Candles and air

The idea

Oxygen is the component of air that supports combustion, so a flame without a continuous supply of air goes out; a burning candle forms carbon dioxide from the carbon of the wax and oxygen.

Under a tumbler

Two candles burn on two dishes. A teacher covers candle B with a glass tumbler. Candle A stays open to the air.

What will happen to candle B?

• It keeps burning like candle A
• It goes out after some time
• It burns more brightly

Two candles

Press the buttons one by one. Watch candle A and candle B.

No air, no flame

The carbon dioxide was formed from the carbon in the wax and the oxygen from the air. The component of air that supports combustion is oxygen.

Oxygen is required for combustion.

• What | What we saw | It tells us
• Candle A, open | kept burning | It had air.
• Candle B, under the tumbler | went out after some time | It had no continuous supply of air.
• Lime water in the dish | turned milky | Carbon dioxide was formed.

Notes

A covered candle goes out after some time because it does not have a continuous supply of air. The component of air that supports combustion is oxygen. Oxygen is required for combustion.

Check yourself

Why does candle B go out after some time?

The component of air that supports combustion is _.

The lime water in the dish turned milky. What does this show?

A person's clothes catch fire, and a blanket is wrapped round the person. Why does the fire go out?

• The glass takes the heat away from it. The glass is not the problem. The candle needs a continuous supply of air.
• It has no continuous supply of air — correct. Yes! A covered candle does not get new air, so the flame goes out.
• Lime water is poured on it at the start. Lime water is added only at the end, to test the gas. The flame went out because the air did not keep coming.
• nitrogen. The component of air that supports combustion is oxygen.
• carbon dioxide. Carbon dioxide forms when the candle burns. It is not what the burning needs.
• oxygen — correct. Yes! Oxygen is required for combustion.
• Carbon dioxide is present — correct. Yes! It formed from the carbon in the wax and the oxygen from the air.
• More oxygen is present in it. The lime water test is for carbon dioxide, not oxygen.
• The tumbler had no air at all. There was air in it. Milky lime water shows carbon dioxide.
• It brings more oxygen to the fire. It does the opposite. A blanket cuts off the supply of air.
• It cuts off the supply of air — correct. Yes! With no air, the fire goes out. A synthetic blanket or cloth should never be used: it can melt and stick to the skin.
• It blows the flame away. Wrapping does not blow anything away. It cuts off the supply of air.