AQA GCSE CHEMISTRY - CHEMICAL ANALYSIS

What is a pure substance?

A single element or compound that is not mixed with any other substance.

How can pure substances be identified?

By measuring their melting and boiling points and comparing to known values.

What is a formulation?

A mixture that has been designed as a useful product.

Give seven examples of formulations

Fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods.

What is the purpose of chromatography?

To separate mixtures and help identify the substances in them.

What is meant by the stationary phase?

The part of chromatography that does not move (e.g. paper).

What is meant by the mobile phase?

The part of chromatography that moves through the stationary phase (e.g. solvent).

What property is used to separate substances in chromatography?

The different solubilities of substances in the solvent and how strongly they stick to the paper.

What is an Rf value?

The ratio of the distance moved by a substance to the distance moved by the solvent.

How is an Rf value calculated?

Rf = distance moved by substance ÷ distance moved by solvent.

Do the Rf values of a compound change in different solvents?

Yes.

What will the chromatogram of a pure substance look like?

It will show only one spot.

Describe how paper chromatography can be used to separate mixtures

Draw a pencil line near the bottom of the paper, place spots of mixture on it, dip into solvent (without submerging the spots), and let the solvent rise to separate the substances.

Describe how you could analyse a chromatogram

Compare the Rf values of the spots with known values to identify substances.

Describe the test for hydrogen

Insert a lit splint; a 'squeaky pop' sound confirms hydrogen.