We often describe substances as being either 'soluble' or 'insoluble', but this is a simplification. No substance is totally insoluble – there will always be some of it in solution. We refer to such compounds as being sparingly soluble. Lead(II) iodide, PbI₂, is an example of a sparingly soluble solid.

If solutions of lead(II) nitrate and potassium iodide are mixed, a precipitate of lead(II) iodide is formed:

Pb²⁺(aq) + 2I^–(aq) _↓ PbI₂(s)

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Lead(II) iodide can be produced that contains radioactive ¹³¹I^– ions. In an experiment, radioactive lead(II) iodide is added to water and the radioactivity of the solution measured over time. The results of the experiment are shown in the following graph.

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What does the initial part of the graph indicate about the location of radioactive iodide ions when the solid lead iodide is added to water?

Some radioactive iodide ions move from the solid into solution.

What further information can you get from the graph?

After a time the concentration of the radioactive iodide ions in solution becomes constant.

In a saturated solution of a sparingly soluble solid such as lead(II) iodide, a dynamic equilibrium exists between ions in the solid lattice and ions in solution:

PbI₂(aq) _∏ Pb²⁺(aq) + 2I^–(aq)

What is meant by a 'saturated solution'?

A saturated solution will not dissolve any more solid at that temperature.

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What can you say about the rate of movement of ions from solid to solution and from solution to solid in a saturated solution?

The rate of movement of ions is the same in both directions.