Monday, 23 March 2020

Year 10 Cooling by evaporation - why does hand sanitiser feel cold?

Look at this photograph of the ball bearing model of Kinetic Theory. Kinetic Theory says that all atoms are always moving. But look carefully at the ball bearings in the photograph. Most of them are at the bottom. They have low energy. A few are at the top because they have high energy. In fact, one or two had so much energy that they jumped out of the tube.
Now, the kinetic energy of atoms tells you the temperature. High kinetic energy = high temperature. Low kinetic energy = low temperature. If the high energy atoms jump out, then what is left are the low energy atoms. High energy atoms jumping out is called EVAPORATION. If you put hand sanitiser onto your hands, it evaporates. the high energy atoms jump out into the air leaving the low energy atoms behind. Low energy = low temperature. Hand sanitiser feels cold.