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

Here are some key characteristics of pure substances:

* Constant composition: The ratio of elements in the substance is always the same.

* Uniform properties: All parts of the substance have the same physical and chemical properties.

* Defined melting and boiling points: Pure substances melt and boil at specific temperatures.

* Cannot be separated by physical means: Methods like filtration, distillation, or evaporation won't change the composition of a pure substance.

Examples of pure substances:

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

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

Examples of substances that are NOT pure:

* Mixtures: A mixture is a combination of two or more substances that are not chemically bonded. They can be separated by physical means. Examples include saltwater, air, and sand.

* Solutions: A solution is a homogeneous mixture where one substance is dissolved in another. Examples include saltwater and sugar dissolved in water.

Important note: It's important to remember that even pure substances can exist in different states of matter (solid, liquid, gas). For example, water can be a solid (ice), a liquid (water), or a gas (steam).