The quantity of heat needed to change unit mass of a substance from liquid to vapour without changing the temperature, is called

The quantity of heat required to change the physical state of a substance without a change in its temperature is known as latent heat [2]. When this quantity is specifically measured for a unit mass (such as 1 kg) of the substance, it is termed 'specific latent heat'. In thermodynamics, the adjective 'specific' indicates that the quantity is divided by mass. Specifically, the heat needed to transition a unit mass from liquid to vapour at its boiling point is the specific latent heat of vaporization [1]. During this phase change, the energy is used to overcome intermolecular forces rather than increasing the kinetic energy of the molecules, which is why the temperature remains constant [2]. In contrast, specific heat refers to the heat required to raise the temperature of a unit mass by one degree [3].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Latent Heat > p. 294
2. [3] https://en.wikipedia.org/wiki/Specific_heat_capacity