Which of the following is not a mixture?

Options

A. Soil
B. Air
C. Steam
D. Milk

Explanation

To determine which of the following is not a mixture, we first need to understand the definition of a mixture in chemistry. A mixture is formed when two or more substances are combined, and these substances can retain their individual properties. Mixtures can be classified into two types: homogeneous and heterogeneous.

1. **Identify the options**: We need to look at the given options to identify which one is not a mixture. Common examples of mixtures include air, soil, and milk.

2. **Analyze each option**:
- **Soil**: Soil is a mixture of various components such as microorganisms, insects, chemical elements, and organic matter. Therefore, soil is a mixture.
- **Air**: Air is composed of different gases, including nitrogen, oxygen, and carbon dioxide. Thus, air is also a mixture.
- **Milk**: Milk contains fats, proteins, and water, making it a mixture as well.
- **Steam**: Steam is the gaseous form of water, which is represented chemically as H2O. In this case, hydrogen and oxygen are chemically bonded to form a compound, not a mixture.

3. **Conclusion**: Since steam (H2O) is a compound and not a mixture, the correct answer to the question is that steam is not a mixture.

### Final Answer:
The option that is not a mixture is **steam (H2O)**.