School Science Lessons
2024-06-04bb
Heat sources, Candles, Combustion
(topic08)
Contents
8.1.0 Candles
8.2.0 Combustion
8.3.0 Heat sources
8.1.0 Candles
Candle wax is a mixture of different alkanes that are solid at room temperature.
| See diagram 3.1.2.6: Candle burner.
| See diagram 4.20: Copper coil candle snuffer.
Experiments
8.1.1 Aluminium foil below candle flame
8.1.2 Burn a candle in a falling plastic bottle
8.1.3 Burn a candle, lighted inside a jar
8.1.4 Burn a floating candle under a jar
8.1.5 Burn a long candle standing under a jar
8.1.6 Burn long and short candles over water
8.1.7 Burn one, two and three happy birthday cake candles inside a jar
8.1.8 Burn short and tall candles in separate jars
8.1.9 Burning candle, rising water
8.1.10 Burn candles in closed containers
8.1.11 Burning candles over water
8.1.12 Candle flame consists of burning vapours
8.1.13 Candle flame forms carbon dioxide
8.1.14 Candle flame forms water
16.2.5 Candle in a bottle, candle in dropped jar
8.1.15 Candle, paraffin wax
8.1.16 Carbon, soot from a candle flame
8.1.17 Collect and weigh the gaseous products of a burning candle
8.1.18 Dark region of a candle flame
8.1.19 Egg in a candle flame
8.1.20 Floating tea candle
8.1.21 Hottest part of a candle flame
8.1.22 Melt candle wax
8.1.23 Parts of a candle flame
8.1.24 Prepare beeswax candles
8.1.25 Prepare lampblack
8.1.26 Re-lighting candles
8.1.27 Rocking candle
8.1.28 Test gases from the candle wick
8.1.29 Test for carbon dioxide from a burning candle
8.1.30 Candle flame, (primary)
8.2.0 Combustion
8.2.1 Carbon dioxide is a product of combustion
8.2.2 Combustion, conditions for combustion, to ignite, ignition point
8.2.3 Oxygen gas is necessary for combustion
8.2.5 Respiration is a form of combustion
8.2.5 Sugar with potassium chlorate, spontaneous combustion
8.3.0 Heat sources
8.3.0 Heat sources
8.3.1 Light a match of a box of safety matches
8.3.2 Study a match flame.
8.3.3 Study a flame in a gas stove.
8.3.4 Burn (Monopoly money, fake money)
8.1.1 Aluminium foil below candle flame
Cut a slot in a piece of aluminium foil and slide it just below the base of the flame and above the melted wax.
The flame dies down or becomes extinguished, because the foil conducts away the heat so you cannot ignite the gases.
8.1.2 Burn a candle in falling plastic bottle
See diagram 6.35.6: Burning candle in falling plastic bottle.
Attach a candle to the screw top of a plastic bottle.
Darken the room.
Light the candle, attach the screw top, and drop the plastic bottle.
The candle flame heat the air around the wick, but the hot air falls with the same speed as the candle.
So the wick remains surrounded by the hot air and the products of combustion of the candle wax vapour.
Immediately after the plastic bottle is dropped, the candle flame becomes dimmer as a blue spherical disc.
The flame receives only a small supply of oxygen by diffusion, because the carbon dioxide remains around it and is replaced by the oxygen when it rises.
Hot carbon dioxide, and some water vapour, accumulate at the top over the candle until the flame is extinguished.
8.1.3 Burn a candle, lighted inside a jar
See diagram 6.35.4: Ignite candle inside jar.
This is a more accurate way of doing the experiment, because the candle is not already burning when the inverted jar is placed over it.
So you are comparing the volumes of room temperature air before combustion and after combustion.
1. Use adhesive tape to attach the head of a match to a candle wick.
2. Place an inverted jay over the candle.
3. Light the match by focussing sunlight on it.
4. Leave the apparatus to cool to room temperature.
5. Measure the increase in water level inside the inverted jar.
The increase is about 5%, not 20%.
8.1.4 Burn a floating candle under a jar
See diagram 6.35.0: Candles burning in closed containers.
1. Fix a short candle to a cork.
Put water in the trough so that the water level is about half the length of the candle.
Put water in the trough.
The short candle on the cork floats in the water.
2. Measure the depth of the water in the trough.
Measure the length of the jar.
The jar is full of air so you are really measuring how much air in the jar.
Light the candle.
3. Place an inverted jar over the candle quickly so that the mouth of the jar is under water.
4. While the candle is alight, gas bubbles come out from under the jar and rise through the water in the trough.
The candle flame heated the air in the jar causing the air to expand.
Lighter gases go to the top of the jar, because gravity exerts a stronger pull on denser gases.
As these colder, denser gases move downward, lighter gases take their place at the top of the jar.
5. The candle flame gets smaller, splutters, then goes out, because some oxygen was converted to carbon dioxide gas.
The blackened wick is carbon.
6. The volume of gases in the jar decreases, because the gases in the jar cool, some water vapour condenses inside the jar and on the
wick and some carbon dioxide dissolves in the water.
The gases in the jar cool and contract to a smaller volume than before so the air pressure in the jar becomes less than the atmospheric pressure.
The air pressure on the surface of the water in the trough pushes the water up into the jar.
The water level rises in the jar and drops slightly in the trough.
7. The decrease of volume inside the jar is about 20%.
Some people wrongly think that the candle flame has "consumed" all the oxygen and that the amount of oxygen consumed = the decrease in volume of gases in the jar, thus showing that air contains
20 % oxygen.
8.1.5 Burn a long candle standing under a jar
See diagram 6.35.0: Candles burning in closed containers.
1. Stand a long candle in a trough.
Put 3 rubber plugs on the bottom of the trough so that the rim of a jar may rest on them and allow water to enter under the jar.
2. Put water in the trough so that the water level is about half up the length of the candle.
Measure the depth of the water in the trough.
Measure the length of the jar.
3. Light the candle.
Quickly place an inverted jar over the lighted candle so that the mouth of the jar is under water, resting on the three rubber plugs.
