Themes > Science > Chemistry > General Chemistry > Matter > Elements, Compounds, and Mixtures > Mixtures Vs. Compounds

The law of constant composition states that the ratio by mass of the elements in a chemical compound is always the same, regardless of the source of the compound. The law of constant composition can be used to distinguish between compounds and mixtures of elements: Compounds have a constant composition; mixtures do not. Water is always 88.8% O and 11.2% H by weight regardless of its source. Brass is an example of a mixture of two elements: copper and zinc. It can contain as little as 10%, or as much as 45%, zinc.

Another difference between compounds and mixtures of elements is the ease with which the elements can be separated. Mixtures, such as the atmosphere, contain two or more substances that are relatively easy to separate. The individual components of a mixture can be physically separated from each other.

Chemical compounds are very different from mixtures: The elements in a chemical compound can only be separated by destroying the compound. Some of the differences between chemical compounds and mixtures of elements are illustrated by the following example using raisin bran and "Crispix.".

Raisin bran has the following characteristic properties of a mixture.

• The cereal does not have a constant composition; the ratio of raisins to bran flakes changes from sample to sample.
• It is easy to physically separate the two "elements," to pick out the raisins, for example, and eat them separately.

Crispix has some of the characteristic properties of a compound.

• The ratio of rice flakes to corn flakes is constant; it is 1:1 in every sample.
• There is no way to separate the "elements" without breaking the bonds that hold them together.

Information provided by: http://chemed.chem.purdue.edu