AskScience

Heat does two things:

1. Everything moves and vibrates faster. That means that bonds will be broken more often and you will reach your 'final' state earlier, aka salt dissolves faster. In addition, Ion Bonds require you to stay still more or less (it's a solid), whereas ions being complexed by water is a fluid state. This brings us to 2.: Higher temperature further pronounces the effect of entropy, aka multiple ions are preferred over one large crystal. There may be more to it though!