4. While the candle is alight, gas bubbles come out from under the jar and rise through the water in the trough.
5. The candle flame gets smaller, splutters, then goes out.
6. The water level rises in the jar and drops slightly in the trough.
7. Some water has condensed inside the jar and on the wick.
8. The decrease of volume inside the jar is about 20%.
8.1.6 Burn long and short candles over water
See diagram 6.35.7: Burn long and short candles over water.
1. Attach a tall candle and a short candle to the bottom of a trough.
Add water to the trough and note the water level.
Add ink or cochineal to colour the water.
Light both candles.
Put an inverted jar over the burning candles.
The tall candle extinguishes first then the short candle.
Hot gas and less dense products of combustion including carbon dioxide gas and water vapour have filled the jar from the top down to extinguish the tall candle flame first.
The cloud of hot carbon dioxide and water vapour gets thicker and so descends to reach the tall candle flame first.
Some hot gases push out under the rim of the jar to form bubbles around the jar in the trough.
When the candles are extinguished, the hot gases cool and contract to form a partial vacuum and the water level rises inside the jar.
This experiment shows that in the above experiments the lighted candle did not "use up the oxygen in the jar", because when the tall candle goes out the short candle keeps burning for some time.
Candles will not burn when air has lost about 30% of its oxygen.
8.1,7 Burn one, two and three happy birthday cake candles inside a jar
See diagram 6.35.5: Burn candles over water.
1. Fix the candles to Plasticine (modelling clay), at the bottom of the tray, then do the experiment as before.
2. The air in the jar is less heated with smaller candles so you probably do not see bubbles coming out from under the jar.
The jars contained about the same amount of oxygen, but the jar with the three candles showed the greatest water rise, because more heat was produced by the candles in it.
8.1.8 Burn short and tall candles in separate jars
See diagram 6.35.3: Burn long and short candles over water.
1. Burn two candles over water.
Attach a tall candle and a short candle to the bottom of a trough.
Add water to the trough and note the water level.
Light both candles.
Put a large jar upside down over the candles.
The tall candle extinguishes first then the short candle.
Hot gas products of combustion, including carbon dioxide gas, have filled the jar from the top down to extinguish the candle flames.
Some hot gases push out under the rim of the jar to form bubbles around the jar in the trough.
When the candles are extinguished, the hot gases cool and contract to form a partial vacuum, and the water level rises inside the jar.
2. In the jar over the tall candle, the water rises almost immediately, mostly before the flame is extinguished.
In the jar over the short candle, the water hardly rises until the flame dies, then rises rapidly.
When the candle goes out, most of the cooling of gases occurs at the top of the jar, where the air is hottest and contact with the glass is greatest.
The taller candle begins to dim earlier, caused by the smothering cloud of carbon dioxide, so the flame gives off less heat.
So the rate of cooling is largely determined by the temperature of the air at the top of the jar.
The flames of shorter candles are farther away from the top, so they take longer to heat up the air up there.
By the time a short candle completely goes out, much carbon dioxide has been produced, mostly lying close to the water.
After the air cools, there is more opportunity for the carbon dioxide to dissolve into the water than with a tall candle.
This may contribute to the rapid rise after a short candle is extinguished, because the air is losing carbon dioxide.
8.1.9 Burning candle, rising water
See diagram 8.3.0: Candles burning in closed containers.
1. Attach a tall candle and a short candle to the bottom of a trough.
Add water to the trough and note the water level.
Light both candles.
Put a large jar upside down over the candles.
The tall candle extinguishes first then the short candle.
Hot gas products of combustion including carbon dioxide gas have filled the jar from the top down to extinguish the candle flames.
Some hot gases push out under the rim of the jar to form bubbles around the jar in the trough.
When the candles are extinguished, the hot gases cool and contract to form a partial vacuum and the water level rises inside the jar.
Some decrease in volume will be caused by the candle wax burning to form carbon dioxide and water.
Some of the carbon dioxide will dissolves in the water from the trough and the water vapour formed will condense to form liquid water.
More air escaped from the jar in the beginning due to large amount of heat released by the two candles.
2. When we ignite the candle, the stearin (purified fatty acids), reacts with oxygen (in excess), to produce carbon dioxide and water.
The burning causes air currents to shape the candle flame and ensure complete combustion at the bottom and the outer surface of the flame.
The hot air and products of combustion rise up above the flame.
3. When a jar is placed over the burning candle the hot gases in the jar expand and pushing some of the air out of the jar as bubbles in the water.
As soon as the rim of the jar touches the water, the burning occurs in a closed environment.
Further pressing the jar down into the water helps to retain the hot air in the jar under a pressure greater than atmospheric pressure and balanced by the pressure of the depth of water.
The burning of hydrocarbon in the jar produces more molecules of carbon dioxide and water than the molecules of oxygen consumed in the reaction.
The increased heat and number of molecules increases the pressure in side as a result if not careful some bubbles of gas will escape from the jar.
Over the time the oxygen in the jar is reduced and conditions for burning are changed.
3. Burning under reduced oxygen may not produce carbon dioxide rather a little carbon monoxide.
When the candle is put out, the temperature decreases followed by also a decrease in pressure due to condensation of water vapour and decreased quantity of air due to thermal expansion during the process of placing the jar on the candle.
The overall situation is a decrease in pressure inside the jar as compared to atmospheric pressure so despite water being heavier than air, it is pulled into the jar.
A negligible amount of carbon dioxide is dissolved in the water during 30 - 40 minutes, the time the experiment usually takes for performing in a classroom situation.
4. If the number of candles is increased in the jar, the heat produced is more therefore more air is likely to escape from the jar due to thermal expansion during the process of pacing the jar over them.
Therefore, more water will rise in the jar with more candles.
The nature and quantity of the products will depend upon the composition of candle material.
However, it is assumed that combustion of saturated hydrocarbons is taking place during burning.
4. For the paraffins in the stearin candle, chain length, n = about 30 cm.
During combustion the solid stearin combines with oxygen gas to form carbon dioxide + water vapour.
So the expansion of gases caused by this chemical reaction = 4/3 = 1.3'.
2CH2 (s) + 3O2 (g)--> 2CO2 (g) + 2H2O (g)
However, after the candles are extinguished, drops of water appear on the inside of the jar caused by condensation, so 3 volumes of oxygen have produced 2 volumes of carbon dioxide, a contraction of 2/3.
