crystals in an unglazed clay pot and leave it in the sun.
Salt crystals appear first in the middle of the dish.
Around them no crystals appear, but there is really a thin lay of salt is deposited.
Salt crystals appear on the raised outer edge of the dish when surface tension causes the salt solution to rise up the porous edge slightly then evaporate, leaving a thick layer of salt crystals.
3.1.10 Prepare crystals with different shapes.
See diagram 3.2.47: Different shapes of crystals.
Note these crystal shapes by putting 2 drops of the warm concentrated solution on to a microscope slide and view with a magnifying glass or microscope.
Cubic crystals: sodium chloride and potassium chloride
Tetragonal crystals: nickel sulfates, potassium nitrate, zinc sulfate
Monoclinic crystals: a potassium chlorate, sodium sulfate
Triclinic crystals: copper (II) sulfate
Octahedral crystals: sodium chloride crystallizes from alkaline urea or ammonia solution
Funnel-shaped crystals: mixture of sodium chloride and alum solution.
3.1.11 Prepare double salt crystals
1. Prepare crystals of alum.
Add 3.0 grams of alum to 10 ml of warm water and heat with a small flame.
Pour the hot solution into an evaporating dish.
Observe the crystals forming after about 30 minutes.
Leave the solution to evaporate.
Wipe off any tiny crystals and keep the large crystals.
Suspend the best crystal in a supersaturated solution by a coarse thread.
Large crystals of hydrated aluminium potassium sulfate, KAl(SO4)2.12H2O, will form.
2. Prepare crystals of ammonia alum.
Dissolve 1 cm of ammonium sulfate in 1 cm of water by shaking.
In another test-tube dissolve 5 cm of aluminium sulfate in 5 cm of water.
Mix the two solutions in an evaporating dish and leave the mixed solutions to evaporate.
Diamond-shaped white crystals of the double salt of aluminium, NH4Al(SO4)212H2O, deposit in the dish.
3. Prepare crystals of ammonium iron (II) sulfate (ferrous ammonium sulfate).
Dissolve 1 cm of ammonium sulfate in 1 cm of water by shaking.
In another test-tube dissolve 2 cm of iron (II) sulfate in 2 cm of cold water.
Mix the two solutions in an evaporating dish and leave the mixed solutions to evaporate.
Green crystals of (NH4)2SO4.FeSO4.6H2O deposit on the bottom of the dish.
The flat square crystals of ammonium iron (II) sulfate do not oxidize or turn yellow when exposed to air.
Large crystals can be grown.
4. Prepare crystals of copper ammonium sulfate.
Dissolve 1 cm of ammonium sulfate in 1 cm of water by shaking.
In another test-tube dissolve 2 cm of copper sulfate in 3 cm of cold water.
Mix the two solutions in an evaporating dish and leave the mixed solutions to evaporate.
Light blue crystals of CuSO4.(NH4)2SO4.6H2O deposit in the dish.
3.1.12 Prepare invert sugar, C12H24O12, HFCS
Prepare invert sugar by heating sucrose + water + cream of tartar or citric acid. (fresh lemon juice), to boil for 20 minutes at 114oC.
Invert sugar is used in confectionery, sorbets and ice cream as a substitute for corn sugar to prevent crystallization and give a smooth mouth feel, Invert sugar is used as a hygroscopic humectant to keep products moist and to assist browning caramelization (Maillard reaction), and to enhances aromas.
C12H22O11 + H2O --> C6H12O6 + C6H12O6
sucrose + water --> glucose + fructose
Invert sugar is 50% glucose + 50% fructose,
HFCS, high fructose corn syrup
HFCS 55, 55% fructose, used in soft drinks
HFCS 42, 42% fructose, used in processed foods.
3.1.13 Prepare large and small crystals.
See diagram 3.2.49: Seed crystal in a saturated solution.
1. Prepare a saturated solution of potassium alum crystals [Al2(SO4)3.K2(SO4).24H2O] by leaving crystals to dissolve over hours in a warm place.
Stir the solution during this time so that it becomes saturated.
Pour off the warm saturated solution into a beaker.
When the saturated alum solution is cold, some original crystal should remain undissolved.
You can also add drops of sulfuric acid or sodium bisulfite solution to keep the solution clear.
Suspend a heavy object by cotton in the liquid.
Diamond-shaped alum crystals form on the cotton.
The crystals have 8 sides, but some "corners" may be missing.
Detach some well-shaped crystals from the cotton and leave them overnight in the saturated solution to grow.
Select the largest crystal, tie cotton around it and suspend the crystal in the saturated solution.
After some weeks, the selected crystal can grow too big for the beaker.
2. Repeat the experiment with copper (II) sulfate crystals.
3. Use phenyl salicylate, C13H10O3, salol, to show cooling rates affects crystal size.
3.1.14 Prepare seawater crystals.
Salinity varies from 32-38 parts per thousand.
Seawater contains mainly ions of Na and Cl, also Ca, Mg and K, and dissolved gases, O2, N2 and CO2.
Heat seawater to just below boiling point.
When the liquid has decreased by one tenth of its volume, add more seawater and continue heating.
When the crystals begin to appear, stir with a glass rod.
Push back into the liquid the crystals that appear on the sides of the beaker.
The mixture becomes a paste.
Heat the paste gently and stir until all the water evaporates and white crystals of salt remain.
The salt contains mainly sodium chloride with magnesium chloride and other salts in small quantities.
Dry the sea salt and leave it exposed to the air.
The salt becomes damp again, because the magnesium chloride is deliquescent and attracts water from the atmosphere.
Also, the magnesium chloride gives sea salt a slightly bitter taste.
Compare the crystals from seawater with crystals from a packet of table salt.
A typical composition of seawater:
| Sodium chloride 77.74% | Magnesium chloride 10.9% | Magnesium sulfate 4.7% | Calcium sulfate 3.6% | Potassium sulfate 2.5% | Calcium carbonate 0.34% | Magnesium bromide 0.22% |.
3.1.15 Prepare split crystals.
See diagram 3.2.51: Split a crystal with a razor blade.
Split crystals of calcite (calcium carbonate) or sodium chloride along their plane of cleavage with a one-sided razor blade or a solid scalpel, i.e. not with a detachable blade.
Cleavage is an important identifying property of minerals.
Put the edge of the blade on the crystal while holding the face of the blade vertical and parallel to the planes of the two opposite sides of the crystal.
Tap with a small hammer on the top of the blade.
If the least possible force is used, the crystal splits down the plane of cleavage.
When the blade is not directed correctly, the crystal crumbles instead of splitting into two parts.
Galena (lead sulfide, PbS) occurs as cubic crystals that are easily split along the three cleavage planes at right angles to each other.
Mica has one perfect base cleavage, so it can be split into very thin flexible sheets.
3.1.16 Prepare stalactite crystals.
Half fill two beakers with concentrated sugar or baking soda or Epsom salts solution.
Put the beakers on the table about 10 cm apart.
Cut a piece of woollen thread, or any thread that will soak up water, about 30 cm long and attach a paper clip to each end of the thread.
Put each end of the woollen thread in one of the beakers with the ends of the thread under the solution.
Adjust the distance between the beakers so that the thread dips down between the beakers without touching the table.
Place a saucer or Petri dish below the dipped down thread to catch any drips.
After some days crystals appear on the lowest part of the thread between the beakers.
The saturated solution has moved along the thread by capillarity until reaching the lowest point on the thread.
Here water is lost by evaporation and crystals like stalactites form.
Leave the experiment in place for weeks.
A stalactite grows down from the thread and a stalagmite grows up from the saucer.
After a long time the stalactite and stalagmite will join to form a column.
Remember that the stalactite must hold "tight" to the thread and the stalagmite "might" grow up to meet the stalactite to form a column.
33.1.17 Prepare sugar crystals from brown sugar
Activated carbon decolorizes and refines brown sugar solution by adsorbing coloured impurities on it.
After filtering, concentrating and cooling the white (refined) sugar crystals form.
