Why does the temperature stay the same when melting?

Answer 1

The molecules in a solid are held by strong intermolecular bonds. For the solid to melt, these bonds have to be broken. Since energy is needed to break the intermolecular bonds, the thermal energy supplied at the melting point is used to do the work to break the intermolecular bonds between the molecules of the solid. Once the intermolecular bonds are broken, the molecules can then move out of their fixed positions. Hence it can then be said that the solid has melted, which is the change of state from solid to liquid. This explains why temperature remains constant during the melting phases.

Answer 2

When liquid molecules come together and solidify, intermolecular bonds are formed. As the intermolecular bonds are formed, thermal energy is then released, which explains why the temperature remains constant during the freezing phases.

Answer 3

Temperature is a measure of thermal energy. If you add energy to a system and it doesn't have anywhere else to go, it will go into thermal energy and the temperature will increase. If you cool a system down, you're removing energy from it, which generally comes from the thermal energy.

However ... if you're under conditions where a phase changes is possible ... the energy can go into/come from the phase change rather than from thermal energy. In the case of liquid water, 1 calorie of energy is roughly the amount of thermal energy that represents a change of 1 degree Celsius for 1 gram of water. But, if the water is at the boiling point ... it takes about 540 calories per gram to transition from the liquid state to the vapor state. Adding heat energy at that point doesn't raise the temperature, it just goes into converting the water to water vapor.

Similarly, when water vapor condenses back to a liquid, it releases about 540 calories per gram. So as you cool water vapor (remove energy from it) the temperature decreases to the boiling point, and then the vapor starts condensing (and releasing a lot of energy in the process).

The same thing happens with melting/freezing, though in that case it's about 80 calories per gram.