5. Previously, teachers taught that the candle become extinguished, because all of the oxygen under the inverted jar "was used up", i.e. converted to carbon dioxide, and so the decrease in volume of air under
the jar after the candles are extinguished indicating the proportion of oxygen in the air.
However, some oxygen remains in the inverted jar as can be demonstrated by testing with yellow phosphorus.
6. The rapid rise of water level in the jar after the candles are extinguished is caused by decrease in pressure as the hot gases cool and the condensation of water vapour.
The amount of condensation of water will depend upon the temperature difference between initial and final temperatures of the air in the jar.
Since air is above water, therefore saturated water vapour pressure is considered in the beginning of the experiment.
Increase in temperature, during the candle burning, will make air unsaturated to accommodate additional water vapours especially produced as a product of burning.
A decrease in temperature over time after the candle is off to the initial temperature will help water vapour to condense.
This condensation will decrease the pressure inside the jar and will help water rise in the jar.
The amount of water vapours condensed during a small change of temperature as usually occurs in this experiment may even be too small to notice.
A little carbon dioxide dissolves in the water during the experiment.
A jar full of carbon dioxide inverted over a trough of water does not completely dissolve after some days.
7. To study the level of water rise when the candle was put out as soon jar touched the water, a floating candle was used and it was made to sink as soon as jar touched the water in the trough.
It was found that water did rise to some extent, indicating that some air escaped from the jar, because hot air and burning products entered the jar from the candle during the process of placing the jar
over the candle.
The oxygen in the jar after the candle was extinguished produced rust in steel wool, reacted with yellow phosphorus to produce white smoke of oxide and supported survival of a mouse and insect for a long
time.
8. To test whether the presence of carbon dioxide or lack of oxygen extinguishes the candle, remove the carbon dioxide from the jar by using sodium hydroxide solution in the trough in place of water.
Also you can spray cotton wool with sodium hydroxide and attach it to the bottom of the jar before it is inverted on the candle.
The candle burning time was almost doubled indicating that it is thepresence of carbon dioxide that extinguishes the candle.
When a candle burning under a jar inverted over water in a trough was repeated using two and three candles.
The level of water in the jar increased with an increase in number of candles.
This finding was used to emphasize that more oxygen is escaped from the jar before or during the burning of candles.
However, it is not true that more oxygen was consumed with the increase in the number of candles.
8.1.10 Burn candles in closed containers
See diagram 8.3.0: Candles burning in closed containers.
When you light a candle wick, some heat from the burning wick melts the wax at the top of the candle.
The melted wax rises in the wick, because of capillary forces and some of that wax evaporates to form a vapour that rises then burns with a bright flame.
The flame of a burning candle in a closed container will go out after some time, not because all the oxygen is used up, as some teachers used to think.
The flame goes out, because the hot carbon dioxide and water vapour produced by the ignition accumulates down from the top of the container.
They displace other gases, including oxygen, and finally stifle the flame.
20.1.4 Burn candles over water
See diagram: 3.1.4.5: Burning candle over water.
1. Fill a trough with water and a float a burning candle in it or attach
burning candles to the bottom of the trough.
8.1.11 Burning candles over water
See diagram 8.3.0: Candles burning in closed containers.
See diagram: 3.1.4.5: Burning candle over water.
1. Attach a tall candle and a short candle to the bottom of a trough.
Add water to the trough and note the water level.
Light both candles.
Put a large jar upside down over the candles.
The tall candle extinguishes first then the short candle.
Hot gas products of combustion including carbon dioxide gas have filled the jar from the top down to extinguish the candle flames.
Some hot gases push out under the rim of the jar to form bubbles around the jar in the trough.
When the candles are extinguished, the hot gases cool and contract to form a partial vacuum and the water level rises inside the jar.
2. Fill a trough with water and a float a burning candle in it or attach burning candles to the bottom of the trough.
Invert a large beaker over the candle or candles.
Note the level of the water inside the beaker.
At first, the candle keeps burning and the volume of air inside the beaker increases, caused by the heat from the candle, until some air escapes from below the beaker to form bubbles in the trough.
The candle flame is extinguished when all the oxygen in the air inside the beaker is converted to carbon dioxide and carbon monoxide and some smoke may issue from the wick from the carbon of partially oxidized hydrocarbons.
The level of the water inside the beaker rises to above the original level.
3. Some decrease in volume will be caused by the candle wax burning to form carbon dioxide and water.
Some of the carbon dioxide will dissolves in the water from the trough and the water vapour formed will condense to form liquid water.
More air escaped from the jar in the beginning due to large amount of heat released by the two candles.
4. When we ignite the candle, the stearin (purified fatty acids) reacts with oxygen (in excess) to produce carbon dioxide and water.
The burning causes air currents to shape the candle flame and ensure complete combustion at the bottom and the outer surface of the flame.
The hot air and products of combustion rise up above the flame.
When a jar is placed over the burning candle the hot gases in the jar expand and pushing some of the air out of the jar as bubbles in the water.
As soon as the rim of the jar touches the water, the burning occurs in a closed environment.
Further pressing the jar down into the water helps to retain the hot air in the jar under a pressure greater than atmospheric pressure, and balanced by the pressure of the depth of water.
5. The burning of hydrocarbon in the jar produces more molecules of carbon dioxide and water than the molecules of oxygen consumed in the reaction.
The increased heat and number of molecules increases the pressure in side as a result if not careful some bubbles of gas will escape from the jar.
Over the time the oxygen in the jar is reduced and conditions for burning are changed.
Burning under reduced oxygen may not produce carbon dioxide rather a little carbon monoxide.
When the candle is put out, the temperature decreases followed by also a decrease in pressure due to condensation of water vapour and decreased quantity of air due to thermal expansion during the process of placing the jar on the candle.
The overall situation is a decrease in pressure inside the jar as compared to atmospheric pressure, so despite water being heavier that air, it is pulled into the jar.
A negligible amount of carbon dioxide is dissolved in the water during 30 - 40 minutes, the time the experiment usually takes for performing in a classroom situation.
If the number of candles is increased in the jar, the heat produced is more therefore more air is likely to escape from the jar due to thermal expansion during the process of pacing the jar over them.
