A compound is a substance that contains two or more elements chemically combined in fixed proportions by mass. Citric acid contains carbon, hydrogen, and oxygen atoms chemically combined in the ratio of 3:4:6, respectively. The chemical formula for citric acid is C6H8O7.

A mixture is a combination of two or more substances that are not chemically combined. The composition of a mixture can vary. For example, salt water is a mixture of water and salt. The amount of salt in salt water can vary.