1. Put 5 to 10 g of brown sugar in a small beaker.
Dissolve the sugar in 40 mL water by heating.
Add 0.5 to 1.0 g of activated carbon while constantly stirring.
Filter the suspension when it is still hot to obtain a colourless solution.
If the filtrate appears yellow, add a little more activated carbon, heat and filter the suspension again until the filtrate becomes colourless.
By heating, concentrate the filtrate in a small beaker on a water bath until the volume of the solution is reduced by about one quarter.
White sugar crystals separate out of the liquor after cooling it naturally.
2. Dissolve 100 g of brown sugar in 90 mL water.
Add calcium hydroxide solution until the solution turns red litmus blue.
Filter the solution then heat the filtrate with absorbent charcoal while it is still hot.
Evaporate at reduced pressure at 50 to 65oC.
Put the syrup in a refrigerator for several days to form crystals.
3.1.18 Prepare sugar crystals from sugar cane juice
In production of cane sugar, the solution is clarified by adsorbing impurities on bone char, a type of carbon made by heating bones in the absence of air.
Add calcium hydroxide to sugar cane juice or sugar beet juice until red litmus turns blue.
Leave it to stand overnight.
Filter by suction.
Evaporate in reduced pressure at 50 to 65oC until it becomes a brown syrup that is so viscous that it scarcely flows.
Put the syrup in a refrigerator for several days to form crystals.
Use a centrifuge to separate crystals from the liquor.
3.1.19 Prepare candy with sucrose
1. Use the solubility of sucrose to make toffee candy.
Prepare toffee or candy by heating the sucrose solution with milk or butter until it boils.
Heat to 125 o C.
Pour it into paper cups and leave to cool.
Constant stirring and temperature control is essential.
The actors in movies who appear to fall through a sheet of glass without hurting themselves, fall through a sheet of clear toffee called "sugar glass".
3.1.20 Prepare crystal rope
Make a saturated solution of sucrose (cane sugar).
Tie a weight to a cotton thread and suspend it so that the weight and most of the thread are in the solution.
As the solution cools, crystals form on the thread and weight.
Take out the thread and weight.
Hold the weight and set fire to the cotton.
A rope of sugar crystals remains.
3.1.21 Prepare crystals
See diagram 35.8.3: Crystal habit.
See diagram 3.2.49: Seed crystals in saturated solution.
See diagram 3.2.50: Crystal crown.
Teach the children to dissolve pure substances and make crystals.
Use alum, Epsom salts, common salt, sugar, small spoons, a cup.
There are two main ideas here: dissolving and crystals.
When substances are dissolved in water, you cannot see them.
However, if you dissolve alum, Epsom salts, common salt or sugar, you can prove that these substances are still in the water, , because you can taste them.
If you boil away the water from the dissolved substances by heating or leaving it in the sun then the dissolved substances
reappear as crystals.
Crystals have a glassy look and a certain shape and colour.
However, the crystals of common salt or sugar you buy are mostly broken into small pieces.
Use common salt, potassium permanganate (Condy's crystals) urea, magnesium sulfate crystals (Epsom salts), a container to boil water in, a flat dish, small spoons, a cup.
Be careful! Let the children taste sugar or table salt, but not other crystals!
1. Show some common salt.
Let them taste it.
It tastes salty.
Pour a few spoons of water into a cup.
Stir some salt into the water and show this to them.
Can you see the salt now? [No.]
Is the salt still there? [Let them taste the water. Yes, you can still taste the salt.]
Explain that the salt is dissolved in the water.
2. Add more salt to the water and keep stirring.
When you can dissolve no more salt in the water, pour the water into two tin lids.
Leave one lid in the sun.
Heat the other lid very slowly over a flame.
What happens? [Water boils off, then crystals appear.]
3. Let the children look at the crystals with a magnifying glass.
What did you see? [Shiny solids with flat sides, sharp corners, white colour.]
Explain that all crystals are shiny, have flat sides, shapes and colours.
Each kind of crystal has a special shape.
Here is a sand (quartz) crystal. It has six sides.
4. Let the children make sugar crystals and observe them.
5. Look at the tin lid left in the sun.
Do salty crystals appear? [Yes]
Where does this happen naturally? [In rock pools near the sea.]
3.1.22 Prepare sodium polyacrylate gels, (ghost crystals)
"Expanding Spheres", super absorbent polymers, (toy product to amuse children)
"Super Slurper Polymer", hydrophilic sodium polyacrylate (toy product to amuse children) Be careful! Sodium polyacrylate can irritate the eyes and nostrils.
Sodium polyacrylate, sodium prop-2-enoate, sodium poly (acrylic acid), super absorbent polymer, "waterlock".
The monomer is polymer: [-CH2-CHCOONa-]n.
Fibres + sodium hydroxide, then polyacrylic acid --> sodium polyacrylate, a cross-linked acrylic acid polymer sodium salt.
It has a very high molecular weight, white powder, is very soluble in water and forms a linear anion polymer.
Sodium polyacrylate is used as a thickening agent, in urine test kits, in baby disposable diapers (nappies) (fully-loaded diaper feels damp and not wet, so still comfortable to baby), in tampons for menstruation, (but may cause toxic shock syndrome if not changed daily), as "water crystals" to store water in soils, e.g. "Aqua Crystals", as "magic instant snow powder" novelty and movie set decoration (super slurper), and as "ghost crystals", because they become invisible in water, having the same refractive index.
Sodium polyacrylate can absorb 800 × its own weight of water.
Experiments
1. Attach a pin to a crystal of sodium polyacrylate.
Tie a string around the crystal and lower it into water.
The crystal disappears, but the pin remains suspended in water.
2. Dissolve some powder or gel form of sodium polyacrylate in alcohol.
The solution turns a deep magenta colour until the alcohol evaporates.
(Magenta is a brilliant red aniline dye derived from coal tar.)
3. The disappearing milk trick is an experiment to amuse children, which uses a drinking glass with frosted sides or some decoration so that the audience cannot see into the bottom of the glass.
Put about 1 teaspoon of sodium polyacrylate from a disposable nappy nto the drinking glass without the audience knowing.
Pour milk into the drinking glass.
Attempt to pour milk out of the drinking glass.
No milk comes out of the glass, because it had been absorbed by the sodium polyacrylate to form a gel that sticks in the drinking glass.
3.2.1 Ammonium iron (II) sulfate or ammonium chloride, invisible writing ink
Use ammonium iron (II) sulfate solution or ammonium chloride solution for ink.
On heating, the secret message appears as brown black or yellow brown writing.
3.2.2 Cane sugar, invisible writing ink
Use cane sugar solution, or sucrose solution in water.
Write on a piece of cardboard.
Read the secret message by burning paper and rubbing the ash on the cardboard.
3.2.3 Cobalt (II) chloride solution, invisible writing ink
Cobalt (II) chloride, crystals CoCl2.6H2O
1. Novelty weather indicators contain cobalt (II) chloride that turns blue to show that the weather is fine and turns pink to show that the weather is wet.
Dissolve crystals of cobalt (II) chloride-6-water and use for ink.
Use a small paint brush or a chewed end of a match to paint the ink on the paper, then leave to dry.
Dilute cobalt salt solutions are almost invisible.
However, writing with them can be seen by writing on paper then heating it over a flame or by wrapping the paper over a 100-watt light globe or by ironing the paper with a hot iron, not a steam iron.
The secret message appears as blue writing.
If the message is painted over with water, it disappears again.
Some spies use a mixture of cobalt chloride and glycerine.
However, if the paper with the revealed message is set aside, the cobalt salt may absorb water from the atmosphere and the writing becomes invisible again.
There is no chemical reaction, just a dilution or evaporation effect.
Add some sodium chloride to the cobalt chloride solution to allow the writing to appear after heating then fade many times.
2. Make a weak solution of cobalt chloride by adding one or two crystals to half a test-tube of water and shaking it.
The solution should be very pale pink, almost colourless.
Using a pen with a clean nib and containing no ink, or an old fashioned dip pen, write a message with the invisible ink you have made, and allow the writing to dry.
