Define first order phase transition?

Explanation

### Step-by-Step Solution:

1. **Understanding Phase Transition**:
- A phase transition refers to the transformation of a substance from one phase to another, such as from solid to liquid (ice to water) or from liquid to gas (water to vapor).

2. **Defining First Order Phase Transition**:
- A first order phase transition is characterized by a discontinuity in the first derivatives of the Gibbs free energy (G) with respect to temperature (T) or pressure (P).

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