How does a safety match ignite?


Lighting a match may be easy but there is nothing simple about the cascade of chemical reactions that it sets off. In his latest technology explainer, Jake Port details this ordered sequence of events.


RUBBERBALL/MIKE KEMP

It's all over in a fraction of a second. A brilliant flash of light and the match is on fire, ready to start a BBQ, light a candle or ignite a Bunsen burner. But how does it work?

The safety match is actually far more complex than some volatile chemicals on a stick. It is a highly ordered chemical sequence that ensures that the match will light, stay lit and eventually go out.

So what is this chemical cascade?

A match is lit by striking it against the side of the matchbox. The reddish material coating the side of the box is made of red phosphorus, a stable form of the highly volatile white phosphorus made infamous during the Vietnam war. It is mixed with abrasives such as tiny fragments of glass to create heat through friction.

When you strike a match the red phosphorus is converted to white phosphorus for a fraction of a second, just enough to cause a spark of heat by reacting with oxygen in the surrounding air.

This heat then ignites the first chemical on our journey – potassium chlorate. This is an oxidising agent, fuelling the spark into flame using oxygen stored in its molecular structure.

While this produces the distinctive flame when a match is struck, it will quickly burn out if it can’t ignite the next step in the reaction.

Antimony trisulfide – the same stuff that gives gold-coloured fireworks their colour – is ignited by the potassium chlorate and continues to burn steadily. The antimony is responsible for keeping the match head lit long enough for the underlying wood to catch fire and for you to light that candle.

The distinctive smell of a safety match is also thanks to the antimony, which when oxidised, forms sulfur oxides.

The match is now lit and ready to go to work. Paraffin wax coating the match ensures that the flame travels down the match in a controlled fashion.

The story isn’t quite over yet, however.

When you blow out the match, you might have noticed that there isn’t a lot of smoke and the match head doesn’t glow as recently burning wood would. This is because the match is soaked in ammonium phosphate, which smothers the residual heat and reduces the amount of smoke when the flame has been blown out.

The smoke consists of water vapour and tiny amounts of the chemicals in the match head that did not combust, ejected during the previous reactions.

Want to see the reaction in slow motion? Check out this video by Reactions.

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