8.1 Purity, formulations and chromatography

8.1.1 Pure Substances

- In chemistry, a pure substance is a single element or compound not mixed with any other substance.
- In science we would not refer to a substance such as milk as pure because it is a mixture of a number of different substances. This is different from everyday language, where a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.

Identifying Pure/Impure Substances

- Pure substances have specific melting and boiling points. These can be used to distinguish pure substances from impure substances.
Pure and impure substances

Substance Melting point (°C)
A 42
B 104
C 76-82
D 35


8.1.2 Formulations

- 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.


8.1.3 Chromatography

- Chromatography can be used to separate mixtures and can give information to help identify substances.
- It involves a stationary phase and a mobile phase.

- The ratio of the distance moved by a compound (centre of spot from origin) can be expressed as its Rf value (retention factor).
Chromatography Rf Equation - If a pigment does not travel very fast, it will not travel far up the paper and have a lower Rf value.
- The pigment could have a strong attraction to the stationary phase (with strong intermolecular forces) and/or a weak attraction to the mobile phase.
- If a pigment does travel quite fast, it will travel further up the paper and have a higher Rf value.
- The pigment could have a weak attraction to the stationary phase (with weak intermolecular forces) and/or a strong attraction to the mobile phase.
- This means that different compounds have different Rf values in different solvents, which can be used to help identify the compounds.

Chromatography Experiment

Chromatography Experiment - The black dot is a mixture, because it separates out into multiple colours.
- Yellow is likely to have the strongest attraction to the stationary phase, as it travelled the least.
- Blue is likely to have the strongest attraction to the mobile phase, as it travelled the most.

Rf Calculation

- The solvent front is 10.5cm from the starting line.
- Yellow is 2cm from the starting line.
- Purple is 6.5cm from the starting line.
- Blue is 9cm from the starting line.
- Using the Rf equation: