The quantity of heat required to change the unit mass of a solid substance, from solid-state to liquid state, while the temperature remains constant, is known as

Options

A. latent heat
B. sublimation
C. hoar frost
D. latent heat of fusion

Answer

To solve the question regarding the quantity of heat required to change a unit mass of a solid substance from solid-state to liquid state at constant temperature, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Process**:
- The question describes a phase change from solid to liquid. This process occurs at a constant temperature, which is characteristic of phase transitions.

**Hint**: Identify the type of phase change being described in the question.

2. **Defining Latent Heat**:
- The heat required for a substance to change its state without changing its temperature is referred to as latent heat.

**Hint**: Recall the definition of latent heat and its significance in phase changes.

3. **Identifying the Specific Latent Heat**:
- When a solid changes to a liquid, the specific latent heat involved is called the "latent heat of fusion." This term specifically refers to the heat required to convert a unit mass of a solid into a liquid at its melting point.

**Hint**: Differentiate between latent heat of fusion (solid to liquid) and latent heat of vaporization (liquid to gas).

4. **Temperature Consideration**:
- The latent heat of fusion occurs at a specific temperature, which is the melting point of the solid. For many substances, this is typically around 0 degrees Celsius for ice.

**Hint**: Remember the melting point of common substances to contextualize the phase change.

5. **Conclusion**:
- Therefore, the quantity of heat required to change the unit mass of a solid substance from solid-state to liquid state while the temperature remains constant is known as "latent heat of fusion."

**Final Answer**: Latent heat of fusion (Option D).