Question

why does the temp remain constant during melting and heating

Answer 1

Because changing the pressure changes the phase transition temperature. For example, water boils at 100 C at 1 atm. If you increase the pressure, the boiling temperature will increase. Therefore, to keep on boiling the water, you will need to increase its temperature, otherwise you will not be able to vaporize all the water

Answer 2

Point out that when water freezes, the water molecules have slowed down enough that their attractions arrange them into fixed positions. Water molecules freeze in a hexagonal pattern and the molecules are further apart than they were in liquid water. Note: The molecules in ice would be vibrating.