If the solution was weak enough, your writing will be invisible.
Heat the paper, but not over a flame.
Note whether the writing now shows.
Breathe on the visible writing.
Describe what you see.
Heat the paper to make the writing blue.
Breathing on the blue writing makes it invisible again.
3. Using a dilute solution of cobalt chloride as ink and a matchstick as a pen, write a word on a piece of paper.
When this dries the writing becomes almost invisible.
Warm the paper over a small flame to see the word again.
In some novelty cards the colour of the word changes with the weather.
In fine weather, it is blue and in wet weather it is pink.
CoCl2 (s) [blue] + 6H2O (l) <--> CoCl2.6H2O (s) [pink].
3.2.4 Gold in aqua regia, invisible writing ink
To make a very expensive invisible ink, dissolve gold in aqua regia (1 part concentrated HNO3 + 3 parts concentrated HCl) and let dry in the shade.
Wet the paper with a sponge wetted in a solution of tin in aqua regia.
Purple writing appears.
3.2.5 Iron (II) sulfate, invisible writing ink
Using a wood spill, write on a sheet of plain paper with iron (II) sulfate solution.
Leave the paper to dry and then heat it by holding it in front of a fire or over a flame.
The writing appears yellow or brown.
3.2.6 Lemon juice, invisible writing ink
Write with the lemon juice and heat the paper as before.
Note the brown colour of the writing.
3.2.7 Milk, invisible writing ink
Use milk solution in water.
Write on a piece of cardboard.
Read the secret message by burning paper and rubbing the ash on the cardboard.
3.2.8 Phenolphthalein, (uncoated laxative tablets), invisible writing ink
Sprinkle sodium bicarbonate or washing soda solution on the secret writing to make it appear pink.
3.2.9 Prepare red invisible ink with phenolphthalein
Write on paper with a solution of phenolphthalein.
Let the paper to dry and the writing becomes invisible.
Wet a cloth or sponge with weak sodium carbonate solution and smear it over the paper.
The writing shows up in red letters.
3.2.10 Plant gall ink
For over 1000 years in the Northern hemisphere ink was made from a mixture of an iron salt, e.g. iron (II) sulfate and infusion of plant galls.
The galls are formed by plants in reaction to attack by gall wasps. e.g. Biorhiza, Amphibolips.
The galls used were from the oak tree, Quercus, oak apple gall, oak marble gall and other nut galls, e.g. Rhus, Caesalpinia.
The galls contain tannic acid C76H52O46, (gallotannic acid).
The reaction of the iron salt with tannic acid produced a grey liquid that would turn purple-black with the addition of a gum, e.g. gum acacia, gum arabic.
Sometimes carbon was added to the ink.
The ink becomes darker and harder with exposure to the air as Fe2+ converted to Fe3+, so keep it in tightly-sealed bottles.
3.2.11 Prepare black ink from iron (II) sulfate and oak galls
Make black ink from iron (II) sulfate and oak galls, the round nut-like growths found on the branches of oak trees, from which they can be collected in autumn.
Crush or cut up one or two galls and boil the pieces with water in a beaker or small pan.
The water extracts tannic acid from the oak galls.
Strain off or filter the solution of tannic acid.
Prepare a solution of iron (II) sulfate in cold water and mix it with an equal amount of the cold oak gall solution.
This forms iron (II) tannate in the liquid.
Write on paper with the liquid.
The writing shows little colour, but when left for a day or two turns black, because iron (II) tannate on exposure to air forms black iron (III) tannate, ferric tannate.
In commercial blue-black ink, it contains a blue dye in addition to the iron (II) tannate.
The dye acts as temporary colouring matter until the black colour of the iron (III) tannate develops.
Make blue-black ink by adding to the ink prepared as above a solution of log wood or methylene blue, two common dyes.
If methylene blue is used, however, the ink should be stored in a dark cupboard, because light makes the colour fade.
Use iron (II) sulfate if the solution is first oxidized to iron (III) sulfate by boiling it with drops of hydrogen peroxide.
3.2.12 Alum solution, invisible writing ink
Alum crystals, K2SO4Al2(SO4)3.2H2O
1. Let the ink dry on the paper then heat the paper over a warm stove until the dry invisible letters become visible as dark carbonized areas.
The alum dehydrates the cellulose in the paper by acting as a proton donor to form H2O from the -OH groups of the cellulose.
2. The experiments below can also be done on a clean egg.
After writing on the egg, wait to allow the chemical to sink in, then wash the egg leaving the secret writing chemical still on the inside of the eggshell.
Then treat the egg as below.
3.2.13 Sodium chloride, invisible writing ink
Use a saturated solution of sodium chloride for ink.
Rub the dry paper with a soft lead pencil.
The secret message appears as a darker pencil mark where the pencil scrapes on the salt crystals.
3.2.14 Starch, cornstarch suspension, invisible writing ink
Starch (C6H10O5)n, starch maize, starch potato, soluble starch, starch solution 2% W / V
Use liquid starch for ink.
Test the paper with iodine solution to be sure that it does not contain starch.
Write on a sheet of paper with a weak solution of boiled starch.
When the writing is dry, develop it by brushing over the paper a weak solution of iodine.
The writing stands out in blue-black lettering.
Read the secret message.
The writing appears dark blue on light blue paper.
The writing disappears if the paper is warmed, but returns when the paper is cooled.
3.2.15 Vegetable juice, onion juice, red cabbage juice, invisible writing ink
These juices convert paper to substances similar to cellophane with ignition temperature lower than paper.
When the paper is gently heated, the parts written on turns brown.
Most organic liquids will char on heating as some of the organic molecules are reduced to carbon with loss of water.
Also, add dilute iodine solution to see white writing on a light blue background or use red cabbage water to make the secret writing appear red.
3.2.16 Watermarks
Watermarks are a kind of secret writing caused not by chemicals, but by physical forces.
Experiment
Wet a sheet of paper on a glass surface.
Place a dry sheet of paper over it and use an H pencil to write on it while pressing down firmly.
Remove the top dry paper and note the writing on the wet paper.
Hang up the wet paper to dry and note the writing disappears, but reappears when the paper is wet again.
The pressure on the paper from the pencil has compressed the thickness and density of fibres of wet paper, so that they reflect light in a different way when wet again.
They may be viewed by transmitted light by holding the wet paper up to the light or viewed by reflected light over a dark background.
Watermarks have been used to identify government paper and to prevent forgery.
They may be seen in some banknotes, postage stamps and in some special paper to identify the brand.
3.2.17 Electric writing, sodium chloride with litmus paper
See diagram 3.4.1: Electric writing.
When an aqueous solution of table salt is electrolysed, the sodium hydroxide solution produced around the cathode can make red litmus paper turn blue.
The chlorine gas formed at the anode has a bleaching effect.
2NaCl (aq) + 2H2O ---> 2NaOH (aq) + H2 (g) + Cl2(g)
Experiment
Soak a piece of red litmus paper in dilute sodium chloride solution and fix it on a sheet of glass.
Connect the terminals of a 6 volt battery to pencil "leads".
Touch the paper with the leads.
Write in blue with the lead attached to the +ve terminal.
Write in white with the lead attached to the negative terminal.
3.2.18 Prepare yellow invisible ink, with copper (II) sulfate
Copper (II) sulfate with ammonium chloride
Add 1 mL of powdered copper (II) sulfate and 1 mL of ammonium chloride to a test-tube nearly full of water.
Dissolve the substances by holding the thumb over the end of the test-tube and shake the test-tube upside.
Use a small paint brush to write on a piece of paper with the faint blue solution.
When the writing has dried, it is invisible.
Warm the paper in front of a flame to make the writing appear in yellow letters.
CuSO4.5H2O + 2NH4Cl --> (NH4)2SO4 + CuCl2 + 5H2O
Copper (II) chloride solution is green-blue.
3.2.19 Prepare invisible inks
Teach the children to prepare invisible inks.
1. To make brown writing, squeeze lemon juice into a bowl and add water to make the invisible ink.
Write a secret message on thick paper or unglazed paper, e.g. drawing paper, using a wide tipped pen or a toothpick.
