Duration: 30 minutes
Outcomes: Have an understanding of the conditions necessary for combustion to occur.
What you need
The experiment should be carried out in an open space with little or negligible wind.
Note: 1. Do not allow the kids to do this experiment on their own.
2. Don’t forget to blow out the candle when done with the experiment!
Question to consider
Have you ever wondered why fire extinguishers are able to put out fire?
Encourage the child to make deductions based on the experiment
Observation: The candle relights.
Explanation: There are three things needed to make a fire: fuel (which may be a solid, liquid or vapor), oxygen and a heat source. If any of the above three are missing, there can be no fire. But in some cases, there are other conditions that must be met.
Most flammable liquids and solids will not burn unless they are first vaporized. Using a candle as an example, we all know that you can hold a lit matchstick close to the candle side and the candle won’t catch fire. The wax may melt, but even the liquid wax will not burn.
When we light the candle at the candle wick, the flame of the match first heats the wax in the wick (or near the wick) to a temperature at which it first liquefies and then becomes a gas. Once the wax has become a gas and mixes with the oxygen in the air, the heat from the match ignites the mixture of gaseous wax.
The burning is supported by the oxygen around it. Once the candle is burning, the flame of the candle melts more wax which rises up by capillary action through the wick. At the top of the wick, it is further heated to become a gas. The gaseous wax is also ignited by the existing flame. The process becomes self-sustaining.
So, how is it possible to relight it through the smoke?
The trail of smoke released by the smoldering wick of a candle still has wax that hasn’t fully burned. When you hold a fire source to the wisps of smoke, they can reignite and cascade back to relight the candle! This also explains why if a piece of wood is used instead of a candle, it won’t relight. The trail of smoke released by the piece of wood does not contain fuel.
The three conditions necessary for combustion form the basis for using fire extinguishers to quench fire.
Most fire extinguishers contain carbon dioxide gas or chemicals that smother the fire. That is, they form a layer of foam or powder on the fire that cuts off the fuel from the oxygen supply around it thus putting out the fire. Remember, oxygen supports burning.
Fuel is needed to make a fire. So, with the two effectively separated, burning can no longer take place.
Fire extinguishing. One of the conditions necessary for combustion is eliminated for the fire to stop.
Key words: combustion, relighting, wax, smoke
Key concepts: heat transfer, combustion,
Resources for further reading/ exploration:
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