An atom is the smallest part of
an element that can exist. Chemical reactions always involve the formation of one or more
new substances, and often involve a detectable energy change.
Compounds contain two or more elements chemically combined in
fixed proportions and can only be separated into elements by chemical reactions.
A mixture consists of two or more elements or compounds not
chemically combined together. The chemical properties of each
substance in the mixture are unchanged.
Mixtures can be separated by physical processes such as filtration,
crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical
reactions and no new substances are made.