Question sent by REMEI CALABUIG (Albaida, Valencia). EDUARDO MARTÍNEZ answers:
The burning sensation is a sensory reaction to a process of heat transfer from a heat source —whether a piece of cutlery or tinfoil taken out of the oven—, to a cold source, i.e. our fingers.
This process shows two different faces: how much heat is transferred and at what speed. It is in the first where we can find the answer to our question: the burning sensation is caused by the damages in our tissues caused by the heat. If a large enough amount of heat is transferred from the hot source to the cold source, there is a sudden evaporation of the tissue water, with the subsequent destruction of the biological structures of the epidermis.
For water to evaporate, and for the destruction of biological macromolecules, large quantities of heat are needed. If we compare a hot piece of cutlery with tinfoil, both recently taken out of the oven, tinfoil is a depository that has stored a smaller amount of heat than the piece of cutlery, especiallydue to the differences of their masses. Tinfoil, therefore, in spite of being very hot, has not enough heat stored in it as to produce skin burns; in fact, occasionally, we don’t even feel it hot when we take it out of the oven. On the contrary, a hot piece of cutlery, with a greater mass, can store enough heat as to produce a skin burn.
We have also said that, in the process of transferring the heat, another factor takes part, and that is the transfer speed. In this case, this factor plays a minor role, but it is important in other situations, and we may have the chance to get to it some other time.
Eduardo Martínez Tamayo. Institute of Materials Science. University of Valencia.