Why is mixture called impure substance?

Explanation

### Step-by-Step Solution:

1. **Definition of Pure Substance**: A pure substance is defined as a material that consists of only one kind of atom or molecule. This means that it has a uniform and definite composition throughout.

2. **Characteristics of Pure Substances**: Pure substances have a fixed composition and specific properties that do not change. For example, pure water (H2O) always has the same chemical composition and properties.

3. **Definition of Mixture**: A mixture, on the other hand, is a physical combination of two or more substances. In a mixture, the components can be elements or compounds.

...