purity

In chemistry, the extent to which a substance consists only one type of material. Pure substances may be simple elements made up of atoms such as carbon or silver. They may be molecules, like oxygen or hydrogen, or they may be compounds like water. Compounds are pure substances because each particle is the same type.

Pure substances have individual properties and characteristics. These properties can be both physical, such as boiling point and melting point, or chemical such as the way they react with other substances. Mixtures contain more than one type of substance and so have the properties different from those of each component. An example would be the air in the atmosphere, which is a mixture of many gases including nitrogen, oxygen, carbon dioxide, and the noble gases (rare gases). The process of purification separates the components of a mixture, and removes impurities from a substance.

Throughout history people have tried to purify substances ranging from metals in metal ores to medicines from plants. Nowadays there are two main reasons to purify mixtures: to obtain one or more of the components pure from a manufacturing process; or to analyse a mixture to check what is in it, such as food analysis or the mapping of the human genome (involving the separation of the individual sections of DNA, the chemical of life).

Testing for purity There are now many ways to tell whether or not a substance is pure. For solids the simplest test of purity is its melting point. A small sample is carefully heated until the solid just melts. All of the solid will melt at one temperature, known as the melting point. Pure solids have a sharp melting point whereas impure solids do not. An impure solid melts below the melting point of a pure solid. This idea is used when the roads are salted in the wintertime. Pure ice melts at 0°C/32°F, but if salt is added, an impurity, the ice will melt at a lower temperature, below 0°C/32°F. This is known as depression of the freezing point.

A purity test for liquids is the boiling point. Pure water boils at 100°C/212°F at normal atmospheric pressure, but if an impurity such as salt is added, the boiling point is higher than this. This is known as elevation of the boiling point.