Heat energy, usually. However, some substances will change states under the influence of UV or other radiation.Answer
The energy needed to change a material from a solid to a liquid is called the 'latent heat of fusion', or just the 'heat of fusion'. It is equal to the amount of energy or heat given off by the same material to change it from a liquid back into a solid.
Each material has its own heat of fusion. Take solid water (ice) at 0o Celsius, the freezing point. Water's heat of fusion is 79.71 calories per gram. That means that you will need to add 79.71 calories of heat to each gram of ice (solid water) to turn it into liquid water. Interestingly, once that's done, the water will still be at 0o Celsius! The heat was used just to change the water's state from solid to liquid. Once the ice is liquid, then any more heat added will increase the temperature of the liquid, and each calorie of heat will increase the water's temperature about 1o Celsius.
This is a similar concept to a material's 'heat of vaporization', which is the amount of energy required to change a liquid to a gas.
The amount of heat necessary to change a substance from a solid to a liquid or vice versa is commonly called the heat of fusion. It is more properly known as the standard enthalpy of fusion, or also the latent heat of fusion, or the enthalpy change of fusion. The specific temperature at which the change occurs is defined as the melting point of that substance. A link can be found below.
The amount of energy a substance must absorb in order to change from a solid to a liquid is the heat of fusion. A change in which a system absorbs energy from its surroundings is endothermic change.
The heat of fusion, also known as the enthalpy of fusion is how much energy is required to melt a solid to a liquid. This is the energy required to change the phase of the material - not the temperature. In fact at the melting point the heat capacity becomes infinite because all the energy is being used to change state and not temperature.As energy cannot be created or destroyed it is reasonable to assume it is the same amount of energy for a liquid to turn into a solid, which it is.So if energy is needed to change the phase of ice into water, the same amount of energy is released when water changes back into ice.
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