Retrograde solubility Īccording 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. The positively charged ionized species CaOH + has been detected in the atmosphere of S-type stars. It has also been known to arise in burning coal dumps. The mineral form, portlandite, is relatively rare but can be found in some volcanic, plutonic, and metamorphic rocks. In the laboratory it can be prepared by mixing aqueous solutions of calcium chloride and sodium hydroxide. Ĭalcium hydroxide is produced commercially by treating (slaking) lime with water: Strong hydrogen bonds exist between the layers. The structure is identical to that of Mg(OH) 2 ( brucite structure) i.e., the cadmium iodide motif. SEM image of fractured hardened cement paste, showing plates of calcium hydroxide and needles of ettringite (micron scale)Ĭalcium hydroxide adopts a polymeric structure, as do all metal hydroxides. When heated to 512 ☌, 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: Ca(OH) 2 → CaO + H 2OĬalcium hydroxide reacts with hydrogen chloride to first give calcium hydroxychloride and then calcium chloride. Limewater turns milky in the presence of carbon dioxide due to the formation of insoluble calcium carbonate, a process called carbonatation: 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. This behavior is relevant to cement pastes. At high pH values due to a common-ion effect with the hydroxide anion, its solubility drastically decreases. Calcium hydroxide solutions can cause chemical burns. With a solubility product K sp of 5.02 ×10 −6 at 25 ☌, its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction:Īt ambient temperature, calcium hydroxide ( portlandite) dissolves in water to produce an alkaline solution with a pH of about 12.5. Limewater, also called milk of lime, is the common name for a saturated solution of calcium hydroxide.Ĭalcium hydroxide is poorly soluble in water, with a retrograde solubility increasing from 0.66 g/L at 100 ☌ to 1.89 g/L at 0 ☌.
Calcium hydroxide is used in many applications, including food preparation, where it has been identified as E number E526. It has many names including hydrated lime, caustic lime, builders' lime, slaked lime, cal, and pickling lime. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed with water. Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH) 2.
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