As soon as the ink dries it becomes invisible.
Send the secret message to someone who knows the secret of reading it!
Hold the secret message over a hot light bulb or near a candle flame.
The ink becomes brown and the message can be read.
Juice of lemons contains many carbon compounds such as sugars.
The heat breaks down the sugars and other compounds into water and carbon.
Pure carbon is black.
The water leaves the paper as water vapour or steam and the carbon remains.
The writing is brown not black, because the carbon is not pure.
You can also use milk, sugar solution, onion juice, and even your own saliva!
2. To make black writing, dissolve alum [potassium alum, AlK(SO4)2.12H2O in water and use as above.
You can also use dilute sulfuric acid, sodium hydrogen sulfate, and copper (II) sulfate.
3. To make blue writing dissolve red cobalt chloride crystals in water to give a pink to colourless solution.
The writing is invisible on pink writing paper, but when you warm the paper it appears brilliant blue.
You can make the writing disappear by breathing on it, but it will reappear when you warm the writing paper again.
4. To make transparent writing use wax or a small piece of candle.
The writing is invisible until you warm the paper.
5. Put one drop of oil and 1 cm of 880 ammonia solution in a test-tube.
Add water and shake the test-tube.
Use the solution to write on paper with a paint brush.
When the paper is dry the writing becomes invisible.
To see the writing again, dip the paper in water.
3.6.5 Prepare fibrous plaster board with plaster of Paris
Plaster of is made by heating the mineral gypsum, (calcium sulfate-2-water), to remove some of its water of crystallization.
This process is called calcination or calcining.
Plaster of is partly dehydrated gypsum 2CaSO4.H2O (s).
Gypsum as a hemihydrate is shown as CaSO4.H2O.
Plaster of Paris is used for making casts, e.g. of the shape of a shell or for keeping broken bones in place.
CaSO4.H2O.2CaSO4.2H2O (s) <--> 2CaSO4.H2O (s) + H2O (l)
gypsum <--> plaster of + water
Mix wet plaster of Paris with fibres.
As the mixture dries, gypsum crystals form again by taking in water, so wet Plaster of Paris does not become dry, because of evaporation.
The setting plaster of Paris gives out heat and expands slightly.
3.3.13 Push pencils through a polythene bag
Half fill a polythene bag with water and tie it shut with one end of a long string.
Suspend the bag by raising the other end of the string.
Very quickly push a sharpened pencil through both polythene bag walls and enclosed water.
Leave the pencil in place and push other pencils through the bag.
Withdraw the pencils and the polythene bag returns to its former shape.
Polythene consists of a web-like matrix of molecules such that the polythene is easily stretched and then can return to its former shape, without tearing.
3.4.1 Glass transition temperature, (Tg)
1. Glass transition temperature, (Tg), is when the molecular structure has macromolecular mobility and the material changes from glassy to rubbery properties.
Soda lime window glass, (about 73% SiO2, 14% Na2O, 7% CaO, 4% MgO, 2% Al2O3), has glass transition temperature, Tg, 564oC, and melting point, Tm, about 1500oC.
At glass transition temperature, the viscosity drops suddenly.
Amorphous polymers below Tg are hard and stiff, but above Tg are rubbery.
The glass transition process is gradual.
Semi-crystalline polymers have less change at Tg and above Tg may not be brittle.
An amorphous polymer has molecular chains in the irregular arrangement at Tg are homogeneous and transparent, because they do not contain crystals to scatter the light.
3.4.2 Chewing gum Tg
Natural chewing gum comes from chicle based on gutta-percha plasticized by triterpenes.
Most chewing gums are based on poly (vinyl acetate), PVA.
If chewing gum is stuck to a carpet, you can freeze it with ice to bring its temperature down to its Tg, then remove it as a solid.
Plastic containers made from recycled plastic has raised Tg, becausethe mix of materials slows molecular movement.
In cold climates, below Tg, the containers become brittle bins and crack easily if dropped.
Calcium may be added to chewing gum only if the chewing gum contains no more than 0.2% residual sugars
Experiment
Tests for chewing gum quality by comparing bubbles
Chew different samples of chewing gum until the taste has gone.
Apply the same exhaling force to make a chewing gum bubble.
Measure the diameter of the chewing gum bubbles.
Note whether the samples of chewing gum are made from chicle based on gutta-percha plasticized by triterpenes or made from poly, (vinyl acetate), PVA.
3.4.3 Cotton, (Tg)
Cotton is a cellulose polymer with Tg of 225oC.
It absorbs water, because the water molecules can slip in between the polymer molecules plasticizes the cotton and lowers the Tg.
When ironing cotton fabric, you can sprinkle with water or use a steam iron to increases the plasticizing effect, then remove by heat to raise the Tg and set the fabric in a new shape.
Nylon fibres have Tg of 50oC and polyester has Tg of 69oC, so lower temperature ironing is needed!
Steam ironing of wool breaks the disulfide bonds that keep wool fibres in shape and then allows them to reform.
3.4.4.0 Breakdown polymers to small molecules
See diagram 3.4.4: Breakdown polymers to small molecules.
Put very small pieces of Perspex or polystyrene in a hard glass test-tube.
Connect a delivery tube to a receiving test-tube that must be cooled thoroughly with cold water, because the fumes are harmful.
Heat the test-tube containing the Perspex gently.
The polymer melts and forms vapours collected in the receiving tube.
Control the heating to enable all the fumes to condense in the receiving tube.
Heat breaks down the polymer to smaller molecules.
3.3.3 Collapse polystyrene beads with propanone
Pour 50 mL of propanone into a beaker full of expanded polystyrene beads used as packing material or into a polystyrene coffee cup in a beaker.
The expanded polystyrene fizzes and shrinks to form a sticky gel.
The expanded polystyrene does not dissolve in the propanone, but just loses the gas that had puffed it out.
3.3.6 Prepare formaldehyde-resorcinol resin
Formaldehyde resorcinol resin is a condensation polymer.
1. Add 2 g of resorcinol to 5 mL of 45% formaldehyde solution in a small beaker and stir the mixture.
Add drops of concentrated hydrochloric acid and stir the mixture.
The mixture suddenly hardens as molecules build up to larger molecules.
Take out this condensation polymer resin and wash it thoroughly.
2. Put 5 mL of 45% formaldehyde solution into a small beaker.
Add 2 g resorcinol and mix very thoroughly with the formaldehyde.
Add a few drops of concentrated hydrochloric acid and stir.
The mixture will suddenly harden as the molecules build up into larger molecules.
Extract this resin and wash very thoroughly.
3.3.7 Prepare nylon polymer
See diagram 16.3.4.7: Condensation polymerization to form a polyamide.
Nylon 6,6 is a polyamide formed by condensation polymerization.
2[HCOOC(CH2)4COOH] + NH2(CH2)6NH 2 --> -C-(CH2)4-C-NH-(CH2)6-NH-C-(CH2)4-C- + H2O
adipic acid + 1,6 diamimo hexane --> nylon 6,6 + water
Nylon 6 is a polyamide formed by ring-opening polymerization.
Wind the polymer onto a glass rod.
Nylons have a structure like a long protein.
Nylons form by condensation of amino group -NH2, of one molecule and carboxylic acid group -COOH, of another molecule.
The 3 main nylon fibres are nylon 6, nylon 6,6 and nylon 6,10.
Nylon 6, 6 ("Bri nylon"), is -NH-(CH2)6-NH-CO-(CH2)4-CO-NH-(CH2)6-NH-.
Nylon 6, 6 forms by condensation of hexane-1,6-diamine, NH2.(CH2)6.NH2 and hexanedioic acid CH2.CH2[COOH)2.
Nylon 6, 6 is thermoplastic and is used for nylon thread, rope, toothbrush bristles, cog wheels, shirts, combs.
Nylon 6.10 is prepared by polymerization of decanedioic acid and 1.6-diaminohexane.
Experiments
Experiment 1.
