Question: What happens in the gray zone between solid and liquid?

James Smallcombe answered on 3 Jul 2020:

To discuss this you really have to think about the individual molecules that make up the material.
These molecules are all held to their neighbours by bonds that take energy to break. As you heat up the solid all of those molecules get more energy and shake a bit in their bonds. The temperature of a material tells us the average energy each molecule is shaking with, but they all have slightly different amounts that change all the time as energy is passed between them. (The distribution of energy is called the Boltzmann Distribution)
Every now and then one molecule in the solid will have enough energy to break its bonds and shoot out of the material. What happens to that molecule is I think your “gray zone”, it depends a lot on the pressure outside the solid. If the pressure is very low, the molecule will fly off and be a gas. If the pressure is higher it will form a liquid when it comes into contact with similar molecules that have similar amounts of energy.
So if the solid is already surrounded by liquid, molecules will just break off one at a time and join the liquid. But for those first few lone molecules as a solid heats up life is a bit weird.