8.1 Purity, formulations, and chromatography
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What is a pure substance?
A pure substance is a single element or compound, not mixed with any other substance.
What is a formulation and how is it made?
A formulation is a mixture of compounds in measured quantities that has been designed as a useful product.
Give examples of formulations.
● Fuels ● Cleaning agents ● Paints ● Medicines ● Alloys ● Fertilisers ● Foods
Describe paper chromatography
• A start line is drawn near the bottom of the paper. The mixture is spotted on the line. • A beaker is filled with a small amount of solvent (it cannot touch or go above the start line when paper is placed in a beaker) • Paper is hung on a rod and placed in a beaker. • Solvent travels up the paper, thus separating the components. • Before solvent level reaches the end, the paper is taken out and the finish line is marked. The paper is dried. • The procedure works when different compounds have different affinities for the solvent/paper. Stronger attraction for the paper - travels slowly with the solvent etc. • Paper is called the stationary phase - it doesn’t move. Solvent is the mobile phase.
How is Rf calculated?
Distance moved by the spot / distance moved by solvent
In a paper chromatography experiment, a compound A was found to have an Rf value of 0.85 - what does it tell you about the compound?
It has a higher affinity for the solvent than for the paper.