Solution 1. Dissolve 1.5 g of sebacoyl chloride in 50 mL cyclohexane, i.e. 15 mol per litre.
Solution 2. Dissolve 2.2 g of hexane-1,6-diamine in 50 mL of deionized water, i.e. 1.4 mol per litre.
Pour 5 mL Solution 2. into a beaker.
Pour 5 mL of Solution 1. on top of Solution 2. using a glass rod, so that the two solutions do not mix.
A grey film of nylon forms at the interface of the two solutions.
Experiment 2
Solution 1. Dissolve 2.0 mL of sebacoyl chloride in 50 mL of n-hexane (hexane).
Solution 2. Dissolve 3 g of hexane-1,6-diamine and 1 g of sodium hydroxide in 50 mL of deionized water.
Add phenolphthalein to make it more visible.
Slowly pour Solution 1. as a second layer on Solution 2.
Use forceps to grasp the polymer film that forms at the interface of the two solutions.
Pull it gently from the centre of the beaker.
Wind it round a glass stirrer or a cotton reel.
Wash it thoroughly in 50% ethanol, then in water until moist red litmus paper does not turn blue.
Experiment 3
Solution 1. Add 2 g of sebacoyl chloride to 20 mL of dichloromethane.
Add phenolphthalein to make it more visible.
Solution 2. Add 2 g of hexane-1,6-diamine to 20 mL of 1 M NaOH solution.
Slowly pour Solution 2. into the Solution 1.
Do not mix the solutions.
Grab the film of polymer between the two solutions with forceps.
Wind the polymer onto a glass rod.
Wash the nylon thread in water.
3.3.8 Prepare rayon
1. Copper (II) sulfate with ammonia solution
BE CAREFUL! Concentrated ammonia gives off choking fumes that stings the eyes.
Do this experiment in a fume cupboard.
During manufacture, cellulose filaments pass through solutions that then coagulate them or cellulose ethanoate (cellulose acetate), ethyl cellulose or cellulose nitrate is dissolved in a solvent.
Rayon contains about 270 glucose units per molecule, but cotton contains 2 000 to 10 000 units per molecule.
The solutions are forced through fine nozzles to form rayon or acetate rayon fibre.
Production of rayon uses treat cellulose from wood pulp with sodium hydroxide and carbon disulfide to produce xanthate that is squeezed to produce threads or cellophane.
Rayon may be called "regenerated fibre" or "artificial silk".
Dissolve finely shredded paper in a saturated solution of copper (II) sulfate in concentrated ammonia solution.
Put the solution in a plastic syringe and squirt into 1 M sulfuric acid.
A blue thread forms that slowly runs white.
The acid solution slowly turns blue.
2. Basic copper carbonate with ammonia solution.
Add 10 g of basic copper carbonate to 100 mL of 880 ammonia solution.
Stir then pour the blue solution containing tetraamminecopper (II) ions into a second beaker.
Slowly add 1.5 shredded cotton wool or filter paper or newspaper and stir for up to an hour until the solution becomes a gel, called viscose.
Put viscose into a hypodermic syringe and inject it under 500 mL of 1 mol per litre sulfuric acid.
The extruded blue fibre turns white as the as the acid neutralizes the tetraamminecopper (II) solution.
3.3.4 Prepare Bakelite plastic
phenol / methanal polymerization.
See diagram 16.3.4.9: Phenol-methanal condensation polymerization.
1. Phenolics, phenolic resins, phenol-formaldehyde resins, cresol-formaldehyde resins, melamine-formaldehyde resins, in Formica, electrical insulation, laminates
Bakelite thermoset from phenol and formaldehyde
Bakelite is named after its inventor, L. H. Baekeland, for the resin made from the reaction of phenol and formaldehyde, first polymer, patented 1907, oldest thermosetting resin.
It is used to make electric plugs, saucepan handles, industrial electrical components, parts of cars, radio and televisions parts, telephones.
2. Bakelite phenol-formaldehyde resins, or phenolics.
It is strong, takes a polish, is a good electrical insulator and is resistant to water, alcohol, and acids, used for light bulb holders, electrical fittings. saucepan handles.
Phenolic resins are used in varnishes and lacquers.
Phenols, hydroxybenzenes, are aromatic compounds with the hydroxyl group attached to the benzene nucleus.
They react as alcohols and as weak acids to from salts.
Phenol (carbolic acid, C6H5O10) is used to make Nylon, phenolic resins and epoxy resins.
It is a strong disinfectant.
As it is a weak acid, it can ionize polymer:
C6H5OH --> C6H5O- + H+.
3. The phenol group C6H5-is the organic group in benzene, C6H6.
3. It may be produced in transparent, clear coloured masses.
When powdered and mixed with various filling materials it may be moulded under heat and pressure.
BE CAREFUL!
Experiment
Teacher demonstration only!
Do the experiment in a fume cupboard.
Use safety glasses and nitrile chemical-resistant gloves.
1. Make a mould in Plasticine by pushing an object into it, e.g. a key.
Put resorcinol in a beaker and add the formalin solution.
Stir the solution until it is clear.
Add 1 mL of dilute hydrochloric acid while stirring and then quickly pour the mixture into the mould.
Leave the plastic to harden for a day or two.
Remove plastic from mould and wash with water.
2. Add 30 mL concentrated sulfuric acid to 30 mL water.
Be careful! Pour slowly and keep stirring!
Then leave to cool to room temperature.
Pour 25 mL of formalin into a disposable container and add 55 mL of glacial ethanoic acid (glacial acetic acid).
Add 20 g of phenol C6H5OH and stir with a disposable glass stirring rod until the phenol dissolved.
Add 60 mL dilute sulfuric acid (diluted above) and keep stirring.
The mixture turns pale yellow then opaque pink, especially around the stirring rod.
Heat is given off.
3. Discard the milky liquid then take out the pink polymer, Bakelite, and heat it with a Bunsen burner flame.
The polymer chars, but does not melt, because it is a thermosetting plastic.
Discard the disposable container and the stirring rod.
3.3.11 Prepare slime, PVA slime
Products to amuse children: 1. "Silly putty", silicone polymer.
2. "Laundry Bag Slime", one bag makes 1 litre of PVA solution for slime.
3. "Make Your Own Slime", polymer powder forms thixotropic slime.
4. "Slime School Lab Pack", Non-Newtonian liquid, pre-dissolved PVA.
Slime
Slime flows like a liquid under normal conditions, but bounces on impact.
Slime is a water-soluble polymer sold by chemical supply firms or novelty firms to amuse children.
The long molecules that comprise slime can slide over and around one another and cover the entire bench if left unguarded.
They can also form temporary cross-linking bonds, which affect the viscosity of the slime.
Polymers are very large molecules made by linked monomers.
Polymer molecules can cross link with weak and strong chemical bonds.
A strand of PVA, molecular weight 78, 000, may contain 1800 monomer units.
Funny worms, magic octopus polymer
Thermoplastic polymers may be treated to form substances to amuse children.
They may be plasticized and "tackified" so that when thrown against a clean wall they stick to the wall and fall slowly, so appearing to crawl down the wall.
1. Test the slime by (1) slowly poking it with a finger (2) quickly poking it with a finger (3) slowly pulling it apart, (4) quickly pulling it apart (5) roll it into a ball and note whether it keeps its shape, (6) roll it into a ball and note whether it bounces when dropped on the bench.
2. Prepare glue slime with 50 mL of 4% poly(vinyl alcohol) (PVA) or use pre-dissolved PVA, and 10 mL of 4% borax solution.
3. Add 1 mL of 4% borax solution to the 50 mL of 4% poly(vinyl alcohol).
With each successive addition of 1 mL of 4% borax solution, note any changes when the gel is stirred slowly, stirred rapidly, poured into another container, rolled in the hands to form a ball of gel, left to stand, formed into a ball and pulled on.
4. Dissolve 2 tablespoons of borax in half a cup of warm water with a spoon.
Add a drop of food colouring.
Pour some PVA glue into a bowl.
Put an equal amount of the coloured water into the bowl and mix.
