Pure Substances and Mixtures

1.1: Pure Substances and Mixture

Often we regard a pure substance as something which is clean and unpolluted and an impure substance as its opposite. However, in science, purity is more than just being clean and unpolluted. Polluted sea water is obviously impure, but to a scientist, so is clean fresh sea water. A pure substance contains only one type of substance. (i.e one type of atom or a molecule)
Pure water is a pure substance but sea water also contains dissolved salts which are referred to as impurity.


1.2 The Need for Pure Substances


Scientists need pure substances to study their properties. Food and medicines have to be tested for purity before they are sold as any impurity in them could be harmful or poisonous. In the electronics industry, microchips have to be made from pure silicon. Any impurity in the chips can greatly reduce their effectiveness.

A pure solid has a fixed and exact melting point. Presence of an impurity lowers the melting point and causes the substance to melt over a range of temperatures. An example of this is the addition of salt to ice. Pure ice melts at exactly 0 degrees celsius. When salt is added, its lowers the melting points to between -5 degree celsius to -25 degree celsius, depending upon the quantity of the salt added. That is why in cold countries, salt is spread on icy roads in winter to melt the ice. This also explains why it is very cold enough for the sea water to freeze.
With liquids, any impurity raises the boiling point. A pure liquid has a fixed and exact boiling point. For example, water boils at 100 degree celsius. However, when salt is added, it raises the boiling point. Mixtures do not have exact properties as their compositions can vary. Because of this, they do not have fixed melting and boiling points. For example, petrol is a mixture and has a boiling point within the range of 35 degrees celsius to 75 degrees celsius. Candle wax is also a mixture and has a melting point wtihin the range if 50degrees celsius to 60 degrees celsius.
Mixtures and formed by a physical change only. Because of this, they are easy to separate into pure substances. The process is called purification and can be achieved by simple physical methods like dissolving, filtering, evaporation, distillation and chromatography.