What are two ways that a substance can be described?

- By its chemical composition. This refers to the elements that make up the substance, and the proportions in which they are present. For example, water is a compound made up of hydrogen and oxygen, in a ratio of 2:1.

- By its physical properties. These are the characteristics of a substance that can be observed or measured without changing its chemical composition. For example, the color, density, and melting point of a substance are all physical properties.