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Once a qualitative understanding of partition equilibria has been established, students can move on to a more quantitative approach with the introduction of partition coefficients.

Students can be given an opportuntity to 'discover' for themselves the constancy in the ratio of the concentration of a solute in two immiscible liquids by providing appropriate data. Look at the data below:

What can you say about the ratio of the concentrations of iodine in tetrachloromethane and water at this temperature?

The ratio of the concentrations is approximately constant.

The partition coefficient is approximately 87. The slight differences between sets of data arise from differences in experimental data and in rounding up of numbers.

Students need to make a link between a qualitative view of solubility in terms of polarity of solvent and the values given by a partition coefficient. You can help them achieve this by asking them to work in groups to predict the approximate value of the partition coefficient of solutes between pairs of liquids. They can then be given the data book values to check their predictions. The following examples can be used for this kind of activity:

What can you say about two solvents for which the partition coefficient for a solute is approximately 1?

The nearer the partition coefficient is to 1 the more similar is the solubility of the solute in each solvent.

This might be a good time to contextualise the topic by exploring applications of partition equilibria such as solvent extraction, pesticides and drug absorption.