5. Add 7g of powdered or crystalline PVA to 100 mL of water on a hotplate with a magnetic stirrer.
The PVA may take hours to dissolve.
Add 10 mL of borax solution to 25 mL of the PVA solution.
Observe the formation of the gel.
6. Add 10 mL of 4% borax solution to 40 mL of PVA wood glue solution.
Observe the formation of the gel.
7. Dissolve a small water-soluble laundry bag in 1 litre of hot water.
Add 10 mL of borax solution to 40 mL of the PVA solution.
Observe the formation of the gel.
8. The "viscosity builder" grade vegetable guar gum is used as a colloid stabilizer in foods, e.g. salad, dressing and ice cream.
Slime is a non-Newtonian fluid.
Dissolve 1 g of "viscosity builder" grade vegetable guar gum in 20 mL of water.
Boil 60 mL of water and add the 20 mL suspension of guar gum while stirring.
Dissolve 0.75 g of borax in 20 mL of water and add to the still warm solution while stirring.
Leave to cool as a green gum and store in a closed container to prevent drying.
9. At boiling point of water, poly(vinyl acetate) PVA, breaks down to give a lower molecular mass polymer.
Dissolve 3% solution of poly(vinyl acetate) in boiling water then cool rapidly.
Add 10 mL of saturated borax solution and food colouring, then stir slowly.
10. Pour 15 mL of borax solution into a plastic bag, e.g. Ziploc bag.
Add 3 drops of food colouring.
Add 60 mL of 50 % white glue mixture (polyvinyl acetate and polyvinyl alcohol).
Close the plastic bag and kneed it for about 10 minutes until the colour is uniform.
Turn the plastic bag inside out to remove the gloop polymer, called GlueP, a semi-solid plastic material with different properties to the ingredients.
11. Mix an equal quantity of the Elmer's glue and water with a popsicle stick.
Add a saturated solution of borax and stir quickly to from a slimy mixture.
Elmer's glue contains the polymer polyvinyl acetate and the borax solution (sodium tetraborate) is a cross-linking substance that binds the polymer chains together to make the glue solution thicker.
The substance called "'Gak"' is a special form of the polymer polyvinyl alcohol, similar to the this mixture made with Elmer's glue.
3.3.9 Prepare sodium alginate polymer
Medicines to relieve heartburn, e.g. Gaviscon, may contain sodium alginate as well as sodium bicarbonate and calcium carbonate.
An alginate is a polysaccharide from a seaweed, mainly D-mannuronic acid and L-glucoronic acid subunits.
Alginates are also used a food thickeners polymer, E400 Alginic acid, (from seaweed) (vegetable gum, thickener, emulsifier) (used in flavoured milk, ice blocks, yoghurt) E401 Sodium alginate (vegetable gum).
Experiment
Pour a stream of the heartburn medicine, or 2 g of the sodium salt of alginic acid in 100 mL of deionized water, into calcium chloride solution, 1g in 100 mL of deionized water.
Worm-like masses form as the polymer becomes cross-linked.
As soon as they form, lift out some worm-like masses, feel their texture, and put them in a saturated sodium chloride solution in a beaker.
Shake the beaker to see the worm-like masses dissolve to form a cloudy solution.
The cross-linking in the presence of Ca2+ has disappeared as Na+ replaces the Ca2+.
3.3.2 Breakdown polymers with heat.
| See diagram 3.2.97: Breakdown of polymers.
| See diagram 1.13a: Simple fume hood.
Do this experiment in a fume cupboard, fume hood.
Put pieces of polymer, e.g. Perspex or polystyrene, in a hard glass test-tube.
Fit a one-hole stopper with a delivery tube.
Be ready to cool the receiving test-tube with cold water, because the fumes produced by the reaction are harmful.
Slowly heat the hard glass test-tube.
The polymer melts then produces vapours to be collected in the receiving test-tube.
Keep heating until all the fumes from the reaction are condensed to the liquid in the receiving test-tube.
The polymer has been broken down by heat to smaller molecules.
3.3.3a Prepare a polymer ball
Mix glue (containing polyvinyl acetate), water and borax in a beaker.
After a short time, take the mixture out of the beaker and knead it with the fingers intoa ballshape.
The ball can bounce.
Corn starch can be included in the mixture.
3.3.5 Prepare casein plastic from milk.
See diagram 3.100: Casein.
Casein is a phosphoprotein thermoplastic polymer that may be used to make insulators, buttons, handles, adhesives and artist's priming paint.
Casein is prepared from the reactions of skimmed milk with ethanoic acid (acetic acid).
If casein is left in formalin solution it hardens.
It can be moulded and made into buttons.
Experiments
1. Separate some milk from the cream, and put 100 mL of this milk into a beaker.
Heat it to about 50oC and add acetic acid or vinegar until casein ceases to separate out.
Remove the lump of casein and squeeze it with the fingers to free it from liquid.
Work it in the fingers until it becomes rubbery.
Casein is a protein polymer containing nitrogen atoms.
If left in formalin solution it hardens.
It can be moulded and made into buttons.
2. Add 1 mL of glacial acetic acid to 10 mL of water.
Heat 200 mL of skimmed milk to 50oC then maintain the temperature.
Do not let the solution boil.
Add drops of the acetic acid solution or vinegar to the warmed milk while stirring.
Leave to stand until the liquid becomes clear and white-yellow lumps of casein curd separates.
Remove the heat and leave to cool.
Use gloves to remove the lumps of casein, wash them under the tap, and squeeze them together until dry and the resulting one lump becomes rubbery.
Mould it into shapes and then expose it to the air for 2 days.
Leave the dried casein in 40% formaldehyde solution (formalin) to harden.
Polish the hard casein plastic with sandpaper.
Add ammonia solution to prepare glue.
Calcium caseinate + 2H+ --> casein + Ca2+.
3. Use acid to make milk curdle.
Use skimmed milk or dried milk that has been reconverted to milk by mixing with hot water.
Leave fresh milk to stand and later pour off the top layer of cream.
Heat half a beaker of the milk, but stop heating before the beaker becomes too hot to hold in the hand.
Pour a test-tube of vinegar into the beaker while stirring the milk with a glass rod.
The milk curdles.
Remove the curd with a spoon onto a newspaper and squeeze out the moisture by folding and pressing the newspaper to leave a spongy mass left of casein.
Milk also curdles when it turns sour.
This reaction is caused by bacteria that change the milk sugar into lactic acid, C3H6O3, that precipitates the casein as a curd.
Pure lactic acid is a white crystalline substance.
4. Heat milk until it is hot, but not boiling.
Pour the milk into the bowl.
Add the vinegar to the milk and stir it up with a spoon for about a minute.
Casein occurs when the protein in the milk reacts with the acid in the vinegar.
The casein in milk does not mix with the acid and so it forms blobs.
Pour the hot milk through the strainer into the sink to leave behind lumpy blobs in the strainer.
When cool enough, rinse the blobs water while you press them together.
Press the blobs into a shape and it will harden in a few days.
5. Into one jam jar put 2 tablespoons of dried skimmed milk powder and mix with about 100 mL of water, about 60oC.
Add 15 mL of vinegar and stir well.
The milk curdles as the vinegar is added to form solid curds and liquid whey.
After about three minutes the liquid and solid will begin to separate out.
Cover the top of the other jam jar with coffee filter paper and filter the milk / vinegar mixture by pouring the liquid into the filter.
Discard the liquid, but retain the solid residue on the coffee filter paper.
To make buttons, take a lump of residue, press into a thin layer and mould into a button shape with holes for sewing on with a needle and thread.
Put the button on a piece of aluminium foil and allow to dry overnight.
To make glue, scrape the residue into a clean container and add 1 tablespoon of warm water and half a teaspoon of bicarbonate of soda.
Mix well for a couple of minutes until mixture is smooth.
3.3.12 Prepare urea-formaldehyde resin.
See diagram 1.13a: Simple fume hood.
Urea-formaldehyde, urea-methanal, non-transparent thermosetting resin
Do not use hydrochloric acid as a catalyst for these preparations, because the carcinogenic bis(chloromethyl) ether may form.
