Before beginning to consider partion equilibria students need a sound understanding of the basic ideas on which the concept is based. Simple introductory experiments on what kind of solids dissolve best in different solvents is a good place to start. This can be followed by asking students to add different liquids together to bring out the ideas of miscible and immiscible liquids. Students should be encouraged to talk about what is happening in terms of bond breaking and making the link between what they observe and what is happening on a molecular level.

The partition of iodine between water and an organic solvent such as cyclohexane is a good place to begin the study of partition equilibria. 

[INSERT PE_EU_03_ima]

How might you expect students to explain the difference in solubility of iodine in water and cyclohexane?

You might expect students to talk about bonds between molecules and to point out that there are weaker instantaneous dipole-induced dipole bonds between molecules of iodine and cyclohexane and stronger hydrogen bonds between molecules of water. 

It is usual to use potassium iodide solution rather than water because iodine is not very soluble in water. 

Why does iodine 'dissolve' in potassium iodide solution?

Iodine combines with iodide ions to form the I₃^– ion. The I₃^– ion is in equilibrium with iodine and iodide ions: I₂(aq) + I^–(aq) _∏ I₃^–(aq)
Athough you may wish to mention this equilibrium it's important that students focus on the simple partition of iodine between water and cyclohexane.

Iodine can be shown to dissolve separately in potassium iodide solution and in cyclohexane. The two solutions can then be added to each other. 

What important points about the partition of iodine does this approach illustrate?

It illustrates that:
 - iodine can move in both directions between liquids;
 - the same dynamic equilibrium is set up in both cases;
 - the equilibrium can be established starting with the iodine in either liquid.