Elements compounds and mixtures

Elements

• Are pure substances that contain only one type of atom

Compounds

• Contain two or more different elements chemically bonded together in a fixed ratio

Mixtures

• Contain two or more substances that are not chemically bonded together and can be separated by physical methods

A pure substance has a fixed (sharp) melting point and boiling point, whereas a mixture melts and boils over a range of temperatures. This difference in melting and boiling behaviour provides a reliable test for purity — a sharp melting point indicates a pure substance, while melting over a range indicates that impurities are present.

Five key separation techniques apply to different types of mixtures.

Simple distillation

• Separates a solvent from a solution by boiling off the solvent and then condensing the vapour back into a liquid

Fractional distillation

• Separates two or more miscible liquids with different boiling points using a fractionating column

Filtration

• Separates an insoluble solid from a liquid using filter paper, leaving the solid as residue while the liquid passes through as filtrate

Crystallisation

• Recovers a dissolved solid (solute) from a solution by evaporating some of the solvent and then allowing slow cooling so that crystals form

Paper chromatography

• Separates the components of a mixture based on their different solubilities in a solvent
• The resulting chromatogram reveals how many components are present in the mixture

Rꜰ values, calculated by dividing the distance moved by the substance by the distance moved by the solvent front, are used to identify each component when compared with known values obtained under the same conditions.

Rꜰ = distance moved by the substance ÷ distance moved by the solvent front