Substance is the material, or matter, of which something is made. Substances are physical things that can be seen, touched, or measured. They are made up of one or more elemental parts. Iron, aluminium, water and air are examples of substances.

A substance is a pure single form of matter. It cannot be separated into other kinds of matter by any physical process. The substance must be composed of atoms and molecules. The substance must have a particular amount of weight, mass and volume.

Carbon dioxide is a pure substance because it has a fixed composition no matter where it was taken. Each molecule of carbon dioxide will always have 1 carbon and 2 oxygens.

A substance that has a fixed chemical composition throughout is called a pure substance such as water, air, and nitrogen. A pure substance does not have to be of a single element or compound.

Examples of Pure Substances All elements are mostly pure substances. A few of them include gold, copper, oxygen, chlorine, diamond, etc. Compounds such as water, salt or crystals, baking soda amongst others are also grouped as pure substances.

Example of impure substance are as follows: Salt mixed in water. Chocolate chips dipped in cookies. Fruit salad. Muddy water.

Wood is not a pure substance but a homogenous mixture because it is composed of compounds such as cellulose, hemicelluloses, and lignin which are made up of elements such as carbon, hydrogen, and oxygen which are found in varying concentrations in the wood body.

The compound acetylsalicylic acid (the active ingredient in aspirin) is a substance because it is a chemical compound that cannot be further reduced to other materials by physical means. However, the aspirin that contains the acetylsalicylic acid is a mixture. Substances may be either elements or pure compounds.