Phase changes, also known as phase transitions, are changes in the physical state of a substance that occur when a certain amount of energy is added or removed. During a phase change, the temperature of a substance remains constant, even though heat is being added or removed. This is known as the latent heat of a substance.

There are several different types of phase changes, including melting, freezing, vaporization, and condensation. During a melting phase change, a solid is transformed into a liquid by adding heat. During a freezing phase change, a liquid is transformed into a solid by removing heat. Vaporization is the process by which a liquid is transformed into a gas by adding heat, and condensation is the process by which a gas is transformed into a liquid by removing heat.

In each of these phase changes, the temperature of the substance remains constant until the phase change is complete. This is because the energy added or removed during a phase change is used to overcome the forces that hold the particles of the substance together. These forces, known as intermolecular forces, include van der Waals forces, hydrogen bonding, and dipole-dipole interactions.

The energy needed to overcome these forces and complete a phase change is known as the latent heat of the substance. The latent heat of a substance is different for each phase change, and is dependent on the specific type of intermolecular forces present in the substance.

For example, the latent heat of fusion (the heat needed to melt a substance) is higher for substances with strong intermolecular forces, such as water, compared to substances with weaker intermolecular forces, such as mercury. This is why it takes more heat to melt a block of ice than it does to melt a piece of metal.

In conclusion, the temperature of a substance remains constant during a phase change because the energy added or removed is used to overcome the intermolecular forces that hold the particles of the substance together. The amount of energy needed to complete a phase change is known as the latent heat of the substance, and is dependent on the specific type of intermolecular forces present in the substance.