A pure substance is a material that has a constant composition and properties throughout. This means it is made up of only one type of atom or molecule and cannot be broken down into simpler substances by physical means.

Here are some key characteristics of pure substances:

* Fixed composition: The ratio of elements in a pure substance is always the same.

* Uniform properties: The physical and chemical properties of a pure substance are consistent throughout the entire sample.

* Can be a single element or a compound: Examples include gold (element), water (compound), and table salt (compound).

* Can be separated by chemical reactions: A pure substance can only be broken down into simpler substances by chemical reactions, not physical processes like filtration or distillation.

Here are some examples of pure substances:

* Elements: Oxygen (O2), gold (Au), iron (Fe)

* Compounds: Water (H2O), table salt (NaCl), sugar (C12H22O11)

It's important to note that the term "pure" is relative. In practice, most substances contain some impurities. However, they are still considered pure substances if the impurities are present in very small amounts and do not significantly alter the substance's properties.