In my kitchen cupboard I have olives in brine and pickled onions in vinegar. There is no doubt that when I go to remove an olive or onion with my fingers (I know I should use a fork) the vinegar feels warmer than the brine. I even filled a jar with water and left it overnight in the cupboard to check. It felt colder than the vinegar. And friends have verified my finding. Surely the vinegar should be at cupboard temperature? Has anybody any idea what鈥檚 going on?
鈥� I suspect vinegar feels warmer than brine because water is better at conducting heat. Remember that room temperature is around 20 to 25 掳C, lower than our body temperature, which is around 37 掳C. So, for example, metal and stone objects feel colder than wood or plastic because they conduct heat away more quickly. Touch a wooden object at room temperature and the part in contact with your skin rapidly warms to body temperature. Touch a metal object and body heat is conducted away, so the object continues to feel cold.
鈥淭ouch a metal object and your body heat is conducted away, so the object feels cold鈥�
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Pickling vinegar is about 18 per cent acetic acid to water, and pure acetic acid has a thermal conductivity of around 0.2 watts per kelvin-metre, whereas the conductivity of water 鈥� pure or salty 鈥� is about three times as great at 0.6. This is enough to make a noticeable difference to how warm vinegar and brine feel.
Incidentally, one place where metal objects do feel warm to the touch is in trains in summer. If I ever have the misfortune to travel on the Central line in August, the metal handrails in the carriages feel noticeably warm. This is because the temperature in the carriage is close to body temperature, so body heat is not conducted away.
Alan Harding, Stansted, Essex, UK