The equilibrium expression that governs this reaction is called the solubility product, or K sp . K sp is a constant value for any substance, and if known, can be used to calculate the solubility of that substance in water. Calculating the solubility of calcium carbonate for a system open tot he air is more difficult than the simple example we just did. We will deal with this in a future lecture. Since most natural waters contain at least a few dissolved salts, we should try to understand what causes this error.
Salts modify the properties of water by reducing an ion's activity in solution.
This reduced activity results in greater than expected solubility. Activity is related to ionic strength, and these effects can by quantified if an ion's activity coefficient is known. Activity coefficients for some important ions are shown in the following table.