Therefore, more water will rise in the jar with more candles.
6. The nature and quantity of the products will depend upon the composition of candle material.
However, it is assumed that combustion of saturated hydrocarbons is taking place during burning.
For the paraffins in the stearin candle, chain length, n = about 30.
During combustion the solid stearin combines with 3 volumes of oxygen gas to form e volumes of carbon dioxide e gas + 2 volumes of water vapour.
So the expansion of gases caused by this chemical reaction = 4/3 = 1.3'
2CH2 (s) + 3O2 (g) --> 2CO2 (g) + 2H2 (g)
However, after the candles are extinguished, drops of water appear on the inside of the jar caused by condensation, so 3 volumes of oxygen have produced 2 volumes of carbon dioxide, a contraction of 2/3.
7. Previously, teachers taught, (and some still teach), that the candle become extinguished, because all of the oxygen under the inverted jar "was used up", i.e. converted to carbon dioxide, and so the decrease in volume of air under the jar after the candles are extinguished indicating the proportion of oxygen in the air.
However, some oxygen remains in the inverted jar as can be demonstrated by testing with yellow phosphorus.
8. The rapid rise of water level in the jar after the candles are extinguished is caused by decrease in pressure as the hot gases cool and the condensation of water vapour.
The amount of condensation of water will depend upon the temperature difference between initial and final temperatures of the air in the jar.
Since air is above water, therefore saturated water vapour pressure is considered in the beginning of the experiment.
Increase in temperature, during the candle burning, will make air unsaturated to accommodate additional water vapours especially produced as a product of burning.
A decrease in temperature over time after the candle is off to the initial temperature will help water vapour to condense.
This condensation will decrease the pressure inside the jar and will help water rise in the jar.
The amount of water vapours condensed during a small change of temperature as usually occurs in this experiment may even be too small to notice.
9. Some teachers believe that all the oxygen is consumed during combustion before the candle is extinguished and the water rises in the jar to fill in vacuum created by consumption of oxygen.
They do not expect the air to escape from the jar as a result of thermal expansion.
They believe that one candle will burn longer in the jar than two candles.
The water level in jars with one or two candles will rise to the same level, because the amount of oxygen in the jars is the same, about 20%.
A little carbon dioxide dissolves in the water during the experiment.
A jar full of carbon dioxide inverted over a trough of water does not completely dissolve after some days.
To study the level of water rise when the candle was put out as soon jar touched the water, a floating candle was used and it was made to sink as soon as jar touched the water in the trough.
It was found that water did rise to some extent, indicating that some air escaped from the jar, because hot air and burning products entered the jar from the candle during the process of placing the jar over the candle.
The oxygen in the jar after the candle was extinguished produced rust in steel wool, reacted with yellow phosphorus to produce white smoke of oxide and supported life of a mouse and insect for a long time.
To test whether the presence of carbon dioxide or lack of oxygen extinguishes the candle, remove the carbon dioxide from the jar by using sodium hydroxide solution in the trough in place of water.
Also, you can spray cotton wool with sodium hydroxide and attach it to the bottom of the jar before it is inverted on the candle.
The candle burning time was almost doubled indicating that it is the presence of carbon dioxide that extinguishes the candle.
When a candle burning under a jar inverted over water in a trough was repeated using two and three candles.
The level of water in the jar increased with an increase in number of candles.
This finding was used to emphasize that more oxygen is escaped from the jar before or during the burning of candles.
However, it is not true that more oxygen was consumed with the increase in the number of candles.
8.1.12 Candle flame consists of burning vapours
1. Blow out a candle flame then quickly insert a lighted taper into the rising vapours.
The candle lights again.
2. Use an L-shaped glass tube to lead vapours from a burning candle into a cool beaker.
A grey-white vapour condenses into a solid.
8.1.13 Candle flame forms carbon dioxide
Place a glass funnel over the candle flame.
1. Hold a lighted match in the hot air coming out of the stem of the funnel.
The match goes out.
2. Fix a test-tube over the stem of the funnel to collect some hot air.
Invert the test-tube, add limewater, seal the end of the test-tube and shake it.
The limewater turns cloudy, indicating carbon dioxide.
8.1.14 Candle flame forms water
1. Hold a very cold beaker over a candle flame.
Water droplets form inside the beaker.
2. Sprinkle ice cubes with salt then wrap them in aluminium foil.
Hold the foil bundle over a candle flame and note the water droplets forming on the aluminium foil.
16.2.5 Candle in a bottle, candle in dropped jar
Drop, throw up and throw a bottle containing a lighted candle.
Drop a closed jar containing a lighted candle.
Throw a jug with a lighted candle into the air.
A candle in a dropped chimney goes out due to absence of convection currents.
8.1.15 Candle, paraffin wax
See diagram 3.1.2.6: Candle burner.
Candle wax is a mixture of different alkanes that are solid at room temperature.
Candles are usually made of paraffin wax that is a residue from the distillation of petroleum.
With enough air, the wax burns to form carbon dioxide and water.
With insufficient air, the wax burns to form carbon monoxide and smoke containing carbon.
The teardrop-shaped flame is called a diffusion flame, because oxygen gas diffuses in from the air to the combustion region and hydrocarbon vapour diffuses out wards form the wick.
Heat radiated from the burning wick melts the wax drawn up the wick by capillarity.
The melted wax vaporizes to form a cloud of hydrocarbon molecules that diffused into the flame and are broken down into small molecules by the intense heat of the flame.
The smaller molecules react with oxygen.
The smoke from the flame contains carbon particles (soot), water vapour and various products of the reactions of the hydrocarbon particles with oxygen gas.
Experiments
1. Cut the top off one of a clear plastic soft drink bottle and fill it with water.
Float a candle on the water.
Light the candle.
A cup of molten wax forms around the wick.
As the candle flame burns the wax melts and moves up the wick by capillarity, then is converted to a vapour by the heat of the flame.
The vapour rises and burns to form more flame.
The ascending current of air, produced by the heat of the candle, keeps the outside edge cool, and forms a cup for the melted wax around the wick.
The rising vapour draws up cold air containing oxygen gas.
2. See the shape of the flame with three regions:
* The innermost part is a dark area, the shape of the flame around the wick.
