We can use the K_sp value to predict whether a precipitate will form when solutions containing appropriate ions are mixed. We will illustrate this by looking at what happens when equal volumes of solutions containing different concentrations of Ca²⁺  and CO₃^2– ions are mixed.

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What assumption are we making about the other ions in solution besides Ca²⁺ and CO₃^2–?

We're assuming that the other ions do not form an insoluble compound. 

Why are the concentrations of the ions halved when the solutions are mixed?

The concentrations of the ions are halved because each solution is diluted by the equal volume of the other solution.

How could you calculate the maximum concentration of calcium and carbonate ions that can exist in solution without causing a precipitate?

The maximum concentration of ions is when the ionic product equals K_sp. To calculate this concentration we need to find the square root of K_sp.

How would you describe such a solution?

When the ionic product of ions is equal to K_sp the solution is saturated.

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Coral reefs are made up of calcium carbonate. What can you say about the concentrations of calcium and carbonate ions in sea water that are needed for coral to grow?

The ionic product of [Ca²⁺] and [CO₃^2–] around the coral must be greater than K_sp(CaCO₃).