Hot enough to burn

The idea

The minimum temperature at which a substance catches fire is called its ignition temperature; a substance can start burning even without a flame once it is heated to this temperature.

Paper in the air

A sheet of paper is a combustible substance. It lies in the air, and the air has oxygen. Yet it does not catch fire.

What could make the paper catch fire?

• Only a flame touching it
• Heating it enough, even with no flame
• Nothing can make it burn

Heat the paper

Try the three ways, one by one. Watch the paper and its temperature column.

Ignition temperature

No flame touched the paper under the focused sunrays. It burned because it was heated until its temperature was high enough.

The minimum temperature at which a substance catches fire is called its ignition temperature.

• What happens to the paper | Its temperature | Result
• Left in the air | stays low | does not catch fire
• Sunrays focused on it | rises with time | catches fire when hot enough
• A lit matchstick | reaches it very quickly | catches fire almost at once

Notes

The minimum temperature at which a substance catches fire is called its ignition temperature. A substance can start burning even without a flame once it is heated to this temperature.

Check yourself

The minimum temperature at which a substance catches fire is...

Sunrays are focused on a sheet of paper. What happens to its temperature?

A lit matchstick makes paper catch fire almost at once, because its temperature is _ the ignition temperature of the paper.

Which of these can make paper catch fire without any flame touching it?

• the temperature of the room. Paper can stay in a room for any length of time without catching fire.
• the temperature of boiling water. This is not about boiling water. It is called the ignition temperature.
• its ignition temperature — correct. Yes! The lowest temperature at which a substance catches fire.
• It goes down with time. Focused sunrays heat the paper, so its temperature goes up.
• It rises with time — correct. Yes! After some time it becomes so hot that it starts burning.
• It stays the same. Focused sunrays heat the paper, so its temperature goes up.
• higher than — correct. Yes! It is already higher, so the paper catches fire almost at once.
• much lower than. A matchstick that cool could not make the paper catch fire.
• exactly the same as. The temperature of the lit matchstick is higher than that.
• Keeping it in the air for a long time. Paper can be kept in the air for any length of time without catching fire.
• Focusing sunrays on it — correct. Yes! The sunrays heat it up to its ignition temperature.
• Keeping it in a cold room. Cooling does not bring it up to its ignition temperature.