It is not luminous and consists of the vapour from the molten wax.
* The coloured part of the flame is orange-yellow to blue near the bottom.
It is where some combustion occurs.
* The outer, almost colourless region of the flame is where most combustion occurs, because more air (oxygen gas), is available.
Blow out the candle then ignite again the vapour quickly with a lit match.
The flame will go down and ignite again the candle.
Complete combustion of the wax hydrocarbon should produce carbon dioxide and water only, but the candle flame is not hot enough to allow complete combustion so a mixture of
gases and tiny specks of black carbon (soot), forms.
The glowing carbon particles glow and are the main emitters of candle light.
Hold a white plate above the flame to see the black soot.
Suspend a suspended spiral of paper above the candle flame.
The spiral turns, because of the force of the rising hot gases from the candle flame.
3. Relight a candle.
Light a match, then blow out the candle, keeping the match lit.
Then immediately bring the burning match close to the smoking candle wick and observe closely.
Note when the candle flame reignites.
4. Repeat this experiment with a cold candle that has not been recently burning.
A wax vapour still exists in the space between the hot wick and the match flame.
Candle wax, or paraffin, is a mixture of high molecular weight saturated hydrocarbons consisting mostly of long chains of (-CH2-) units.
The simplest hydrocarbon, methane, burns as follows:
CH4 + 2O2 --> CO2 + 2H2O
A single (-CH2-) unit burns as follows:
2CH2 + 3O2 --> 2CO2 + 2H2O.
8.1.16 Carbon, soot from a candle flame
The soot deposited is the carbon used in the manufacture of inks and motor tires.
Whenever fuels, e.g. kerosene (paraffin oil), or coal or wood, burn with insufficient oxygen, similar deposits of carbon (soot), can be seen.
1. Hold a glass rod in the centre of the flame.
The rod becomes coated with a sooty black film called lamp black (carbon black).
Carbon deposits on the glass rod, because not enough oxygen is available for complete combustion.
2. Hold a wire gauze heating mat over the candle flame.
The wire gauze cools the flame by conduction and carbon, soot, deposits.
3. Sprinkle flour on the candle flame.
The flour particles sparkle as they catch fire to leave specks of carbon.
4. Bend lemon peel near a candle flame.
The squeezed peel emits oil and water.
Some of the oil burns in the flame to leave carbon particles.
8.1.17 Collect and weigh the gaseous products of a burning candle
See diagram 3.2.29: Gaseous products of burning candle.
Candle wax is a mixture of different alkanes (paraffins) saturated hydrocarbons with general formula CnH2n+ 2 that are solid at room temperature.
Soda lime is a grey-white mixture of sodium hydroxide and calcium hydroxide as granules or powder that absorbs the products of combustion, carbon dioxide and water.
Use soda lime instead of sodium hydroxide, because soda lime is not deliquescent.
Experiments
1. Weigh a candle, C1.
Weigh a U-tube containing granules of soda lime, U1.
Put a candle under an inverted glass filter funnel connected to one arm of the U-tube.
Attach a filter pump to the other arm to draw air through the U-tube.
Light the candle and turn on the filter pump to draw air over the candle.
Let the candle burn for five minutes.
Extinguish the candle and disconnect the filter pump.
Weigh the candle again, C2.
The candle has lost weight, C1-C2.
When the U-tube is cool, weigh it again, U2.
The U-tube containing the soda lime has gained weight, U2-U1.
2. The U-tube gains more weight than the candle loses weight (U1-U2) > (C2-C1) for two reasons:
* The candle wax combines with oxygen in the air to form carbon dioxide gas and water.
*The air sucked in by the filter pump contains some water vapour absorbed by the soda lime.
3. To measure the weight of water absorbed from the air, in a control experiment, repeat the experiment for the same period, but without the candle.
8.1.18 Dark region of a candle flame
See diagram 3.1.2.3: Burn gas from the cone of the flame.
Hold a glass tube so that it slants upwards and the bottom end is as close as possible to the wick.
Light a match and hold it close to the gases coming out of the end of the tube.
Gases burn at the end of the glass tube.
These gases have come from the dark region of the flame where there is not enough air to burn them.
8.1.19 Egg in a candle flame
Hold an egg near the top of a candle flame.
The egg becomes covered in black soot.
Put the egg in a dish of water.
The egg now looks like a silver mirror.
A layer of air as bubbles has formed between the soot and the shell of the egg.
Light reflects back from the bubbles.
Leave the egg in the water.
Gradually all the bubbles dissolve and the egg looks black again.
8.1.20 Floating tea candle
A tea candle has about 3 cm diameter, 1.4 cm height and weighs about 10 g.
Some people put them in a cut down plastic drink bottle to serve as a cheap lantern that is not blown out by the wind.
Float a lighted tea candle in water.
The flat top of the candle wax forms a cup of molten wax around the wick.
The burning candle should balance symmetrically when floating and the cup of molten wax is also symmetrical.
As the wick burns, the wax nearby melts and molten wax is drawn up through the wick by capillarity.
As the molten wax nears the flame it evaporates and the vapour rises and ignites.
The ascending current of air above the flame keeps the outside edge of the candle wax cool forming a cup for the molten wax around the wick.
The draft of ascending hot gases draws up cooler air alongside the body of the candle and supplies oxygen to maintain the burning of the vapour.
8.1.21 Hottest part of a candle flame
See diagram 3.1.2.4: Bunsen burner flame, Candle flame.
Push a piece of cardboard sideways into the flame.
The outside of the flame forms a sooty ring as it scorches the cardboard.
So the hottest part of a candle flame is near the edge of the flame where there is enough oxygen to burn all the vaporized candle wax completely.
The burning gases rise to the tip of the flame so that is the hottest part.
The flame near the wick is yellow, because there is not enough oxygen for complete combustion and this causes the deposit of black (unburnt), carbon on the wick.
8.1.22 Melt candle wax
Most candle waxes melt at about 60oC.
Do not melt candle wax over direct heat, because the vapour may ignite.
If it ignites, smother the flames with a lid, fire blanket, sodium carbonate powder, or moist towel, do not use water.
Melt candle wax in a heat resistant container in gently boiling water, in an electric frying pan or over a hot plate.
