S3.6.6 Chapter 6

Burning and fire safety

Why does a candle go out under a jar, and why is water wrong for some fires?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A fire needs only something to burn. Take that away and the fire goes out.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A fire needs fuel, oxygen and heat. Take away any one of them and the fire goes out.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Water puts out every fire, because water is wet.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Name the three things a fire needs: fuel, oxygen and heat
Explain how each way of putting out a fire works
Write a word equation for burning a fuel

Fire cooks food and gives light. It also burns down homes. To use it safely, you need to know what a fire is made of and how it stops.

Draw it first

Draw the fire triangle

1 Draw a triangle. Write fuel, the thing that burns, on one side. Write oxygen on another side and heat along the bottom. Write fire in the middle.
2 Draw it again. Take away the fuel. There is no fire.
3 Draw it again. Take away the oxygen. There is no fire.
4 Draw it again. Take away the heat. There is no fire.

This is the fire triangle. A fire needs fuel, oxygen and heat, all three at once. Take away any one side, and the fire goes out.

Burning is a chemical change. A fuel is something that burns, such as wood, wax or gas. It joins with oxygen from the air and gives out heat and light. The gases it makes float away.

A jar over a candle holds only so much air. The flame uses the oxygen, and when too little is left it goes out.

A bigger jar holds more air, and more oxygen. So the flame lasts longer. This is a fair test, because only the jar changes.

Each way of putting out a fire takes away one side of the triangle. Water takes away heat. A lid or sand smothers a fire. It covers it so no air reaches it, and that takes away oxygen. Turning off the gas takes away the fuel.

Three ways to put out a fire, and the side of the triangle each one takes away.

But water is not right for every fire. Oil is lighter than water, so it floats on it. Water poured on burning oil sinks, turns to steam and throws burning oil about. For an oil fire in a pan, turn off the heat. Slide a lid or a baking tray over it. Never use water. Leave the lid on until the pan is cold, or the oil may flare up again. If the fire is big, get out and call for help.

Water also carries electricity. Never pour water on an electrical fire. If it is safe, switch off the power first. Otherwise get away and get help.

Two fires where water is the wrong choice.

Now the reaction. Wax and cooking gas are made of carbon and hydrogen. Wood is made of carbon, hydrogen and oxygen. When these burn, the carbon joins with oxygen to make carbon dioxide. The hydrogen joins with oxygen to make water vapour.

Burning a fuel made of carbon and hydrogen.

A candle burning. The wax seems to vanish, but it has become gases.

These equations hold when there is plenty of air. With too little air, a fire makes soot and carbon monoxide. Carbon monoxide is a poison gas, and you cannot see or smell it. Never burn charcoal, wood or gas in a closed room. Always let fresh air in.

A burning candle gets shorter. The wax has not been destroyed. It has become carbon dioxide and water vapour, which go into the air. Burning adds carbon dioxide to the air.

See it three ways

With things

Cover a burning candle with a jar. The flame goes out after a few seconds. The jar did not cool it or remove the wax. It kept fresh air away.

As a picture

In words

The flame uses oxygen. Under a jar there is only a little. When too little is left, the flame goes out.

Try it

Time a flame under two jars

You need: a candle on a saucer, two glass jars, one small and one big, a clock or a way to count seconds, a grown up with matches.

Be safe: Only a grown up lights the candle. Do not touch the jar while it is hot. Never leave a flame alone. Keep paper and cloth away.

A grown up lights the candle. Everyone else stays back. Tie long hair back.
Lower the small jar over the flame. Count the seconds until the flame goes out. Write it down.
Wait until the jar has cooled. A grown up lights the candle again. Do the same with the big jar. Compare the times.

What you will see: The flame lasts longer under the big jar, because there is more air and so more oxygen. Your times will differ from the picture.

If you cannot try it, this is what happens.

A wrong idea many people have

Water puts out every fire.

Why it feels right: Water is what we throw on a campfire, and fire engines carry it.

The test that breaks it: Water takes away heat. It does not stop an oil fire from burning. Burning oil floats, and hot oil and water spit and splash. Water also carries electricity.

What is true: Pick the way that takes away one side of the triangle safely. A lid for an oil fire. Switch off the power for an electrical fire.

Key words

fuel: something that burns, such as wood, wax or gas.
fire triangle: the three things a fire needs: fuel, oxygen and heat.
smothers: covers a fire so no air reaches it.

Back to your guess

The answer is B: A fire needs fuel, oxygen and heat. Take away any one of them and the fire goes out.

A fire needs fuel, oxygen and heat. A jar takes away oxygen. Water takes away heat, but it makes some fires worse, so it is not right for every fire.

Keep it

Copy this into your exercise book.

A fire needs fuel, oxygen and heat. Take away one and it goes out. Never use water on oil or electrical fires. Fuel + oxygen → carbon dioxide + water.

Draw: Draw the fire triangle with fuel, oxygen and heat on its sides.