A pure substance is a single kind of matter that cannot be separated into other kinds of matter by any physical means. A pure substance always has a definite and constant composition. A mixture is a physical combination of two or more pure substances in which each substance retains its own chemical identity.
A pure substance is a single substance on its own. Elements and compounds are pure substances, but mixtures are not. Compounds are very different from the elements they contain. But a mixture resembles the substances it contains. The properties of the mixture are a kind of average of the properties of the substances in it. So a mixture of sugar and water is both sweet and wet.