8.1.23 Parts of a candle flame
See diagram 3.1.2.4: Bunsen burner flame, Candle flame.
1. A candle contains wax made from petrochemicals.
The wick is lighted, and this melts the wax.
The evaporated wax rises and catches fire.
As the vapours rise higher, they stay longer in the hot regions of the flame and start burning completely with oxygen gas.
The candle flame has three regions.
The inner zone appears black, contains unburned wax vapours and is the least hot region of the candle.
The middle zone is where the wax vapours start burning giving a yellowish flame of partially burnt gases, because of insufficient gases for complete combustion.
The flame is a luminous region, but not very hot.
The outer zone is where the wax vapours have enough oxygen gas to burn completely.
The flame appears blue and the temperature is very high.
2. Hold a piece of white cardboard behind the flame, so that you can see each part clearly.
The candle flame has three regions.
Each region has the shape of the flame around the wick.
* The innermost region closest to the wick consists of vapours from the molten wax.
It is dark in colour, because air cannot reach that region, so the gases are not burning.
* The second region is bright yellow to orange to blue near the bottom.
It forms much light.
Incandescent soot particles cause some orange and yellow glow.
The red area near the centre of the flame is about 800oC.
The outer orange and yellow areas are hotter than this region.
Some combustion occurs in this region.
* The third region, the outer rim of the flame, is practically colourless.
It is a very faint blue colour and is the hottest part of the candle flame.
The blue colour shows that oxygen is mixing with the wax molecules.
Most of the combustion occurs in this region.
Complete combustion of the paraffin hydrocarbons should produce carbon dioxide and water only, but the candle flame may not be hot enough to produce complete combustion, so intermediate substances form.
Tiny black specks in this region are particles of carbon (soot), that glow on ignition and emit most of the light from the candle.
C + O2 --> CO2 + light energy.
8.1.24 Prepare beeswax candles
Household candles, votive candles for churches and birthday cake candles are usually made of paraffins.
However, specialist suppliers sell different kinds of candle wax and wicks so you can make your own novelty candles.
Foe example, candle paraffin with specific melting points, different waxes for different lights, e.g. beeswax, and different odours for aromatherapy.
Experiment
Heat some beeswax in a tin can floating in hot water.
Put a piece of white cotton thread in the melted wax for a wick.
Use a fork to swirl the wick through the wax, place it to run through the centre and stick out the top by 1 cm.
Let the wax cool until solid.
Light the beeswax candle and compare the flame with the flame of the other candles.
Beeswax comes from bee honeycomb.
It is mainly an ester of palmitic acid, C15H31COOC30H61.
Make candles with the same shape and weight from different waxes, compare their flames and rates of burning.
8.1.25 Prepare lampblack
Lampblack, C, pigment, (80-85% carbon + oils), (combusted mineral oil, turpentine in limited air supply)
Lampblack 1. Hold the bowl of a spoon, bowl entrance down, over the flame of a cheap candle, e.g. happy birthday candles.
The black soot formed in the bowl is lampblack.
2. Suspend a copper bowl, bowl entrance down, over an oil lamp with a sooty flame.
This is a slow process so expect to leave the bowl suspended for at least an hour.
3. Prepare pure lampblack.
Put lampblack in a borosilicate glass bottle, squash it down to a minimum surface area and heat it with a blow torch to combust any substances not carbon,
leaving pure carbon lampblack.
Mix lampblack with methylated spirits and add a water solution of gum Arabic.
8.1.26 Re-lighting candles
See diagram 3.1.2.6: Candle burner.
Re-lighting candles, (happy birthday candles you can't blow out!), "Trick Candles", "Magic candles"
The relatively low autoignition temperature, 473oC, is used in trick happy birthday candles that cannot be blown out.
When a candle is blown out, a glowing ember usually remains in the wick, but it does not provide enough heat to ignite the paraffin.
The wicks of trick candles contain particles of magnesium powder, which may be ignited by the glowing ember to then ignite any remaining paraffin vapour.
Look closely at the wick of a trick candle just before it reignites to see sparks of burning magnesium powder.
Only the magnesium in the glowing ember ignites, so the trick candle can be blown out then reignites many times, because the magnesium in the rest of the wick does not burn, being isolated from the air by the liquid paraffin.
Extinguish the trick candles by putting them in water.
Put away the trick candle for storage only after several minutes and be sure that they are extinguished.
The wicks of re-lighting candles should be < 6 mm.
8.1.27 Rocking candle
See diagram 18.4.2.4: Burn a candle at both ends.
1. Cut wax away from around the wick at the bottom of the candle so that you have the same length of wick sticking out of each end of the candle.
Push a nail or knitting needle through the middle of the candle so that the candle will balance when you place the nail across the sides of two beakers.
Put the apparatus in the sink or, to catch candle drips, put a piece of aluminium foil under it if on the table.
Simultaneously light both ends of the candle.
The burning candle rocks up and down.
When you light both ends, one end is sure to burn faster than the other end.
It loses more candle wax and becomes lighter than the other end that then tilts downwards.
The other end then burns faster, becomes lighter then tilts upwards.
The tilted down ends burn faster, because the flame becomes closer to the wax.
The candle rocks, because its centre of gravity, originally through the
axis of the nail or needle, moves away from the end burning faster.
The centre of gravity continually moves from one side of the axis to the other, like a seesaw.
This apparatus is sometimes called a "perpetual motion machine"!
2. Use a cylindrical table candle, not a tapered candle.
Use a knife to trim each end of the candle to expose 1 cm of wick.
Put the candle on a fulcrum to find the mid-point.
Put two glasses over waxed paper on the table.
Use a hot needle to make a hole through the centre of the candle at right angles to the length.
Push a knitting needle through the hole then balance the candle with the rims of the two glasses supporting the knitting needle.
Light both wicks.
One end of the candle dips down slightly then dips down more, because the flame is closer to the candle wax that at the other end of the candle.
More wax melts and drips off the end of melts and vaporizes so that end becomes lighter and the other end of the candle dips down to become lower.
The candle develops a faster see-saw motion, a simple harmonic motion, as the differences between the weight each end of the candle become less.
