Heat of transition is the heat evolved or absorbed when a substance is converted from

Options

A. Solid to liquid
B. Solid to vapour
C. Liquid to vapour
D. One allotropic form to another allotropic form

Explanation

### Step-by-Step Solution:

1. **Understanding Heat of Transition**:
- Heat of transition refers to the heat that is either absorbed or released when a substance changes from one phase to another or from one allotropic form to another.

2. **Identifying the Phases**:
- The common phases of matter include solid, liquid, and gas.
- The transitions between these phases involve specific terms:
- **Solid to Liquid**: This process is known as **melting** or **heat of fusion**.
- **Liquid to Gas**: This is referred to as **vaporization**.
- **Solid to Gas**: This is called **sublimation**.

3. **Allotropic Forms**:
- Some substances can exist in different structural forms, known as allotropes. For example, carbon can exist as graphite or diamond.
- The transition from one allotropic form to another, such as from graphite to diamond, involves a specific energy change known as **heat of transition**.

4. **Conclusion**:
- Therefore, the heat of transition is the heat evolved or absorbed when a substance is converted from one phase to another or from one allotropic form to another.

### Final Answer:
The heat of transition is the heat evolved or absorbed when a substance is converted from one phase to another (e.g., solid to liquid, liquid to gas) or from one allotropic form to another (e.g., graphite to diamond).

---