Elements, Compounds and Mixtures

Elements

Elements are substances made up from only one type of atom.
There are over 100 different elements, which are organised in the Periodic Table.
Each element has a unique chemical symbol, for instance, Oxygen is represented as O and Hydrogen as H.

Compounds

Compounds are substances in which atoms of two or more elements are chemically combined together.
The properties of a compound are usually different from the properties of the elements it is made from. For instance, hydrogen is a gas and oxygen is a gas, but when these two are combined they form water, which is a liquid.
Compounds have a fixed ratio of atoms, for example, water (H2O) always contains two hydrogen atoms for every oxygen atom.
Chemical bonds hold the atoms together in a compound.

Mixtures

Mixtures are composed of two or more elements or compounds that are physically mixed together but not chemically combined.
The components of a mixture can be separated by physical methods (e.g., filtration, evaporation, chromatography).
Mixtures can vary in the ratio of their components.

Separating Mixtures

Filtration is used to separate an insoluble substance from a liquid.
Crystallisation or evaporation can be used to separate a soluble substance from a solution.
Distillation can separate a liquid from a solution. This is based on the principle that different liquids boil at different temperatures.
Chromatography separates substances based on their solubility in a solvent and their attraction to the paper.

Pure Substances and Formulations

A pure substance has a sharp melting point or boiling point, whilst a mixture melts or boils over a range of temperatures.
A formulation is a complex mixture designed to have a useful product. Examples include fuels, cleaning agents, paint, medicine, alloys, and fertilisers.
Each component in a formulation contributes to the properties of the final product, so it is important to control the quantities used.