Composition of mixtures

Pure Substances

• Consist of a single type of atom, molecule, or formula unit
• Always exhibit a fixed percent composition by mass
• The constancy of composition in a pure substance is what allows chemists to assign definite chemical formulas

Mixtures

• Combine two or more pure substances in variable proportions
• Their overall elemental composition by mass changes whenever the ratio of components changes
• The variability in a mixture is what makes quantitative analysis of mixtures both necessary and informative

Elemental Analysis

• Provides the experimental mass percentages of each element in a sample
• Enables chemists to determine the relative numbers of atoms in a substance and to assess its purity
• When experimental percent composition values match the theoretical values calculated from a known formula, the sample is confirmed as pure
• Deviations between experimental and theoretical values signal the presence of impurities

In mixture problems, the mass contribution of each element from each component can be summed to find the overall composition, or the process can be reversed — using measured elemental masses to solve for the unknown proportions of components in the mixture.