Define latent heat. What are the `S.I`. Units of latent heat?

Answer

### Step-by-Step Solution:

1. **Definition of Latent Heat**:
Latent heat is defined as the amount of heat energy required to change the phase of a substance without changing its temperature. This occurs during the transition phase of a substance, where it changes from one state (solid, liquid, or gas) to another.

2. **Understanding Transition Phase**:
The transition phase refers to the process when a substance changes from one state to another. For example, this can be the transition from solid to liquid (melting), liquid to gas (vaporization), or gas to liquid (condensation).

3. **Types of Latent Heat**:
- **Latent Heat of Vaporization**: This is the heat required for the transition from gas to liquid.
- **Latent Heat of Fusion**: This is the heat required for the transition from liquid to solid.

4. **Formula for Latent Heat**:
The heat required during a phase change can be calculated using the formula:
\[ Q = mL \]
where \( Q \) is the heat absorbed or released, \( m \) is the mass of the substance, and \( L \) is the latent heat.

5. **Alternative Definition**:
Latent heat can also be described as the heat that must be added to or removed from a unit mass of a substance to convert it from one phase to another without changing its temperature.

6. **SI Units of Latent Heat**:
The SI unit of latent heat is derived from the formula and is expressed as:
\[ \text{Latent Heat (L)} = \frac{Q}{m} \]
Thus, the SI unit of latent heat is Joules per kilogram (J/kg).