It’s Getting Hot in Here

Fact: A lit cigarette is as hot as lava.

It’s become normal to light up a cheeky one in the smoking area after a few pints, but few realise that the end of a lit cigarette burns at around 900°C – roughly the same temperature as molten rock when it erupts from the top of a volcano.

Why this ridiculously high temperature? It’s because of the smoker: when you take a drag on a cigarette, you’re pulling air through the end of the cigarette. The oxygen in this air fuels the fire burning at the tip, causing it to burn hotter: think about how fanning a fire can help get it started.

So what about that age-old question – can a cigarette ingite petrol? On the face of it, the answer is simple. The auto-ignition temperature of petrol (i.e. if you heat petrol to this temperature it will catch fire by itself, even if there is no flame to set it alight) is at most 500°C. Cigarettes burn at up to 900°C. If we do the maths:

900 > 500

[900 is bigger than 500]

So it should light. Except time and time again experiments have proved that a lit cigarette cannot light a bucket of petrol. Even mythbusters tried it, so it must be true.

The reason for this paradox is still debated, but the most likely cause is that when a lit cigarette hits a puddle of cold petrol, the heat at the tip of that cigarette is very quickly spread throughout a large volume of liquid petrol. Imagine pouring a cup of boiling water into a cold bath – that boiling water alone can burn you, but once its heat is spread out throughout the bath it barely raises the overall temperature at all. This means that while the tip of the cigarette is theoretically hot enough to light petrol, because that heat is spread out over a large volume there is no one section of the puddle that gets hot enough to light.

However, the auto-ignition temperature of petrol gases are much lower than liquid petrol itself, so a lit cigarette held just above a can of petrol that has been sitting in the sun all day is could cause a fire by igniting the petrol vapours that are evaporating off, and in turn ignite the petrol itself. It’s unlikely, but don’t go chucking lit cigarettes around in petrol stations – there’s still a risk things could get hectic.

‘Another one of Mr Shaunak’s Little Bites of Science’
Image under Creative Commons license

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