Some students find the common ion effect particularly difficult to grasp. This may be because they haven't sufficiently understood that the actual concentration of cations and anions can vary in a saturated solution as long as the ion product is equal to K_sp.

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One way of illustrating this idea is to use the analogy of a 'seesaw'. In this analogy the height of one end of the seesaw above the ground represents the concentration of cations and the height of the other end above the ground represents the concentration of the anions.

If we multiply the height above the ground of one end of the seesaw by the height above the ground on the other end, what does this value represent in the analogy?

The product of the heights above the ground represents the solubility product.

If we use this analogy to represent the situation when silver chloride is added to water until a saturated solution is formed the seesaw will look like this:

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If, however, silver chloride is added to a solution of silver nitrate the seesaw will look like this:

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How could the seesaw analogy be extended to represent a solid with a different value of K_sp?

The pivot of the seesaw will be at different heights above the ground for solids with different K_sp values.

What would the seesaw look like if used to represent a saturated solution of lead(II) chloride, PbCl₂, in water?

The [Cl^– (aq)] end of the seesaw will be twice as high as the [Pb²⁺(aq)] end.

How might the analogy in this example be used to illustrate how the value of K_sp can be calculated?

To find the value of K_sp, the concentration of [Pb²⁺(aq)] must be multiplied by the square of the concentration of [Cl^–(aq)].