**Explanation:**

A pure substance is a type of matter that has a constant composition and cannot be separated into simpler substances by physical methods. It consists of only one type of particle, either atoms or molecules, and has specific chemical and physical properties.

There are two types of pure substances:

1. **Elements:**
- An element is a substance that consists of only one type of atom.
- It cannot be broken down into simpler substances by chemical methods.
- Examples of elements include hydrogen (H), oxygen (O), carbon (C), etc.
- Each element has unique properties, such as melting point, boiling point, density, and reactivity.

2. **Compounds:**
- A compound is a substance that consists of two or more different elements chemically combined in a fixed ratio.
- It can be broken down into simpler substances by chemical methods.
- Examples of compounds include water (H2O), carbon dioxide (CO2), sodium chloride (NaCl), etc.
- Compounds have different properties than the elements that make them up.

Therefore, a pure substance can be either an element or a compound. It cannot be a heterogeneous mixture because a heterogeneous mixture consists of two or more different substances physically mixed together, and these substances can be separated by physical methods. A pure substance, on the other hand, cannot be separated into simpler substances by physical methods.

To summarize:
- A pure substance is a type of matter with a constant composition.
- It can be either an element (consists of only one type of atom) or a compound (consists of two or more different elements chemically combined).
- A pure substance cannot be a heterogeneous mixture, which consists of physically mixed substances that can be separated by physical methods.

As they are found in the pure form without additional components or impurities,both an element and a compound can be a pure substance.