The rate at which a substance reacts, depends on its:

Options

A. Active mass
B. molecular mass
C. Equivalent mass
D. Atomic mass

Explanation

To solve the question "The rate at which a substance reacts, depends on its:", we will analyze the options provided and determine which one is correct based on the principles of chemical kinetics and the law of mass action.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the factors that influence the rate of a chemical reaction. The options provided are active mass, molecular mass, equivalent mass, and atomic mass.

2. **Law of Mass Action**: According to the law of mass action, the rate of a chemical reaction is directly proportional to the active masses (or concentrations) of the reactants. This means that the more concentrated the reactants are, the faster the reaction will proceed.

3. **Defining Active Mass**: Active mass refers to the effective concentration of a reactant in a chemical reaction. It is often expressed in terms of molarity (moles per liter) and represents the amount of substance that is available to react.

4. **Evaluating the Options**:
- **Active Mass**: This is the correct answer as it directly relates to the concentration of reactants and their ability to react.
- **Molecular Mass**: This is the mass of a single molecule and does not directly influence the rate of reaction.
- **Equivalent Mass**: This relates to the amount of substance that can donate or accept a certain number of electrons in a reaction, but it does not directly determine the rate of reaction.
- **Atomic Mass**: This is the mass of individual atoms and is not relevant to the rate of reaction.

5. **Conclusion**: Based on the law of mass action and the definitions provided, the rate at which a substance reacts depends on its **active mass**.

### Final Answer:
The rate at which a substance reacts depends on its **active mass**.

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