Which of the following statements is correct?

Options

A. A pure substance must contain only one type of atom.
B. A mixture containing two compounds must be heterogeneous.
C. A heterogeneous mixture must contain at least three elements.
D. A homogeneous mixture must be uniform.

Explanation

To determine which of the statements is correct, we need to analyze each option one by one based on the definitions and characteristics of pure substances, mixtures, homogeneous mixtures, and heterogeneous mixtures.

### Step-by-Step Solution:

1. **Analyze the First Statement**:
- The first statement claims that "A pure substance must contain only one type of atom."
- **Evaluation**: A pure substance can be made up of only one type of atom (like gold, Au) or one type of molecule (like water, H2O). Therefore, this statement is incorrect because it does not account for pure substances that are molecules.

2. **Analyze the Second Statement**:
- The second statement claims that "A mixture containing two compounds must be heterogeneous."
- **Evaluation**: This is not necessarily true. For example, when common salt (NaCl) is dissolved in water (H2O), it forms a homogeneous mixture. Thus, this statement is also incorrect.

3. **Analyze the Third Statement**:
- The third statement claims that "A heterogeneous mixture must contain at least 3 elements."
- **Evaluation**: This statement is incorrect. A heterogeneous mixture can be formed with just two elements. For example, mixing mercury and bromine creates a heterogeneous mixture. Therefore, this statement is false.

4. **Analyze the Fourth Statement**:
- The fourth statement claims that "A homogeneous mixture must be uniform."
- **Evaluation**: This statement is correct. By definition, a homogeneous mixture has a uniform composition throughout, meaning the components are evenly distributed.

### Conclusion:
After evaluating all four statements, the only correct statement is the fourth one: "A homogeneous mixture must be uniform."

### Final Answer:
**The correct statement is: 4. A homogeneous mixture must be uniform.**

---