Latent Heat — Definition

Definition

Imagine you're heating a pot of ice on a stove. You'll notice that even though you're continuously supplying heat, the temperature of the ice-water mixture stays at $0^circ\text{C}$ until all the ice has melted.

Only after all the ice has turned into water does the temperature of the water start to rise. Similarly, when water boils, its temperature remains at $100^circ\text{C}$ (at standard atmospheric pressure) until all the water has converted into steam.

This 'hidden' heat, which doesn't cause a temperature change but instead facilitates a change in the physical state (phase) of the substance, is called latent heat.

The term 'latent' literally means 'hidden'. So, latent heat is the heat energy that is absorbed or released by a substance during a phase transition without any change in its temperature. This energy is not used to increase the kinetic energy of the molecules (which would lead to a temperature rise) but rather to overcome or establish the intermolecular forces that hold the molecules together in a particular phase.

For example, when ice melts, the latent heat of fusion provides the energy needed to break the rigid bonds in the ice crystal structure, allowing water molecules to move more freely as a liquid. When water boils, the latent heat of vaporization provides the energy to completely overcome the attractive forces between water molecules, allowing them to escape as individual gas molecules.

There are primarily two types of latent heat relevant for NEET:

1
Latent Heat of Fusion ($L_f$) — This is the amount of heat energy required to change a unit mass of a substance from solid to liquid state at its melting point, or the heat released when a unit mass changes from liquid to solid at its freezing point. For water, the latent heat of fusion is approximately $3.34 \times 10^5,\text{J/kg}$ (or $80,\text{cal/g}$). This means $3.34 \times 10^5,\text{J}$ of energy is needed to melt $1,\text{kg}$ of ice at $0^circ\text{C}$ into water at $0^circ\text{C}$.
2
Latent Heat of Vaporization ($L_v$) — This is the amount of heat energy required to change a unit mass of a substance from liquid to gaseous state at its boiling point, or the heat released when a unit mass changes from gas to liquid at its condensation point. For water, the latent heat of vaporization is approximately $2.26 \times 10^6,\text{J/kg}$ (or $540,\text{cal/g}$). This means $2.26 \times 10^6,\text{J}$ of energy is needed to convert $1,\text{kg}$ of water at $100^circ\text{C}$ into steam at $100^circ\text{C}$.

The total heat ($Q$) absorbed or released during a phase change for a mass ($m$) of a substance is given by the simple formula: $Q = mL$, where $L$ is the specific latent heat (either $L_f$ or $L_v$). Understanding latent heat is crucial for analyzing heating curves and solving problems involving phase changes, which are common in NEET Physics.