Why can you smell things hot things but not smell cold things?

Its like water. Can you smell water? NO! So the cold things have ice on them, and the ice is made of water. You can't smell it. You can't smell boiling water either, so that argument doesn't hold.

Other things smell when its cold like petrol (gasoline).

Answer

When things are heated the molecules vibrate more rapidly, some molecules may gain enough energy the become a gas, but not all of them or your chocolate bar would evaporate! The gas molecules can diffuse through the air and up your nose where they can be sensed by chemical receptors. This makes cooking food that is very hot smell great.

If the object is cold the molecules will have less energy and fewer will become gaseous meaning fewer molecules reach your chemical receptors so the object will be harder to smell. Therefore you can't smell frozen foods very well at all.

Some molecules evaporate very easily such as petrol molecules, this means that you can smell them even though it is cold. Many molecules we just arn't very good at sensing, like water and oxygen, I guess we just don't have the correct chemical receptors. Imagine if we could smell how humid the day was for example, it would be a pain really and take a lot of brain power.