Some students find it difficult to relate what they see at the macroscopic level with what is happening at the sub-microscopic level in a saturated solution of a sparingly soluble salt because nothing appears to change. One way of helping them to see the link is to ask questions like the ones below, which are designed to test their understanding.

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• Is anything changing?
• How could we get more solid to dissolve without opening the flask?
• What expression represents the relationship between the ionic salt and the ions in solution?

Look at the following conversation between two students about dissolving sparingly soluble solids.

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What misconception does student B have about sodium chloride and solubility products?

The student hasn't realised that it is not appropriate to apply the solubility concept to very soluble salts such as sodium chloride.

In what way has student B not quite understood how to calculate the value of a solubility product?

The method that student B suggests works when the salt contains cations and anions of equal charge but doesn't work in other cases. Calculation of K_sp depends on the formula of the salt. For a salt such as PbCl₂, for example, K_sp is calculated by multiplying the concentration of lead ions by the square of the concentration of the chloride ions in the saturated solution.

In what way is student B incorrect in his account of the common ion effect?

A common ion in solution reduces the solubility of a salt but it doesn't prevent it dissolving altogether.