8.1.28 Test gases from the candle wick
Light the candle, let it to burn for five seconds and then blow out the flame.
Immediately, light a match and hold it near the smoke, vapour trail, coming from the wick.
A flame will race back along the vapour trail and reignite the candle.
This shows that the gases from the wick are flammable.
8.1.29 Test for carbon dioxide from a burning candle
Cut off each end of a plastic drink bottle to make a cylinder.
Place the cylinder vertically around the candles.
Pour sodium bicarbonate solution then tartaric acid solution into the water around the candles.
The acid reacts with the base to form bubbles of carbon dioxide gas.
As the cylinder fills with carbon dioxide gas the short candle flame then the long candle flame will be extinguished as the carbon dioxide gas displaces the air upwards.
Try to relight the candle with a match or taper.
The flame is extinguished when it reaches the carbon dioxide layer.
Make a loop with a piece of wire, dip it in a soap or detergent solution and blow a small bubble so that it falls gently into the cylinder.
The bubble will stop falling when it reaches the carbon dioxide gas layer.
8.2.1 Carbon dioxide is a product of combustion
1. Put some limewater in a test-tube.
Put some carbon on a deflagrating spoon and ignite it.
While it is burning, lower it into the test-tube just above the lime water.
When burning stops, cover the test-tube and shake it.
The limewater now has a milky colour, a test to identify the gas carbon dioxide gas.
2. Repeat the experiment with small quantities of fuels, e.g. wood, coal, and kerosene.
All the common fuels are mixtures and contain compounds of carbon.
8.2.2 Combustion, conditions for combustion, to ignite, ignition point
Combustion is the burning, usually in oxygen gas, of a substance releasing heat energy and light energy.
Ignition temperature is the temperature at which the substance ignites, e.g. sulfur must reach a temperature of about 400oC before it will burn.
To ignite means to makes something intensely hot by the action of fire, to heat something to the point of combustion or chemical change, to set fire to something, to catch fire and begin to burn, to cause
an electric arc, to start combustion in the cylinder of an internal combustion engine.
The ignition point is the temperature at which the rate of reaction is high enough to produce more heat than is lost to its surroundings.
When the heat energy accumulates it increases the rate of reaction, so a fire occurs.
Experiments
1. Put small quantities (that can be put on your little finger nail) of sulfur, magnesium and carbon on a lid of a jam tin.
Put the lid on a tripod and heat the centre of the lid with a Bunsen burner flame.
Each chemical should receive equal heating.
Note the order in which the different substances ignite.
In this experiment the order of ignition temperatures should be as follows: sulfur, magnesium, carbon.
2. Repeat the experiment with small quantities of paper, wood and coke (petroleum coke).
Heat the centre of the lid and note the order in which any of the materials catch alight.
Your ignition temperature order should be as follows: paper, wood, coke.
3. Put some kerosene in a small tin and ignite it with a Bunsen burner flame.
With the kerosene still burning, float the tin on a mixture of ice and water.
The kerosene stops burning, because the ice water mixture removes heat from the burning substance and cools it below its ignition temperature.
Firemen use water in the same way to control fire and put out fires.
8.2.3 Oxygen gas is necessary for combustion
1. Ignite a small coil of magnesium wire in a crucible.
Pour sand on it while it is still burning.
When you cut off the supply of oxygen gas, the burning stops.
2. Play a candle flame oil to the bottom of an evaporating basin.
Note what forms on the basin.
The deposit is carbon that has not been burned to gaseous products, because not enough oxygen gas was available for complete combustion.
You may see similar deposits of carbon or soot when you burn fuels like kerosene, coal and wood with insufficient oxygen.
3. Put some wood shavings and a piece of wood on a metal lid.
Heat the lid.
Note which ignites first.
Since oxygen and a solid fuel can interact only at the surface of the solid, the greater the surface area of the solid the more likely the combustion of the solid is to occur.
In coal grinding plants, the wood shavings that had the greater surface area ignited before the piece of wood.
Explosive and spontaneous combustion involving solids occur when the solids are finely divided like a powder and well-mixed with air or oxygen.
Explosions have occurred in coal grinding plants and flour mills when the coal dust or flour has been well-mixed with air.
4. Put two lighted candles on a bench.
Simultaneously, cover one with a small jar and the other with a larger jar.
The candle in the larger jar burns longer.
8.2.4 Respiration is a form of combustion
See diagram 9.155: Respiration of soaked peas.
1. Set up the following flasks:
Flask 1 containing potassium hydroxide,
Flask 2 containing limewater,
Flask 3 containing snails or other small animals,
Flask 4 containing limewater.
Equip each flask with a two-hole stopper.
Connect flask 2 to flask 3, and flask 3 to flask 4 with delivery tubes.
Connect flask 4 to an air pump and air can enter flask 2 through an open glass tube (not as in the diagram).
Notice inlet tubes in each flask reach down to the bottom of the flask.
The openings of the outlet tubes are just below the bottom of the stoppers.
The air pump draws air through the flasks and through the limewater in flask 2 and flask 4.
After some time the limewater in flask 4 turns milky.
2. Repeat the experiment with germinated peas.
Both animals and plants produce carbon dioxide as they respire.
3. Put some germinating seeds in a thermos flask and leave a second thermos flask empty.
Fit each thermos flask with a cork and a thermometer.
Record the temperatures of each flask daily for a few days.
The temperature in the thermos flask containing germinated seeds is higher.
Heat is produced in respiration.
The respiration reaction is exothermic.
8.3.0 Heat sources
A flame is a region where a gas emits light, because of the high temperature.
Burning, i.e. combustion, needs oxygen gas, is exothermic process and has reaction products are carbon dioxide and water.
Spontaneous combustion does not need external heat energy to start it, e.g. white phosphorus in air.
Combustible substances catch fire easily, e.g. paper.
You can smother a flame to cut off the oxygen gas supply and put out the fire.
Water is used to put out fire, because it reduces the temperature of substances below its ignition point.
However, the temperature of burning oil is too high for an oil fire to be extinguished by water.
Most fire extinguishers either reduce the ignition temperature or cut off the oxygen gas supply.
8.3.1 Light a match of a box of safety matches
| See diagram 3.1.2.6a: Matchbox of Safety Matches.
