Which of the following would be described as impure?

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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