Assertion Antiferromagnetic substances can be made paramagnetic by heating.
Reason`(R)`On heating some of the ions change their direction of spin.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Antiferromagnetic substances can be made paramagnetic by heating."
- Antiferromagnetic substances have their magnetic moments aligned in opposite directions, resulting in a net magnetic moment of zero. When heated, the thermal energy can disrupt this alignment.

2. **Understanding the Reason**:
- The reason states that "On heating, some of the ions change their direction of spin."
- When antiferromagnetic materials are heated, the thermal energy can cause some of the magnetic moments (spins) to become disordered. This disorder can lead to some spins aligning in the same direction, which is characteristic of paramagnetic materials.

3. **Connecting Assertion and Reason**:
- The assertion is correct because heating can indeed convert antiferromagnetic materials to paramagnetic ones due to the disruption of the ordered spin arrangement.
- The reason is also correct because the change in the direction of spin due to heating is what leads to the loss of the antiferromagnetic property and the emergence of paramagnetism.

4. **Conclusion**:
- Both the assertion and the reason are true. Furthermore, the reason provides a correct explanation for the assertion because it describes the mechanism (change in spin direction) that leads to the transition from antiferromagnetism to paramagnetism upon heating.

### Final Answer:
Both the assertion and reason are true, and the reason correctly explains the assertion.