Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed with water. It has many names including hydrated lime, caustic lime, builders' lime, slaked lime, cal, and pickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as E number E526. Limewater, also called milk of lime, is the common name for a saturated solution of calcium hydroxide.
Properties
Calcium hydroxide is poorly soluble in water, with a retrograde solubility increasing from 0.66 g/L at 100 °C to 1.89 g/L at 0 °C. With a solubility product Ksp of 5.02 at 25 °C, its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction: Ca(OH)2 → Ca2+ + 2 OH−
At ambient temperature, calcium hydroxide (portlandite) dissolves in water to produce an alkaline solution with a pH of about 12.5. Calcium hydroxide solutions can cause chemical burns. At high pH values due to a common-ion effect with the hydroxide anion, its solubility drastically decreases. This behavior is relevant to cement pastes. Aqueous solutions of calcium hydroxide are called limewater and are medium-strength bases, which react with acids and can attack some metals such as aluminium (amphoteric hydroxide dissolving at high pH), while protecting other metals, such as iron and steel, from corrosion by passivation of their surface. Limewater turns milky in the presence of carbon dioxide due to the formation of insoluble calcium carbonate, a process called carbonatation: Ca(OH)2 + CO2 → CaCO3 + H2O
When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101kPa (normal atmospheric pressure), which decomposes calcium hydroxide into calcium oxide and water: Ca(OH)2 → CaO + H2O
Calcium hydroxide reacts with hydrogen chloride to first give calcium hydroxychloride and then calcium chloride.
Structure, preparation, occurrence
Calcium hydroxide adopts a polymeric structure, as do all metal hydroxides. The structure is identical to that of Mg(OH) (brucite structure); i.e., the cadmium iodide motif. Strong hydrogen bonds exist between the layers.
Calcium hydroxide is produced commercially by treating (slaking) lime with water: CaO + H2O → Ca(OH)2
In the laboratory it can be prepared by mixing aqueous solutions of calcium chloride and sodium hydroxide. The mineral form, portlandite, is relatively rare but can be found in some volcanic, plutonic, and metamorphic rocks. It has also been known to arise in burning coal dumps.
The positively charged ionized species CaOH+ has been detected in the atmosphere of S-type stars.
Retrograde solubility
According to Hopkins and Wulff (1965), the decrease of calcium hydroxide solubility with temperature was known since the works of Marcellin Berthelot (1875) and Julius Thomsen (1883) (see Thomsen–Berthelot principle), when the presence of ions in aqueous solutions was still questioned. Since, it has been studied in detail by many authors, a.o., Miller and Witt (1929) or Johnston and Grove (1931) and refined many times (e.g., Greenberg and Copeland (1960); Hopkins and Wulff (1965); Seewald and Seyfried (1991); Duchesne and Reardon (1995)).
The reason for this rather uncommon behavior is that the dissolution of calcium hydroxide in water involves an entropy decrease, due to the ordering of water molecules around the doubly charged calcium ion, This counter-intuitive temperature dependence of the solubility is referred to as "retrograde" or "inverse" solubility. The variably hydrated phases of calcium sulfate (gypsum, bassanite and anhydrite) also exhibit a slight retrograde solubility due to the presence of the calcium ion. However, other calcium salts like calcium chloride show an increase in solubility since the enthalpy change is larger and dominates the free energy change during dissolution. In all cases, the dissolution is exothermic.
Uses
Calcium hydroxide is commonly used to prepare lime mortar.
One significant application of calcium hydroxide is as a flocculant, in water and sewage treatment. It forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and low toxicity of calcium hydroxide. It is also used in fresh-water treatment for raising the pH of the water so that pipes will not corrode where the base water is acidic, because it is self-regulating and does not raise the pH too much.