Why do our hands burn when we touch ice?
Question sent by RAQUEL RUBIO (València). EDUARDO MARTÍNEZ TAMAYO answers:
The burning sensation we feel when we touch some objects at low temperatures, and also the damages produced, are very similar to the sensation we feel when we touch very hot objects, but not identical. However, the processes that cause biological damages are very different. The heat transference from a hot object (our fingers) to a cold one (ice) can cause the freezing of the water contained inside our cells, forming ice crystals which, if they expand enough, can cause cells to break open and the liquid inside of them is thus released. The result is the destruction of the frozen tissue.
For the cold object to produce damages it must have some particular characteristics —a low temperature and a high thermal conductivity. The speed of heat loss must be higher than the speed of heat release in living tissues (our fingers). The object must have a great capacity to store the transferred heat (big mass and high heat capacity) and the contact time must be long enough (the thermal gradient is usually relatively low compared to high temperature burns). Only then will damages be permanent.
Eduardo Martínez Tamayo. Materials Science Institute of the University of Valencia.