The substance which decreases the rate of a chemical reaction in called a/an:

Options

A. inhibitor
B. poison
C. moderator
D. promoter

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the term used to describe a substance that decreases the rate of a chemical reaction.

2. **Identifying Key Terms**: In the context of chemical reactions, we often encounter terms like catalyst, inhibitor, poison, moderator, and promoter. Each of these has a specific role.

3. **Defining Inhibitor**: An inhibitor is a substance that decreases the rate of a chemical reaction. It works by interfering with the reactants or the reaction mechanism.

4. **Contrasting with Catalyst**: Catalysts are substances that increase the rate of a chemical reaction. Inhibitors are essentially the opposite of catalysts.

5. **Examples of Inhibitors**: A common example of an inhibitor is antioxidants, which are added to food to prevent oxidation and spoilage.

6. **Understanding Other Terms**:
- **Poison**: This term refers to substances that can reduce the effectiveness of catalysts but do not directly decrease the rate of the chemical reaction itself.
- **Moderator**: This term is related to nuclear reactions and refers to substances that slow down neutrons, not applicable to chemical reactions.
- **Promoter**: This term refers to substances that enhance the efficiency of a reaction but do not necessarily decrease the rate.

7. **Conclusion**: Based on the definitions and roles of these substances, the correct answer to the question is "inhibitor".

**Final Answer**: The substance which decreases the rate of a chemical reaction is called an **inhibitor**.

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