Phosphorus burns in chlorine gas to form `PCl_5`. Which type of reaction is this?

Options

A. Combination reaction
B. Displacement reaction
C. Decomposition reaction
D. Neutralization reaction

Explanation

To determine the type of reaction when phosphorus burns in chlorine gas to form phosphorus pentachloride (PCl5), we can analyze the reaction step by step.

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
- The reactants in this reaction are phosphorus (P) and chlorine gas (Cl2).
- The product formed is phosphorus pentachloride (PCl5).

2. **Understand the Nature of the Reaction**:
- A combination reaction occurs when two or more reactants combine to form a single product.
- In this case, phosphorus and chlorine gas are combining to form one product, PCl5.

3. **Classify the Reaction**:
- Since we have two reactants (phosphorus and chlorine) combining to form one product (PCl5), this reaction fits the definition of a combination reaction.

4. **Eliminate Other Reaction Types**:
- **Displacement Reaction**: This involves a more reactive element displacing a less reactive element from a compound. This does not apply here as we are not displacing any element.
- **Decomposition Reaction**: This involves a single compound breaking down into two or more products. This is not applicable since we are not breaking down any compound.
- **Neutralization Reaction**: This involves an acid reacting with a base to form salt and water. There are no acids or bases involved in this reaction.

5. **Conclusion**:
- Based on the analysis, the reaction of phosphorus burning in chlorine gas to form PCl5 is classified as a **combination reaction**.

### Final Answer:
The reaction of phosphorus burning in chlorine gas to form PCl5 is a **combination reaction**.