Compound is a pure substance.

Answer

### Step-by-Step Text Solution

1. **Understanding the Statement**: The statement we need to evaluate is "A compound is a pure substance."

2. **Definition of Pure Substance**: A pure substance is defined as a chemical substance that is made up of only one type of particle. These particles can be atoms, molecules, or ions.

3. **Definition of Compound**: A compound is formed when two or more different elements combine together in a fixed ratio by mass.

4. **Examples of Compounds**:
- **Water (H2O)**: This compound consists of hydrogen and oxygen in a fixed ratio. It is a pure substance because it is made up of only H2O molecules.
- **Glucose (C6H12O6)**: This compound consists of carbon, hydrogen, and oxygen. It is also a pure substance as it is made up of only glucose molecules.
- **Acetic Acid (CH3COOH)**: This compound consists of carbon, hydrogen, and oxygen in a fixed ratio. It is a pure substance because it consists of only acetic acid molecules.
- **Ammonia (NH3)**: This compound consists of nitrogen and hydrogen. It is a pure substance as it is made up of only NH3 molecules.

5. **Conclusion**: Since all compounds are made up of only one type of particle (the molecules of the compound itself), and they are formed from different elements in a fixed ratio, we can conclude that all compounds are indeed pure substances. Therefore, the statement "A compound is a pure substance" is correct.