Which of the following would be described as impure?

Options

A. Crystallized salt
B. Salt solution
C. Rock salt
D. All of these

Explanation

To determine which of the substances listed is impure, we need to understand the definitions of pure and impure substances.

### Step-by-Step Solution:

1. **Definition of Pure and Impure Substances**:
- A pure substance has a uniform and definite composition. This means that all samples of a pure substance have the same chemical properties and composition.
- An impure substance contains one or more impurities, meaning its composition can vary and is not uniform.

2. **Analyzing Crystallized Salt**:
- Crystallized salt is obtained through the process of evaporation. This process typically results in a high level of purity because it separates the salt from other components in the solution.
- Therefore, crystallized salt is considered a pure substance.

3. **Analyzing Salt Solution**:
- A salt solution is a homogeneous mixture where salt is dissolved in water. The composition of the solution is uniform throughout, meaning that it is also a pure substance.

4. **Analyzing Rock Salt**:
- Rock salt (NaCl) is a natural form of salt that has not undergone any chemical purification. It contains various impurities, such as calcium sulfate (gypsum) and other minerals.
- Because of these impurities, rock salt does not have a fixed composition and is therefore classified as an impure substance.

5. **Conclusion**:
- Among the options provided, rock salt is the only substance that is impure due to the presence of natural impurities.

### Final Answer:
Rock salt is the impure substance.

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