If you’re making hot chocolate by stirring flavor mix into hot milk or water, there will be a small amount of additional heat generated as the sugar dissolves, but you’re not likely to notice it.
In strictly scientific terms, the change in temperature of a substance when a solution is formed is known as enthalpic change of solution. This is because some substances absorb, rather than generate heat. When heat is absorbed, the reaction is called endothermic. When heat is generated, it is called exothermic. Gases dissolving in water tend to decrease the temperature of the solution, an endothermic process. The amount of heat generated or absorbed can be measured, and is directly related to the amount of the substance being dissolved. It is more difficult for most gases to form solutions with liquids, because energy is required to form bonds in the solution. Many solid substances, including common household sugar and salt, dissolve rather easily in water, because energy is released, which is why the solution becomes warmer than the water was before the solution was made.
Practical Application of Heat of Solution
Commercial hot or cold packs are made by combining two substances that either absorb or generate heat when forming a solution. You squeeze the pack to break an inner package, allowing the substances to mix as you manipulate the pack. As the solution is formed, the pack becomes either hot or cold, depending on what chemicals are involved.