Question

On a cold morning water vapor in air becomes frost on your window. Is heat flowing into the water or out of the water? Is this process exothermic or endothermic??

Answer 1

I look at it this way. If I have ice and I put in heat, it melts like so.

H2O(solid) + heat = H2O(liquid)
Now look at the reverse equation. It says that If you convert H2O(liquid) to ice (or frost) you produce heat. So that process is exothermic. Right?
melting ice is endotheric because you put heat into the system.
feezing water to form frost is the reverse; i.e., taking heat out of the system which is exothermic.