Elements are substances that are made up of two or more elements chemically combined.

Whereas a compound is made from the chemical combination of two or more substances, a mixture is made from two or more substances that are physically combined.

Heterogeneous mixtures are mixtures that have a uniform composition throughout the entire sample.

Unlike in the example of compounds, the substances that make up a mixture do not lose their individual properties.

The substances in a mixture can be separated by physical means.

When a compound is formed, altogether a new substance is formed and the properties of which are similar to its constituent elements.

All mixtures have variable composition. It means that the substances that are found in the mixture can be mixed with various proportions or concentrations.

Chemical reactions can break more complex substances down into elements, but elements can only be broken down further by nuclear reactions.

Oxygen is a substance because it has a consistent composition and it is an element because it is only made up of one type of atom.