Purity, Separating Mixtures and Chromatography

Pure Substances in Chemistry

In everyday language, “pure” often means natural or uncontaminated. In chemistry, the meaning is much more specific: a pure substance contains only a single element or a single compound, with nothing else mixed in. Pure water contains only \(H_2O\) molecules. Orange juice labelled “pure” in a supermarket is not pure in the chemical sense, because it is a mixture of water, sugars, citric acid and other compounds. This distinction between the everyday and scientific meanings of “pure” is regularly tested.

You can test purity by measuring melting point or boiling point. A pure substance melts and boils at a single sharp temperature. An impure substance (a mixture) melts over a range of temperatures, and its measured melting point will be lower than the expected value for the pure substance. Similarly, mixtures boil over a range of temperatures rather than at a single point.

Formulations

Many useful products are formulations — carefully designed mixtures where each component is present in a precise amount. Formulations include alloys (e.g. steel: iron with a small proportion of carbon and sometimes other metals), paints (pigment, binder and solvent in defined ratios), medicines (active ingredient combined with fillers and stabilisers), fuels (blends of hydrocarbons), and foods such as sauces and dressings. Each component contributes specific properties to the final product, and changing the proportions changes the product’s behaviour.

Filtration and Crystallisation (PAG C3)

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The liquid (the filtrate) passes through the tiny pores in the paper, while the insoluble solid (the residue) is trapped. For example, you could separate sand from salt water using filtration — the sand stays on the paper and the salt water collects in the flask below. This is a key technique in practical activity group PAG C3.

Distillation

Simple distillation separates a solvent from a solution when the solvent alone is wanted as the pure product — for example, collecting pure water from salt water. The solution is heated until the solvent evaporates, the vapour passes through a condenser where it is cooled, and it collects as liquid in a separate container. The dissolved solid remains in the flask.

Fractional distillation is used when the mixture contains two or more liquids with different boiling points. The mixture is heated in a flask fitted with a fractionating column. Liquids with lower boiling points vaporise first, rise to the top of the column, and are collected separately. As the temperature is increased, fractions with progressively higher boiling points are collected. A familiar application is the separation of crude oil into useful fractions (petrol, kerosene, diesel, etc.) in a refinery.

Chromatography (PAG C3)

Chromatography separates a mixture of dissolved substances. It works because different substances travel at different speeds through a stationary phase (such as chromatography paper) when carried by a mobile phase (a solvent). In paper chromatography (PAG C3):

1. Draw a pencil baseline near the bottom of the chromatography paper (pencil is used because ink would dissolve and move up with the solvent).
2. Place a small spot of the mixture on the baseline.
3. Place the paper in a beaker of solvent, making sure the baseline is above the solvent level so the spots do not dissolve directly.
4. Allow the solvent to rise up the paper, carrying the different substances with it.
5. Mark the solvent front before it evaporates.

Substances that are more attracted to the mobile phase (or less attracted to the stationary phase) travel further up the paper. A pure substance produces a single spot; a mixture produces several spots at different heights. The chromatogram can be used to identify substances by comparing spot positions to those of known reference substances run alongside.