Chemical analysis

In chemistry we often talk about the purity of a substance, and use this as a measure of just what is in something. Forgot how you might use the word "pure" in everyday life, and concentrate on the way we use it in chemistry.

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

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.

This is one way that we can use to  determine if water is pure. We know that pure water boils at 100oC, so if a sample of water that is tested boils at 95oC then we know that it isn't pure.


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.



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.

The ratio of the distance moved by a compound (centre of spot from 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.

The key skills that you will have to demonstrate are; your ability to analyse results of chromatography and calculate Rf values.

Here is a typical exam question:

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