Compounds and mixtures differ primarily in two ways: Compounds have a constant composition, whereas mixtures do not, and mixtures can be separated easily, while compounds can only be separated by destroying the compound.
According to the law of constant composition, the ratio by mass of the elements in a chemical compound are always the same, no matter the source of the compound. This law helps to distinguish the differences between mixtures and compounds. The compound water is always 88.8 percent oxygen and 11.2 percent hydrogen by weight. Likewise, brass is a mixture of copper and zinc, with between 10 and 15 percent zinc.