Urea-formaldehyde is very difficult to ignite.
It burns with a yellow flame with blue-green edge, but does not burn after removing the flame.
It burns with has an alkaline / formaldehyde / fishy smell.
1. Place two mL of 40% formaldehyde solution in a boiling tube and add about 1 g urea.
Stir until a saturated solution is obtained.
Add one or two drops of concentrated sulfuric acid.
The mixture suddenly hardens as it builds up to a large molecule.
Extract it and wash very thoroughly.
This is a condensation polymer.
2. Add 30 mL of vinegar to 50 mL of warm skimmed milk while stirring.
Observe lumps of casein forming.
Separate the lumps with a strainer and mix them with 30 mL of warm water.
Add 90 mL of sodium bicarbonate.
Sir the mixture and force it through a sieve.
After leaving to stand overnight, the mixture can be used as an adhesive.
3.Be careful! The solution becomes cloudy and a white powder deposits in the plastic container, because of the formation of the resin.
The solution becomes hot.
Mix 10 g of urea with 20 cc of 40% formaldehyde (formalin) solution in a plastic container that can be thrown away.
Add 1 cc of concentrated sulfuric acid drop by drop and stir the mixture.
4. Make a Plasticine (modelling clay) mould lined with aluminium foil.
Put fibres from a broom in the mould.
Mix urea with twice its weight of formalin and pour it into the mould.
Add drops of dilute sulfuric acid.
Heat in a fume cupboard, fume hood until the solution becomes cloudy, because of the formation of the hard resin.
5. Hold some hard resin with tongs in a Bunsen burner flame.
The resin chars, but does not burn showing that it is a thermosetting plastic.
A condensation polymerization forms with the elimination of water:
(NH2).CO.(NH2) + CH2O --> NH-CO-NH-CH2 + H2O
urea + formaldehyde --> urea-formaldehyde + water
6. Concentrated solutions of formaldehyde very slowly form a white precipitate of paraformaldehyde.
3.6.1 Heat different crystals, water of crystallization
Chemical compounds that contain water molecules in their crystal structures may be known as hydrated salts in the chemicals industry.
Salts containing water of crystallization may be known as hydrated salts.
Heat crystals of bluestone CuSO4.5H2O, borax Na2B4O7.10H2O, common salt NaCl.
Epsom salts MgSO4.7H2O, Glauber's salts Na2SO4.10H2O, green vitriol FeSO4.7H2O, hypo Na2S2O3.5H2O, washing soda Na2CO3.10H2O, and white vitriol ZnSO4.7H2O.
They all contain water of crystallization except common salt.
Test the presence of water with blue cobalt (II) chloride paper, CoCl2.
CoCl2 (s) + 6H2O (l) <--> CoCl2.6H2O (s).
3.6.2 Heat iron (II) sulfate-7-water crystals
The water in the crystal is necessary for the shape and colour of the crystal.
Put crystals in a test-tube.
Heat gently and note any reaction.
The crystals lose water vapour that condenses as liquid in the cool upper region of the test-tube.
The crystals lose their shape and colour.
Fe2SO4.7H20 <--> FeSO4 + 7H2O.
3.6.3 Heat magnesium sulfate-7-water crystals
1. Heat 5 g of Epsom salts crystals, MgSO4.7H2O, in a dry test-tube over a flame.
Hold the test-tube in a paper holder so that it slopes down slightly towards the open end.
The water of crystallization lost forms steam to condense as water in the cooler part of the test-tube to leave anhydrous magnesium sulfate in the test-tube is a white powder.
Repeat the experiment with copper (II) sulfate crystals, CuSO4.5H2O.
2. Relative molecular mass = 246.47. Weigh the hydrated crystals, W1.
Heat the crystals and weigh again.
Continue heating and weighing until the weight does not change, W2. Weight of a water of crystallization = (W1 - W2).
Number of molecules of the crystal, n = (W1 / 246.47).
Number of molecules of water in each molecule of crystal = (W1 - W2) / n = 7.
3.6.4 Make objects and moulds with plaster of Paris
Mix three parts of plaster of Paris with one part of plain flour.
Add enough water to make a stiff dough.
Use this paste to cover objects to be copied or objects to be formed in any shape.
Leave to dry for one day, then brush the surface with a solution of one part alum and ten parts water.
Leave to dry, then paint if needed.
4.1.0 Tests for plastics, burning tests
1. Copper wire burning test
BE CAREFUL! Hold small samples with tongs in a fume cupboard or well-ventilated place.
Hold the burner at an angle at an angle.
Stick a copper wire in a cork.
Heat the wire with a Bunsen burner until any yellow, green or red colour disappears.
Press the end hot wire into the plastic sample, then put the end with molten plastic on it back in the flame.
Observe the colour in the flame, usually yellow, easy or difficult to ignite, melting, residue, fumes and odour.
Remove the burning plastic from the flame and note whether it still burns.
A green colour indicates that the plastic contains a halogen, e.g. chlorine in poly(vinyl chloride) (PVC) or poly(vinylidene chloride) (PVDC).
Cyanide, e.g. from Orlon, may give a positive result.
Before repeating the test with another plastic, again heat the copper wire until the green colour disappears.
2. Ignition burning test
The laboratory must be well ventilated.
Put on protective glasses.
Place a Bunsen burner next to a sink with some water in it.
Light the Bunsen burner.
Hold each of the materials in a test-tube holder over a tin lid or dish, try burning each with the spirit burner flame.
Note how easily or otherwise they burn, whether they leave much ash or char, and whether any easily recognized smell is formed.
Or
Use tongs to hold a test piece of plastic in the outer part of the Bunsen burner flame.
When the test piece catches fire move it over the sink to allow molten drops of plastic to fall into it.
Blow out the flame on the test piece.
Use your hand to gently fan the smoke towards you nose and note the smell.
See diagram 1.13: Smelling a gas.
3. Heating burning test
Gently heat 0.1 g of plastic on a clean spoon over a small colourless Bunsen burner flame until it fumes without ignition.
Remove the spoon from the flame then test the fumes with moist litmus paper.
Note the smell of the burning plastic.
Move the spoon to the hottest part of the Bunsen burner flame.
Observe the following:
* Whether the material burns, and if so, how easily.
* The nature and colour of any flame, a very sooty flame generally indicates an aromatic polymer.
* Whether the plastic continues to burn after removal from the flame.
* The nature of any residue.
Plastics and other materials can also be identified with hot needle tests.
4.6.0 Tests for plastics, thermal behaviour
1. Put test strips of thermoplastics in hot water and compare their softness.
Test the strength of the plastic samples by bending.
2. Put aluminium foil over an electric hotplate in a fume cupboard.
Place plastic granules on the aluminium foil, turn on the hotplate and note which plastics soften or melt on heating.
Turn off the hotplate and leave to cool.
Turn on the hotplate and note whether the plastics soften or melt on reheating.
3. Test for thermoplastic or thermoset plastic
Heat a piece of wire until just below red and press it into the specimen.
The wire penetrates the thermoplastic, but not the thermoset plastic.
4.2.0 Tests for natural fabrics, burning tests
1. Animal fibres burning test
Animal fibres, e.g. wool, contain nitrogen compounds.
When heated they form ammonia.
Wool burns slowly, appearing to melt together.
It chars, and gives a smell of singed hair.
2. Cotton, burning test
Heat a small piece of cotton in a dry test-tube, and hold at the mouth of the test-tube a moist piece of blue litmus paper.
The litmus paper turns red, caused by ammonia.
Cotton is of plant origin so does not contain nitrogen compounds and does not produce ammonia.
You can make a piece of litmus paper blue by pouring a few drops of limewater on it and washing it in water.
Cotton and burns easily, leaving only grey ash.
3. Linen, burning test
It is not of animal origin, does not contain nitrogen compounds, so does not form ammonia.
4. Silk, burning test
Heat a small piece of real silk in a dry test-tube, and hold at the mouth of the test-tube a moist piece of red litmus paper.
