Pure Substance
Pure means that only one substance is present in the material and can be a pure element or compound. A simple physical test for purity, and properties that can help identify a substance, is to measure the boiling point or melting point. Every pure substance melts and boils at a fixed temperature.
If a liquid is pure, it should boil at a constant temperature called the boiling point e.g. water boils at 100oC. An impure liquid will boil at a higher temperature if it contains a dissolved solid impurity e.g. seawater, containing dissolved salts, boils at over 100oC. The boiling then takes place over a range of temperatures. If a solid is pure, it melts sharply at its fixed melting point.
An impure solid melts below its expected melting point and the more impure, the wider the temperature melting range, e.g. a water and salt mixture melts below 0oC.
Impure usually means a mixture of mainly one substance plus one or more other substances physically mixed in.
The purity of a compound is important, particularly in drug manufacture.
Any impurities present may be harmful substances.
Mixtures
A mixture is a substance made up of at least two substances which may be elements or compounds.
They are usually easily separated by physical means e.g. filtration, distillation, chromatography etc. Separation methods are needed to purify materials and separate useful materials.
Pure substances are rare. Most of the things we handle or interact with are impure. Think of the air we breathe in it is not pure. It is a mixture of gases. The other gases in air are useful one way or the other.