Why doesn’t wood melt?

Asked by Matthew Vincent via Facebook

burning wood 7 by BBQ Junkie, on FlickrThere are two things that you should consider really. One of them is what melting actually means. Melting is when a solid transitions to a liquid. Under normal conditions, water does that at 0ºC for example (or 32ºF if you’re North American) but, if you look closely at the way snow “melts” in fact a relatively small amount (most estimates I can find say about 20%) actually melts – turns into liquid – the rest sublimates – goes straight from solid to gas (water vapour) in this case. Dry ice, actually frozen carbon dioxide, all sublimates at room temperature and pressure, which helps it form that spooky low-lying mists that you see in some films, old music videos and the like. So, actually, melting isn’t necessarily all that common.

In the case of wood however, in the open atmosphere it doesn’t melt or sublimate. Rather, thanks to the oxygen in the atmosphere, it burns. Burning, or combustion if you prefer, occurs when oxygen (in this case from the atmosphere) reacts very exothermically [gives off heat] with something and breaks down its structure. You can see this as wood initially chars, then burns turning black and finally burns away completely turning to ash – which is often white. At the same time a lot of gases are given off, which we see as smoke.

I don’t recommend trying this at home – but it’s entirely possible if you put wood in an oxygen free atmosphere and played with the temperature and pressure long enough you could find the conditions under which it melts. But as soon as you heat it up in a nicely oxygenated atmosphere it will combust first.

Answered by Lewis Pike

Article by Lewis Pike

February 6, 2013

Lewis studied BioMedical Sciences at Liverpool John Moores University and got a PhD in Microbiology and Immunology at University of York. He now spends more time helping people understand science writing than he should and wishing his colleagues would write more clearly for the public as well as each other.


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