What is meant by a pure substance?

Explanation

### Step-by-Step Solution:

1. **Definition of Pure Substance**: A pure substance is defined as a material that has a uniform and definite composition. This means that it consists of only one type of particle, which can be either an element or a compound.

2. **Types of Pure Substances**: Pure substances can be classified into two categories:
- **Elements**: These are substances that cannot be broken down into simpler substances by chemical means. Examples include oxygen (O), hydrogen (H), and gold (Au).
- **Compounds**: These are substances formed when two or more elements are chemically bonded together in a fixed ratio. An example of a compound is water (H₂O), which is made up of hydrogen and oxygen.

...