Pure water is composed entirely of water molecules. A sample that is composed of only one substance is called a pure substance. 

Mixtures contain two or more different types of substances. The terms homogeneous and heterogeneous are usually used to describe how the substances in a mixture are arranged:

• Homogeneous Mixture: In a homogeneous mixture, substances are evenly mixed throughout the mixture. Every sample taken from a homogeneous mixture will contain the same substances. Example: salt water.

• Heterogeneous Mixture: In a heterogeneous mixture, substances are unevenly mixed. Samples taken from different parts of the mixture may contain different substances. Example: oil and water.

In summary,

• Pure Water is a pure substance. It contains only water molecules. The terms homogeneous and heterogeneous are not usually used to describe pure substances. 

• A mixture of water and salt is an example of a homogeneous mixture because the salt is evenly mixed throughout the water.

• A mixture of water and oil is an example of a heterogeneous mixture because the water and oil separate into two separate layers and are not evenly mixed.