Friday, 18 December 2015

Christmas Science - Flaming Brandy!

Why can’t you set fire to the brandy on the Christmas pudding without first heating it in a sauce pan?

With Christmas now just a week away my mind turns inexorably towards getting geared up for the day itself. With only a few shopping days left, my book is the perfect present.

Setting fire to the plum pudding is all part of the performance of Christmas, and while it seems like you should just be able to pour brandy over the pud and set a match to it. But that doesn't work.

Brandy is usually only about 50% alcohol or ethanol to be precise. And while ethanol can be ignited it is not the liquid that burns but the vapour. This is true of most flammable liquids - even petrol is not flammable as a liquid, but only as a vapour. So, to set fire to brandy you need lots of ethanol vapour. To create lots of vapour you need to warm the ethanol first. Cold brandy poured over a hot christmas pudding absorbs lots of heat from the pudding, but most of that is sucked up by the water in the brandy, consequently the amount of vapour is not enough to ignite - unless you first warm the brandy.

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.