A pure substance is a material that is made up of only one type of atom or molecule. It has a fixed chemical composition and uniform properties throughout. This means that no matter where you take a sample from, it will always have the same chemical makeup and physical characteristics.

Here are some key characteristics of pure substances:

* Fixed composition: The ratio of elements in a pure substance is always the same. For example, water (H₂O) always has two hydrogen atoms and one oxygen atom.

* Uniform properties: Pure substances have consistent physical properties like melting point, boiling point, density, and color throughout the sample.

* Cannot be separated by physical means: You cannot break down a pure substance into simpler components by using techniques like filtration, evaporation, or distillation.

Examples of pure substances:

* Elements: Gold (Au), Oxygen (O₂), Carbon (C)

* Compounds: Water (H₂O), Salt (NaCl), Sugar (C₁₂H₂₂O₁₁)

What is NOT a pure substance:

* Mixtures: These are combinations of two or more substances that are not chemically bonded. They can be separated by physical means. Examples include:

* Saltwater (salt and water)

* Air (nitrogen, oxygen, and other gases)

* Sand (different types of minerals)

Let me know if you'd like more examples or want to delve deeper into the differences between pure substances and mixtures!