A pure substance consists of only one type of element. A pure substance has a definite chemical composition. A pure substance cannot be broken down into simpler substances. A pure substance is normally found as a solid at room temperature.
I gave my students this multiple choice question: Sulfur (S), oxygen (O2), water (H2O), and sodium chloride (NaCl) are all examples of pure substances. Which of the following describes all pure substances?

Only one answer is correct among these 4 choices:
- A pure substance consists of only one type of element.
- A pure substance has a definite chemical composition.
- A pure substance cannot be broken down into simpler substances.
- A pure substance is normally found as a solid at room temperature.

Could you tell me which one is correct?

A pure substance has a definite chemical composition.