What is a substance?

pure substances are elements and compounds

The word substance (a physical material) originates from Latin substantia, literally meaning "standing under". The word was used to translate the Greek philosophical term ousia.

In Chemistry

It is a material that can undergo various transformations related to artificial or natural phenomena. The term substance specifically applies to a physical material with a definite chemical composition, that cannot be separated into constituent materials by physical means. The term "pure substance" may be used to indicate this. A substance may be an element or compound. Mixtures and solutions are combinations of one or more distinct substances.

A substance (pure substance) may exist as either a solid, liquid, or gas.

Matter of definite composition and properties.

For example, every pure element is a different substance. Every pure compound is a different substance.

Salt water is a mixture of two substances - sodium chloride and water. Its composition and therefore its properties are not fixed.

Gasoline is a mixture of hydrocarbons and (depending on the composition), gasoline's properties can vary.