A roaring fire may make you feel cosy when it’s freezing outside, but open fires are a very inefficient way of warming your house. Did you know that only about 10% of the heat generated warms the room – the rest goes up the chimney. The cherry red glow of a fire shows that the coal is being heated to around 1650°C – so you better make sure it is out before Santa tries to visit. Hot objects glow with different colours depending on their temperature - white-hot objects are around 5000°C whilst anything hotter than this begins to look blue. Fire walking certainly looks impressive, but most people are able to manage it without difficulty. The temperature of the bed of hot coals is actually lower than in your fireplace at about 600°C, and heat is transferred only slowly from the surface of the coals to your feet. Walking quickly, your feet don’t remain in contact with the coals long enough for them to start to burn.

	Find out what is actually burning.
	You will need:
	A candle (and holder) A bowl of water A glass jar big enough to place over the candle.
	Put the candle and holder in the bowl of water and then light it. Place the jar over the top so that the rim of the jar is submerged in the water. You will want to lift the jar slightly at one side off the bottom of the bowl so that water can easily move in and out of the jar.
	As the candle burns you should notice:
	Water starts to condense on the inside of the jar.
	Soot may start to collect on the inside of the jar.
	The level of water inside the jar will rise higher than the level of water outside the jar.
	Eventually the candle will go out, well before it has burnt down.
	Why?
	All of these things tell you something about what is burning. The candle wax, just like coal, oil or gas is made of carbon and hydrogen. As the wax burns, the oxygen in the air combines with the hydrogen and carbon, making water and carbon dioxide. If there is not enough oxygen for the wax to burn completely, pure carbon, or soot is produced. Everything that burns requires oxygen so when the oxygen contained in the jar runs out, the candle will go out even if there is still lots of wax left. There are two reasons why the water rises up into the jar: (1) the oxygen in the air forms carbon dioxide and water which take up less space; (2) when the candle goes out the air in the jar begins to cool and colder air takes up less space than warm air. These two effects combine to reduce the volume of the gases in the jar and the free space is taken up by the water which is pushed into the jar to fill gap.