Day 23: Hand Warmers

Today turns out to be a great laugh. Sophie has decided that we should treat the chemistry in today's challenge like a cookery programme. I have no problem with this as I consider a lot of chemistry to be similar to cooking, and most cooking to be chemistry. Our challenge today is to make a set of hand warmers for the rest of the team, up in the cold on the mountain (the original challenge was to ‘devise a portable heating device').

I decide to look at some exothermic reactions - chemical reactions that give out heat. One such reaction involves dissolving potassium carbonate in water (potassium carbonate is one of the major components of wood ash). Unfortunately, we discover that this doesn't generate much heat - just a one or two degree Celsius temperature rise at the most. The second method I try is much more successful; if you take chalk (calcium carbonate) and heat it in an oven to drive off the carbon dioxide, you're left with calcium oxide, or lime. If you add water to lime, you get a vigorous chemical reaction in which the lime is converted to calcium hydroxide (slaked lime). You can get a temperature rise of 30 or 40 degrees using this method.

The third way of generating heat that I explore is one that I know will generate so much heat that it'll be useless for our purposes. It'll be fun to try, nevertheless. It's a chemical reaction called the thermit reaction. It involves reacting rust (iron oxide) with powdered aluminium (from old drinks cans). Mix these two chemicals together and spark the reaction off, and you get an amazing amount of heat generated; the temperature will rise to something like 1 600° C. The reaction was used to weld railway lines together, and our attempt at it manages to weld two bits of steel to the metal dish containing the chemicals. Impressive it might be, but practical as a hand warmer up in the mountains it ain't. No - the reaction involving lime should do the trick, and should, if we use the correct amounts of lime and water, produce just the right amount of heat.

We have a great time filming the sequences today. Sophie's got a mischievous sense of humour and we have a ball pretending we're filming a cookery programme. As the day wears on, things get sillier and sillier. This is going to be a very funny programme. No bad thing as far as the chemistry's concerned.. I think it's a pity that chemistry gets such a bad deal out of Rough Science. We're not allowed to give too much away as far as the recipes are concerned, as the BBC is, quite rightly, hesitant to enable viewers to repeat some of the reactions themselves. They're quite dangerous, even if you know what you're doing, but they're even more so if you have no idea what's going on. Yet, for me, this is just what makes chemistry interesting - what brings the subject alive - the sight, sound and smells of chemical reactions taking place. Reading about it in a book can never have the same impact.

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