If a solid body is heated with constant power, its temperature initially increases linearly over time according to the equation:
$$
\Delta Q = cm\Delta T
$$
When the melting point is reached, the additional energy goes into breaking the bonds between the particles, so the temperature remains constant and the solid transitions into the liquid state. The specific melting heat $c_\mathrm{s}$ depends only on the mass and is defined by the equation:
$$
\boxed{\Delta Q_\mathrm{s} = m c_\mathrm{s}}
$$
Therefore, the energy required to melt a substance $Q_\mathrm{s}$ is referred to as melting heat. Since this energy does not cause a temperature increase and is therefore "hidden," it is often referred to as latent heat.
After the melting process is complete, the temperature again increases in proportion to the supplied energy, until the boiling point is reached. At this point, the additional energy is used for evaporation, again resulting in a constant temperature.
Analogous to melting, the evaporation heat $\Delta Q_\mathrm{v}$ and the specific evaporation heat $c_\mathrm{v}$ are defined as:
$$
\boxed{\Delta Q_\mathrm{v} = m c_\mathrm{v}}
$$
Once the entire liquid has transitioned into the gas phase, the temperature increases linearly again. The respective specific heat capacities for the solid, liquid, and gas phases, as well as the melting and evaporation heat, differ significantly for the same substance.
An overview of the melting and evaporation heat values for some selected substances can be found in the table on heat capacities.
Both the melting and evaporation processes are fully reversible. This means that during freezing or condensation, the previously absorbed energy is fully released back to the environment.
For example, in the past, buckets of water were often placed in unheated cellars. During freezing in winter, the released melting heat ensured that the air temperature in the cellar did not fall below the freezing point.
A similar method is frost protection irrigation, used to protect crops from frost by spraying them with water. The release of melting heat prevents plant damage.
Another example is the human body’s thermoregulation through the production of sweat via special sweat glands. The technical term for this is transpiration. Especially at high ambient temperatures, the evaporation heat of water is used to cool down the body.
However, in environments with high humidity, the air can no longer absorb moisture, making heat dissipation via sweat evaporation impossible. This is why summer days with high humidity are often harder to endure than those with lower humidity.