Purity, formulations and chromatography

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state — pure milk, for example. Milk is a mixture of many compounds, so it is not pure in the chemical sense at all. Questions in this topic often turn on that difference.

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

You should be able to use melting point and boiling point data to distinguish pure from impure substances. Two things to check every time: whether the value is sharp or a range, and whether it matches the data book value for the pure substance.

A formulation is a mixture that has been designed as a useful product. Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.

Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

You should be able to identify formulations given appropriate information. The test is not whether something is a mixture — it is whether the mixture was designed, with the quantities chosen to give particular properties. A label listing several components with precise percentages is the usual clue.

So paint is a formulation: the pigment gives the colour, the binder makes it stick, and the solvent lets it be spread, each in a measured quantity. Crude oil is not a formulation, even though it is a complex mixture, because nobody chose what went into it. You do not need to know the names of components in proprietary products.

Chromatography can be used to separate mixtures, and can give information to help identify substances.

Chromatography involves a stationary phase and a mobile phase. Separation depends on the distribution of substances between the phases. In paper chromatography the paper is the stationary phase and the solvent is the mobile phase.

You should be able to explain how paper chromatography separates mixtures. The explanation: the solvent moves up the paper, carrying the substances with it. A substance that is more attracted to the solvent than to the paper spends more time in the mobile phase and travels further; one that is more attracted to the paper spends more time in the stationary phase and hardly moves. Because different substances have different distributions between the two phases, they end up at different heights.

The ratio of the distance moved by a compound — the centre of the spot from the origin — to the distance moved by the solvent can be expressed as its Rf value:

Rf = distance moved by substance ÷ distance moved by solvent

Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.

The compounds in a mixture may separate into different spots depending on the solvent, but a pure compound will produce a single spot in all solvents. That last sentence is how chromatography is used to suggest whether a substance is pure — and why "one spot in this solvent" is weaker evidence than "one spot in every solvent tried".

You should be able to interpret chromatograms and determine Rf values from chromatograms, and provide answers to an appropriate number of significant figures. You should also be able to suggest how chromatographic methods can be used for distinguishing pure substances from impure substances.