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. Annually, approximately 125 million tons of calcium hydroxide are produced worldwide.[8] Calcium hydroxide 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. Solubility Calcium hydroxide is modestly soluble in water, as seen for many dihydroxides. Its solubility increases from 0.66 g/L at 100 °C to 1.89 g/L at 0 °C.[8] Its solubility product Ksp of 5.02×10−6 at 25 °C, [1] its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction: Ca(OH)2 → Ca2+ + 2 OH− The solubility is affected by the common-ion effect. Its solubility drastically decreases upon addition of hydroxide or calcium sources. Reactions When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa (normal atmospheric pressure), which decomposes calcium hydroxide into calcium oxide and water:[9] Ca(OH)2 → CaO + H2O When carbon dioxide is passed through limewater, the solution takes on a milky appearance due to precipitation of insoluble calcium carbonate: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) If excess CO2 is added: the following reaction takes place: CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq) The milkiness disappears since calcium bicarbonate is water-soluble. Calcium hydroxide reacts with aluminium. This reaction is the basis of aerated concrete.[8] It does not corrode iron and steel, owing to passivation of their surface. Calcium hydroxide reacts with hydrochloric acid to give calcium hydroxychloride and then calcium chloride. In a process called sulfation, sulphur dioxide reacts with limewater: Ca(OH)2(aq) + SO2(g) → CaSO3(s) + H2O(l) Limewater is used in a process known as lime softening to reduce water hardness. It is also used as a neutralizing agent in municipal waste water treatment.