What happens to the molecules in a substance when energy is added?

It depends what substance you mean and to what extent it is heated. Each material has a generally different melting points, and some changes made to the materials are different. Say you mean metal was heated to melting point, it would of course melt. I just heated some grated cheese in a 1000W microwave and most of it was melted in 50 seconds, it would have completely melted given 20 more seconds. I tried the same thing a while ago with a thin porcelain plate for my beverage and withing 15 or so seconds the plate broke in half, no conatact was made to it by me or pretty much any other substance, it was the heat that had done it. Now, if you heat an egg, it cooks and that is irreversible, it will always be cooked. If you melt metal through heating it, it will be liquid metal and that is a reversible change made, for it will solidify if allowed to cool. Of course some things as well may hardly heat at all, like wood, which is an insultor of heat, it will take hours or less to heat depending on how you heat it, but mostly the wood will just heat up, and soften maybe. That is the same with plastic which I had my ready meal in, it was cooked for 10 minutes and it softened with heating but hardened once more as it was allowed to cool. Another pint is, as I have discovered, as the food gets heated some times the air under the film lid gets hot s well, and when you lift it scolding steam rises and burns your hand. I welcome anyone to correct my answer, it is poorly written and to my mental capacity it is a complicated question to answer.