Striking causes friction to generate heat to raise the temperature of red phosphorus to ignition temperature.
The match head contains mainly potassium chlorate, sulfur, fillers and glue.
The striking surface on the side of the matchbox contains mainly red phosphorus and powdered glass to cause friction.
In 1826 John Walker, London, invented the first matches called "Friction Lights", which were we copied and sold as "Lucifers" in 1829.
The "strike-anywhere" match, "lucifer match" can be ignited by striking the head of the match on any rough surface, e.g. the sole of a boot.
They are still used by campers, but are not allowed in some countries.
First World War soldiers' song:
"Pack up your troubles in your old kit-bag
And smile, smile, smile
While you've a lucifer to light your fag
Smile, boys, that's the style"
The heads of the matchsticks contain potassium chlorate as an outside layer on the phosphorus sesquioxide (P4O6), match head.
Explosions based on collections of match heads are not permitted in schools.
The chemical that initiates the flame, phosphorous sulfide, is in the match head so friction will start it burning.
Safety matches used the strong oxidising agent potassium chlorate to react with sulfur.
The flame formed ignites the wax below the match head then the wooden match stick.
An early brand of safety matches contained:
1. In the match heads
Ammonium dihydrogen phosphate, to prevent afterglow and so no smouldering match to ignite something.
Paraffin wax to coat the matchstick.
Potassium chlorate for the chemical reaction with sulfur.
Powdered glass to initiate heat from friction of about 200oC.
Pigment and glue.
2. In the striking surface on the side of the matchbox
Red phosphorus is stable, but is converted to flammable white phosphorus by friction.
So a famous brand of safety matches made in Sweden is called "Redheads", Powdered glass and glue
8.3.2 Study a match flame.
Below the ignition temperature, e.g. white phosphorus 35oC, a combustible substance in oxygen gas will not catch fire.
The wood used in the match has a similar ignition temperature.
Match head: mainly potassium chlorate and glue.
Striking surface: mainly red phosphorus and powdered glass.
Safety match heads contain potassium chlorate, sulfur, fillers and glue.
The "strike-anywhere" match heads contain potassium chlorate as an outside layer on the phosphorus sesquioxide (P4O6) match head.
Explosions based on collections of match heads are not permitted in schools.
8.3.3 Study a flame in a gas stove.
Rapid combustion releases a large amount of heat in a short time, e.g. Lighting LPG gas in a kitchen stove.
8.3.4 Burn (Monopoly money, fake money)
1. Soak a (Monopoly), bank note, e.g. ten pounds or ten dollars, in 50 mL of water.
Use tongs to hold it in the yellow flame of a Bunsen burner.
It does not ignite unless all the water evaporates, then the paper bank note can reach ignition temperature of about 230oC, (Fahrenheit 451o!).
2. Soak a (Monopoly), bank note, e.g. ten pounds or ten dollars, in 50 mL of ethanol.
Use tongs to hold it in the yellow flame of a Bunsen burner until it ignites.
3. Soak a (Monopoly), bank note, e.g. ten pounds or ten dollars, in a mixture of 25 mL water, 25 mL ethanol and 2 g sodium chloride to colour the flame.
Use tongs to hold it in the yellow flame of a Bunsen burner.
The ethanol ignites, but the paper banknote does not ignite, because it is still wet with water.
8.1.30 Candle flame, (primary)
| See diagram 3.1.2.4: Bunsen burner flame, Candle flame
| See diagram 3.1.2.6: Candle burner
| See diagram 4.20 : Copper coil candle snuffer
Teach the children to study a burning candle.
Fire can be useful.
It can also be dangerous.
Its heat can cook food.
Its heat can also burn you.
In this experiment you will study fire.
You must study it carefully and safely, or you could be burned and scarred.
Always obey these instructions: 1. Fires made from things that burn quickly,
e.g. paper, must be made outside.
Keep away from dry grass.
2. Do not touch hot containers with your hands.
3. Keep a bucket of water and a bucket of dry sand near you to put out
fires.
4. Avoid sudden movements.
5. If you burn yourself, cool the burnt part in the cold water.
Keep it cool for 10 minutes then get first aid from your teacher.
1. Stand a candle on a table so that it is not in a draught.
2. Light the candle.
Watch the flame carefully.
What shape is the flame?
Is the shape always the same?
What colours can you see in the flame?
Is the candle wick straight or bent?
3. Draw a picture of the flame and colour the picture.
4. What happens to the candle?
Does the candle get smaller?
What happens to the wax just below the flame?
Where does the melted wax go?
Graph showing length of candle as it burns, at different times of day:
| 8 a.m. | 10.0 a.m. | 12 noon | 2 p.m. | 4 p.m. |
5. The graph shows how the length of a candle changed while it burned.
How much of this candle burned in each hour?
At what time do you think the candle would have completely burned away?
Could the candle be used as a clock?
6. Candle experiments
Use 2 candles and several jars of different sizes.
6.1 Put a large jar over a burning candle.
Did the candle continue burning? How long did it keep burning?
6.2 Place a smaller jar over a burning candle.
Did the candle continue burning? How long did it keep burning?
6.3 Light two candles.
Cover one candle with a jar.
When the flame goes out, remove the jar and place it immediately over the
second candle.
What do you observe?
8.2.5 Sugar with potassium chlorate, spontaneous combustion
Be Careful!
Do this experiment in a fume cupboard or in the open, or behind a glass screen!
1. The reaction of the concentrated sulfuric acid with the sugar released heat.
The heat then activated the release of oxygen from the potassium chlorate.
The oxygen released by the potassium chlorate further oxidized the sugar.
This further oxidation released so much heat that the sugar bursts into flames.
Mix sugar or powdered sugar (castor sugar) with an equal amount of potassium chlorate crystals in an evaporating dish.
Push a dent in the top of the heap of powder.
Add one drop of concentrated sulfuric acid.
A spontaneous combustion occurs!
Be Careful!
2KClO3 --> 2KCl + 3O2
2. Repeat the experiment with potassium nitrate.
The reaction is slower, because while potassium chlorate loses all its oxygen, potassium nitrate loses only one third of it oxygen.
2KNO3 --> 2KNO2 + O2.