What is happening when I dissolve styrofoam in acetone? (And should this change temperature)

I’ve completed a science experiment on endothermic and exothermic reactions in which I measured the temperature of solvents before adding solutes and measuring again. I found that dissolving styrofoam peanuts in acetone lowers the temperature of the acetone, but my book says that should not happen. Can anyone tell me what is happening when acetone dissolves polystyrene? I thought the reaction would be endothermic because the acetone is breaking down bonds in the styrofoam and using energy to do so and my book did not even explain anything about the reaction.


