A good way of introducing sparingly soluble salts is to demonstrate the formation of lead iodide by mixing potassium iodide and lead nitrate(V) solutions together. An animation can help focus student's thinking about what is happening on a sub-microscopic scale and thus make the link betwen the macro and micro scales.

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The Phet animation, salts and solubility, is particularly useful in helping students gain a qualitative understanding of the solubility of a sparingly soluble solid. Download the animation and use it as described below.

Choose the 'Slightly soluble salts' tab and select the salt silver bromide. Shake the shaker to add the solid salt to the water with a total of about 60 positive and negative ions present.

What features of solubility does the animation illustrate up to this point?

The animation illustrates the following points:

• Below a certain concentraion, the salt is completely soluble in water.
• There is an equal number of cations and anions in solution.
• Eventually a saturated solution is produced and some excess solid remains undissolved.
• There is a dynamic equilibrium in which ions move from solid into solution and from solution to solid.

Press the 'reset all' button and change the solid salt to mercury(II) bromide. Shake the shaker a couple of times.

What do you notice about the number of mercury and bromide ions?

There are twice as many bromide ions as mercury ions. 

Shake the shaker a few more times.

How does the solubility of mercury(II) bromide compare with the solubility of silver bromide?

Mercury(II) bromide is much less soluble than silver bromide.