* Composition of a compound: A compound is formed when two or more elements chemically combine in a fixed and definite ratio. This ratio is always the same for a given compound, regardless of how it's formed. For example, water (H₂O) always has two hydrogen atoms and one oxygen atom.

* Mixture: A mixture is a combination of two or more substances where each substance retains its individual properties. The components of a mixture can be present in any proportion, and their ratios are not fixed. For example, you can make saltwater with varying amounts of salt and water.

In summary:

* Compounds have fixed ratios of elements.

* Mixtures have variable ratios of components.