At a basic level heat is just molecules moving around more. The colder it is, the slower water molecules fly around. Water molecules either bind or breakdown other molecules through typical chemical processes. When water comes in contact with glucose it binds with it and your Hot Chocolate mix starts to become one fluid instead of water and a powder. So the more the molecules are moving around the more likely they are to come into contact with the thing you are dissolving. Fats also undergo chemical changes when they get hot so they may become fluid and mix better in water.