In the universe you can see different states of the same substance. A particular state of a substance is called phase. For example, water can exist as solid (ice), liquid and gas (vapour) states. Therefore, solid, liquid and gaseous forms of water are different phases of water such as solid phase, liquid phase and gaseous phase. The phase change takes place when one phase of a substance changes into other phase (for example the state of water can change from liquid phase to solid phase etc.). The change in phase is called phase transition.
In phase transition, one phase of a material can change into another phase by absorbing or releasing a certain quantity of heat without any change in temperature. For example, ice at \(0{{\kern 1pt} ^ \circ }{\rm{C}}\) is converted into liquid at \(0{{\kern 1pt} ^ \circ }{\rm{C}}\) by absorbing a certain quantity of heat \(Q\). We introduce a new heat called latent heat which is the amount of heat per unit mass required to turn one phase of a substance into another phase without changing its temperature. The quantity of heat per unit mass required to change the phase of a substance from solid to liquid is called latent heat of fusion and the transition is solid-liquid phase transition. If \(m\) is the mass of a substance in its solid form and \(L_f\) is the latent heat of fusion, the quantity of heat which must be absorbed by the substance to turn into liquid at the same temperature as in solid form is
\[Q = {L_f}m \]
In the reverse process that is when the liquid turns into solid,` the same quantity of heat given by the above expression is released by the substance and the quantity of heat in this case is negative:
\[Q = -{L_f}m\]
Note that in phase transition a certain amount of heat is used by the substance to change the phase of the substance not to increase or decrease the temperature of the substance. That means the ice at \(0{}^ \circ {\rm{C}}\) changes into liquid at \(0{}^ \circ {\rm{C}}\) by absorbing a certain quantity of heat. As the solid form of the substance is melting into liquid form, both forms are at the same temperature; the melting temperature is the same as the freezing temperature and the melting and freezing processes are accompanied by absorbing or releasing a certain amount of heat without increasing or decreasing the temperature of the substance respectively. But yes if you supply heat greater than a substance needs to turn completely into another phase, you increase the temperature of the substance after the substance turns completely into another phase. The two phases of a substance at the same temperature is called phase equilibrium. Combining both above expressions for heat required to change the phase gives:
\[Q = \pm {L_f}m \tag{1} \label{1}\]
Similar to the latent heat of fusion there is latent heat of vaporization which is the quantity of heat per unit mass required to change liquid phase to vapour (gas) phase without changing temperature. The transition is liquid-gas phase transition. In the reverse process of liquid-gas phase transition the gaseous phase turns into liquid phase releasing the same quantity of heat absorbed by the liquid to turn into gas. For example, water boils at \(100{}^ \circ {\rm{C}}\) and turns into vapour. Liquid water at \(100{}^ \circ {\rm{C}}\) absorbs a certain quantity of heat depending on its mass to turn into steam at \(100{}^ \circ {\rm{C}}\) and in the reverse process the steam condenses or turns into liquid water at \(100{}^ \circ {\rm{C}}\) releasing the same amount of heat absorbed before. You saw in phase transition that a certain quantity of heat is absorbed or released only to change the phase not the temperature. If \(L_v\) is the latent heat of vaporization and \(m\) is the mass of a substance, the quantity of heat in both processes (as in Eq. \eqref{1}) required for the phase change is:
\[Q = \pm {L_v}m \tag{2} \label{2}\]
In some cases, the solid form of a substance turns directly into gaseous form and this process is called sublimation. The latent heat of sublimation is denoted similarly by \(L_s\).
