The amount of heat necessary to change the state of a substance is called the latent heat. It's the energy required to break or form the bonds between molecules during a phase change, like melting, freezing, boiling, or condensation.

Here's a breakdown:

* Latent heat of fusion: The heat required to change a substance from a solid to a liquid at its melting point.

* Latent heat of vaporization: The heat required to change a substance from a liquid to a gas at its boiling point.

Here's the formula to calculate the heat required for a phase change:

Q = m * L

Where:

* Q is the amount of heat energy (in joules)

* m is the mass of the substance (in kilograms)

* L is the specific latent heat (in joules per kilogram)

Important Note: The specific latent heat is a property of the substance and depends on the type of phase change (fusion or vaporization).

For example, the latent heat of fusion for water is 334 kJ/kg, meaning 334 kJ of energy is needed to melt 1 kg of ice at 0°C.

Key points to remember:

* No temperature change: During a phase change, the temperature of the substance remains constant even though heat is being added or removed.

* Energy is absorbed or released: When a substance changes state from solid to liquid or liquid to gas, energy is absorbed. When it changes from gas to liquid or liquid to solid, energy is released.

I hope this explanation is helpful!