* Pure substances have a fixed chemical composition. This means they are made up of only one type of molecule or atom. Examples of pure substances include:

* Elements: Gold (Au), oxygen (O2)

* Compounds: Water (H2O), salt (NaCl)

* Naturally occurring materials are usually mixtures. They are composed of two or more substances physically combined, but not chemically bonded. Here are some examples:

* Air: A mixture of gases like nitrogen, oxygen, carbon dioxide, and others.

* Rocks: Mixtures of different minerals.

* Wood: A mixture of cellulose, lignin, and other organic compounds.

* Seawater: A mixture of water, salts, and dissolved minerals.

Why are pure substances rare in nature?

Nature is a dynamic process. Elements and compounds interact and react, forming mixtures. Even when pure substances are formed naturally, like crystals of pure quartz, they are often found in combination with other materials.

Exceptions:

While most naturally occurring materials are mixtures, there are some exceptions where pure substances can be found:

* Elemental deposits: Certain elements, like gold or sulfur, can be found in their pure elemental form in deposits.

* Crystals: Some crystals, like diamonds (pure carbon), can form as pure substances.

In summary: The vast majority of materials found in nature are mixtures. They are composed of multiple substances that are not chemically bonded, making them impure.