Heat always flows from a hot area to a colder area.
Explanation:
1. Phase transitions
Examples of phase transitions are the melting of a solid to form a liquid and the freezing of a liquid into a solid.
(a) Heat flow during melting
Heat flows from the surroundings into the solid during melting.
The heat energy overcomes some of the forces holding the molecules next to each other.
It allows the molecules the slide past each other although they aren't completely free of each other.
(b) Heat flow during freezing
Heat flows from the liquid into the surroundings during freezing.
The molecules lose so much energy that they can no longer overcome the forces holding them next to each other.
The molecules stay in fixed positions nest to each other and the substance is a solid.
2. Heat flow from water is placed in freezer
When you place your ice-cube trays of water into the freezer, heat flows from the water into the cold compartment.
Eventually, the water molecules have lost so much energy they cannot overcome their mutual attractions.
They are "frozen" in place, and you have a tray of ice cubes.
3. Heat flow from hot soup to ice cube
When you add an ice cube to hot soup, heat flows from the soup into the ice.
The water molecules gain enough energy to slide away from the neighbours and the ice cube melts.
