Criteria of Purity

Understanding Purity

Purity refers to the absence of any impurities in a substance. A pure substance contains only one type of particle.
An impure substance is a mixture that contains two or more different types of particles.
In Chemistry, purity often refers to the proportion of a single chemical substance within a given sample.

Chemists can identify the purity of a substance using several methods, including melting point and boiling point tests, chromatography, and colorimetry.
Melting and boiling points are often used as they are characteristic properties and pure substances melt and boil at specific, sharp temperatures.

In a pure substance, all particles are identical and are held together by the same intermolecular forces. Therefore, they all require the same amount of energy to break these forces.
When a substance is impure, however, the melting and boiling points are often lower, and the range over which the substance melts or boils is larger.
The reason melting and boiling points are lower when a substance is impure is because the impurities disrupt the regular arrangement of the particles in a pure sample. This leads to a higher likelihood of particles breaking away from the whole and becoming gas or liquid.

Chromatography is a powerful method used to determine purity, and it works by separating the components of a mixture.
The different components in the mixture separate because they move at different rates through the stationary phase.
A pure substance will only produce one spot in chromatography, while an impure substance will migrate to produce several spots.

Colourimetry is a technique used to determine the concentration of coloured compounds in a solution, which can help in identifying impurities.
Spectrometry is a method that involves the interaction of light with matter. This technique can identify the presence of impurities by revealing absorption lines/signals which wouldn’t be present in a pure sample.