Curious Kids: when I swipe a matchstick how does it make fire?

Curious Kids is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

When I swipe the matchstick how does it make fire? Thank you. – Laura, aged 5, Brisbane.

I’m glad you asked this, Laura. I have been interested in the science of fire and fireworks for a long time, and can tell you there is a lot happening in the very short time it takes to light a match.

But first I want to give an important warning: matches are dangerous and they shouldn’t be used without supervision. You can hurt yourself, your friends and family, destroy your home, or damage the environment.

Now, let’s get back to the science.

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Friction

To learn how the match can catch fire, we first need to understand something called “friction”. Friction is when you rub two things together and it creates heat or warmth. Have you ever rubbed your hands together on a cold morning to warm them up? That’s friction.

(For the adults reading, friction converts kinetic energy into thermal energy.)

Friction is important for the first part of lighting a match. You rub the match head against the red strip on the side of the matchbox.

This strip on the box contains a bit of powdered glass to make it extra rough. Scratching the rough match head against the rough strip leads to friction. That creates just enough heat to start a series of chemical reactions.

Chemical reactions

You probably know about chemical reactions. That’s when one chemical interacts with another chemical, and a change occurs. Maybe you’ve added vinegar to bicarb soda to create a mini volcano. That’s a chemical reaction. Heat can help kick off some chemical reactions or make them happen faster.

There are a lot of chemical reactions involved in the lighting of a match.

Surprisingly, the first chemical to react is not on the match, it is on the box!

This chemical is called “red phosphorus”. To our eyes it just looks like a red powder. But if you zoomed right in to see how all its atoms are arranged, it would look like a bunch of triangles and other shapes stuck together into a long chain.

When you rub the match on the box, you get friction, which means you get heat. This heat causes a small amount of the red phosphorus chain to be broken apart.

When that happens, some of the red phosphorous changes into another chemical called “white phosphorus”. It reacts immediately with a gas in the air called oxygen. This will create a lot more heat.

So the story so far: the friction breaks the red phosphorous chain, which allows the white phosphorous to react with oxygen and the match starts to get hot.

But that’s not the end of the story.

Fuel + heat + oxygen = fire

You need three ingredients for a fire: fuel, heat, and oxygen.

Friction and white phosphorus have provided the starting heat, and now the match needs fuel and oxygen to continue to burn.

The fuel comes from the sulfur (that’s another chemical) and wax in the head of the match. It also comes from the wood in the matchstick.

When it comes to oxygen, the match has a secret supply. Stored inside the match head is another chemical called “potassium chlorate”. When it gets hot, it releases a lot of extra oxygen and heat. This makes the match head burn quickly and strongly.

When you put it all together – the heat, the fuel, and the oxygen – you get a flame! And amazingly, all this chemistry happens in a fraction of a second.

‘Strike anywhere’ matches

What I’ve described are safety matches, which are the kind you probably have at home.

But maybe you’ve seen an old cowboy movie, or a cartoon, where a character has lit a match with their boot, a wall, or something else that’s not a matchbox.

These matches are known as “strike anywhere” matches, and they work very similarly to safety matches.

The difference is that the phosphorus component is in the match head rather than on the box.

While this is convenient, it is also much more dangerous!

So please remember — any kind of match can be very, very dangerous, so never use them without adult supervision.

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Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au