Lime (quicklime), CaO, is obtained from the high-temperature decomposition of limestone (CaCO₃). Quicklime is the cheapest source of basic substances, but it is water insoluble. It does react with water, however, producing Ca(OH)₂ (slaked lime). Unfortunately, Ca(OH)₂(s) has limited solubility in water.

When Ca(OH)₂(s) reacts with a soluble carbonate, such as Na₂CO₃(aq), however, a greater amount of Ca(OH)₂(s) dissolves and the solution produced has a much higher pH. Equilibrium is displaced to the right in reaction (18.10) because CaCO₃ is much less soluble than Ca(OH)₂.

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Ca(OH)2(s) Ca²+ (aq) + 2 OH¯(aq) Ksp = 5.5 × 10-6