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We heard that a bowl of Epsom salts placed near a microwave or TV reduces microwave emissions, and these cause the crystals to turn to powder over time. This sounds like an urban myth, but we tried it. To our surprise, the salts turned to powder within a week or two. Why does this occur? Are microwaves really involved?

鈥� Epsom salts are magnesium sulphate heptahydrate, MgSO₄.7H₂O, which is more than half water by weight. This means that they can absorb microwaves of the wavelength used in microwave ovens, as these are designed to heat up water molecules in food. But what almost certainly happens when the crystals are placed in dry air, for example, next to a computer, is that they lose a molecule of water, changing from the heptahydrate to the hexahydrate form, MgSO₄.6H₂O. The hexahydrate is a powder.

I myself used to produce rhombohedral crystals of a similar, but green, heptahydrate for a research project. Sadly, they would only last a week or so before turning into a lime-coloured powder.

Eric Roche, Booragul, New South Wales, Australia

鈥� The absorption of microwaves will dehydrate the crystals and hasten their decomposition to a dry powder. Although the amount of ambient radiation around properly functioning microwave ovens and TV sets should be quite limited, the wavelengths they employ would be easily absorbed by magnesium sulphate, and this explains the rapid decomposition that your questioner observed.

Having said that, factors such as variations in humidity and air temperature cannot be excluded, nor can the fact that magnesium sulphate absorbs high-frequency sound waves 鈥� with the same result as if microwaves had been absorbed. The latter point is particularly relevant if the sample is placed by the TV.

Your questioner and others might care to repeat this experiment, but with the Epsom salts sealed in airtight plastic sandwich boxes, padded with cotton wool to muffle the sound, and with a control sample placed well away from any potential source of microwave or high-frequency transmissions, to see whether a different result ensues.

Hadrian Jeffs, Norwich, Norfolk, UK

鈥� Magnesium sulphate, like many other very soluble salts, forms crystals that include a great deal of water. Some salts such as copper sulphate (CuSO₄.5H₂O) hold on to their water even in hot, dry air and remain beautifully crystalline, but others lose their water of crystallisation and crumble when humidity is low.

A friend of mine in South Africa鈥檚 semi-desert region saw in a shop a half-full sack of washing soda (Na₂CO₃.10H₂O). As far as the shop assistant was concerned, it had 鈥済one off鈥� and 鈥渟hrunk鈥�, so he let my friend take it off his hands at no cost. Had he known his chemistry, he would have realised that Karoo air can desiccate almost anything; there was nothing wrong with the soda.

In warm, dry air Epsom salts also lose water of crystallisation. Microwaves should have nothing to do with this process if your oven is sound, but waste heat encourages air convection and so could speed up drying.

Jon Richfield, Somerset West, South Africa