The litmus paper turns blue, caused by ammonia.
Animal fibres, e.g. silk, contain nitrogen compounds.
When heated they form ammonia.
Silk bums readily, with an orange-yellow flame.
A black bead of ash is formed and a smell of burning hair.
Wool, burning test
Heat a small piece of wool in a dry test-tube, and hold at the mouth of the test-tube a moist piece of red litmus paper.
The litmus paper turns blue, caused by ammonia.
5. Wool or cotton, burning test
Distinguish wool from cotton.
Place a finger width of sodium hydroxide solution in a test-tube and add a strand of wool.
Heat the solution.
Describe what you see.
Repeat the experiment with cotton and see if the same thing happens.
The wool dissolves, the cotton does not dissolve.
Cotton is of plant or vegetable origin, does not contain nitrogen compounds, so does not form ammonia.
Cotton forms acid vapours when heated.
4.3.0 Tests for synthetic fibres, burning tests
4.3.1 Acrylonitrile-butadiene-styrene burning test
Acrylonitrile-butadiene-styrene, has yellow flame with blue base, smoky, burns after removing flame, styrene smell.
4.3.2 Casein, milk casein, burning test
Casein, milk casein is easy to ignite, yellow flame, does not burn after removing flame, burnt milk smell.
4.3.3 Celluloid, cellulose acetate burning test
Celluloid, cellulose acetate is easy to ignite, yellow flame, burns after removing flame, acidic fumes, acetic acid smell.
4.3.4 Epoxy resin, burning test
Epoxy resins, epoxide, is easy to ignite, orange yellow smoky flame, burns after removing flame, acrid smell.
4.3.5 Ethyl cellulose burning test
Ethyl cellulose: if ignited forms drips on ignition, blue-yellow flame with a green base, burns after removing flame, burning wood smell.
4.3.6 Polyamide, nylon, burning test
Polyamide, nylon is easy to ignite and forms a clear melt, blue flame with a yellow tip, does not burn after removing flame, burnt vegetation smell.
4.3.7 Melamine-formaldehyde burning test
Melamine-formaldehyde is very difficult to ignite and forms alkaline fumes, pale yellow flame with light blue-green edge, formaldehyde and fish-like smell.
4.3.8 Nylon 6 and nylon 6.6 burning test
Nylon is easy to ignite and forms a clear melt, blue flame with a yellow tip, does not burn after removing flame, burnt vegetation smell.
Nylon 6 and 6.6 melts and burns in the flame, smoke with white fishy odour like celery, yellow melted falling drops.
Stops burning when removed from flame with small bead on the end that can be stretched into a fine thread.
Residue is a hard round bead.
Note how nylon behaves when heated in the test-tube.
It first melts to a brown liquid, and ammonia is evolved.
It does not burn easily.
Nylon is a synthetic (man-made) fibre that gives ammonia when heated.
However, the way it melts distinguishes it from animal fibres.
4.3.9 Phenol formaldehyde resin, burning test
Phenolics, phenol formaldehyde resin is difficult to ignite and burn, yellow flame, does not burn after removing flame, phenolic / formaldehyde smell.
4.3.10 Polyacrylonitrile, burning test
Polyacrylonitrile is easy to ignite, yellow flame, burns after removing flame, cyanide / burnt wood smell.
4.3.11 Polycarbonates burning test
Polycarbonates are at first difficult to ignite, yellow smoky flame, burns after removing flame, phenolic smell.
4.3.12 Polyesters, burning test
Polyesters melt and burn with black smoke that has a faintly sweet and oily sooty odour like sealing wax, and melted falling drops.
When removed from flame it stops burning with a black bead on the end that can be stretched into a fine thread.
Polyester is a condensation polymer of polyhydric alcohol and polybasic acid, linear polyester is "Terylene", unsaturated polyesters are used in fibre-glass.
4.3.13 Polyethylene, burning test
Polyethylene is easy to ignite and forms a clear melt, yellow flame with blue base, burns after flame removed, burning wax candle smell.
In flame it shrinks, curls and melts and continues to burn slowly with candle wax smell.
When removed from the flame leaves a residue like paraffin wax.
The hot melted substance cannot be stretched, but it can be scratched with a finger nail.
4.3.14 Polymethyl methacrylate burning test
Polymethyl methacrylate is easy to ignite, yellow flame with a blue base.
4.3.15 Polypropylene, burning test
Polypropylene shrinks and burns in flame with smoke that smells like burning asphalt or like candle wax, and continues to burn rapidly when removed from the flame leaving a brown-yellow residue.
The hot melted substance an be stretched into a fine thread.
4.3.16 Polystyrene, burning test
Polystyrene is easy to ignite, blue-yellow smoky flame, burns after removing flame, sweet styrene smell.
4.3.17 Polytetrafluoroethene burning test
Polytetrafluoroethene is difficult to ignite then chars slowly, yellow flame, does not burn after removing flame, acidic fumes, no smell.
4.3.18 Polyurethane, burning test
Polyurethane is is easy to ignite, yellow flame with a blue base, burns after removing flame, acrid smell.
4.3.19 Polyvinyl acetate, burning test
Polyvinyl acetate is easy to ignite and forms a black residue, yellow smoky flame, burns after removing flame.
4.3.20 Polyvinyl chloride burning test
Polyvinyl chloride is easy to ignite, yellow flame with a green base, does not burn after removing flame.
It forms acidic fumes with the acrid smell of hydrochloric acid.
4.3.21 Rayon, burning test
Heat a small piece of rayon in a dry test-tube, and hold at the mouth of the test-tube a moist piece of blue litmus paper.
The litmus paper turns red, caused by ammonia.
Rayon is of plant origin so does not contain nitrogen, so does not form ammonia.
Rayon forms acid vapours when heated.
Rayon burns easily, leaving only grey ash.
4.3.22 Urea-formaldehyde burning test
Urea-formaldehyde is very difficult to ignite, yellow flame with blue-green edge, does not burn after removing flame, has alkaline / formaldehyde / fish smell.
4.4.0 Tests for plastics, transparency and feel
Note whether the plastic is glass clear, translucent, opaque.
However, plastics are usually coloured during manufacture.
Clean the surface of the test strips of any grease and feel them between the first finger and thumb.
Only polyethylene and polytetrafluorethylene have a waxy feel.
Feel the test polymer.
Only polyethylene and polytetrafluorethylene have a waxy feel.
Before the test, clean the surface to remove grease or plasticizers.
4.5.0 Tests for plastics, flotation tests
1. Polyethylene, polypropylene, styrene- butadiene and some types of nitrile will float after wetting it thoroughly, pushing it below the surface of water then released.
You cannot test foam plastics in this test.
Wet test strips of the following plastics and then push them down below the surface of water + a drop of detergent in a beaker.
Some plastics contain additives that affect the density.
The Polyethene, PE strip floats in tap water, because its density is 0.9 g cm3.
Slowly add sodium chloride to the tap water so that it all dissolves.
As the density of the water increases the test pieces start to move up towards the surface of the water.
Note the order the test pieces move up.
The PE test strip moved up first so label it 1., then label the others.
Test strips and order of moving up:
1. Polyethene, PE, polyethylene,
2. Polystyrene, PS,
3. Polymethyl methacrylate, PMMA,
4. Polyvinyl chloride, PVC,
5. Phenolic, PF
6. Polyester, UP
2. Test about 200 cm3 of following plastics in solutions of known density: PET, PVC, PS, HDPE, LDPE, PP EPS
For example PS sample should float in solution 5., but sink in solution 1.
Table 3.104 Tests for plastics in known density solutions
Solution
 Density
 Composition of 1000 cm3 solution
1. 0.79
 Pure ethanol (IMS)
2. 0.91
 471 g (596 cm3) ethanol in 439 cm3 deionized water
3. 0.94
 354 g (448 cm3) ethanol in 586 cm3 deionized water
4. 1.00
 Deionized water
5. 1.15
 184 g K2CO3 in 965 cm3 deionized water
6. 1.38
 513 g K2CO3 in 866 cm3 deionized water