Which of the following conditions is necessary for a chemical reaction?

Options

A. It must be accompanied with change in temperature and pressure.
B. At least one of the reactants must be in a fixed quantity.
C. It must follow the law of conservation of mass.
D. All of the above.

Explanation

To determine which condition is necessary for a chemical reaction, we can analyze the options provided and evaluate their validity.

### Step-by-Step Solution:

1. **Understanding Chemical Reactions**:
- A chemical reaction involves the transformation of reactants into products. This process can occur under various conditions.

2. **Evaluating Option 1**:
- The first option states that a chemical reaction must be accompanied by a change in temperature and pressure.
- **Analysis**: While some reactions do involve changes in temperature or pressure, it is not a universal requirement. Some reactions can occur at constant temperature and pressure.
- **Conclusion**: This option is not necessary for all chemical reactions.

3. **Evaluating Option 2**:
- The second option suggests that at least one of the reactants must be in a fixed quantity.
- **Analysis**: It is not essential for reactants to be in fixed quantities for a reaction to occur. Reactions can proceed with varying amounts of reactants, as long as they follow the law of conservation of mass.
- **Conclusion**: This option is also not necessary for a chemical reaction.

4. **Evaluating Option 3**:
- The third option states that a chemical reaction must follow the law of conservation of mass.
- **Analysis**: The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This principle is fundamental in all chemical reactions, as the total mass of reactants must equal the total mass of products.
- **Conclusion**: This option is indeed necessary for a chemical reaction.

5. **Evaluating Option 4**:
- The fourth option states "All of the above."
- **Analysis**: Since options 1 and 2 are not necessary conditions for a chemical reaction, this option cannot be correct.
- **Conclusion**: This option is incorrect.

### Final Answer:
The necessary condition for a chemical reaction is that it must follow the **law of conservation of mass** (Option 3).

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