Which of the following is thermal dissociation?

Options

A. `2KClO_3 to 2KCl + 3O_2`
B. `NH_4NO_3 to N_2O + 2H_2O`
C. `PCl_5 to PCl_3 + Cl_2`
D. `2Ag NO_3 to 2Ag + 2NO_2 + O_2`

Explanation

To determine which of the following reactions is an example of thermal dissociation, we need to understand the definition of thermal dissociation and how it differs from other types of reactions.

### Step-by-Step Solution:

1. **Understanding Thermal Dissociation**:
- Thermal dissociation refers to the process where a chemical compound breaks down into simpler substances when heat is applied. This is a specific type of decomposition reaction that occurs due to the influence of heat.

2. **Analyzing the First Reaction**:
- The first reaction is described as a thermal decomposition reaction. While this involves breaking down a compound with heat, it is a broader category that includes thermal dissociation. However, it does not specifically identify thermal dissociation.

3. **Analyzing the Second Reaction**:
- The second reaction is identified as a decomposition reaction. Decomposition reactions involve breaking down a compound into simpler products but do not necessarily involve heat. Therefore, this is not thermal dissociation.

4. **Analyzing the Third Reaction**:
- The third reaction is explicitly stated as an example of thermal dissociation. This means that the chemical compound in this reaction is breaking down due to the application of heat, which fits the definition of thermal dissociation.

5. **Analyzing the Fourth Reaction**:
- The last reaction is also described as a decomposition reaction. Similar to the second reaction, it involves breaking down a compound but does not specifically indicate that heat is applied, thus it is not thermal dissociation.

6. **Conclusion**:
- Based on the analysis, the correct answer is the third reaction, which is an example of thermal dissociation.

### Final Answer:
The correct option that represents thermal dissociation is **Option C**.