Calcium hydroxide is a chemical compound with the chemical formula Ca(OH)₂. It is a colorless crystal or white powder, is created when calcium oxide (called lime or quicklime, but unrelated to the citrus fruit (lime)) is slaked with water. A traditional name for calcium hydroxide is slaked lime. It is also called hydrated lime.

If heated, calcium hydroxide decomposes into calcium oxide and water. It is soluble in water and the solution, called lime water, is a medium strong base reacting violently with acids and attacking many metals in presence of water.

Calcium hydroxide is mainly used as a form of lime, in water and sewage treatment and improvement of acid soils. Its strong basic properties also give it varied uses, such as creating milk of lime and in the tanning of leather.

It is also used as whitewash, mortar, and plaster.

An overdose of calcium hydroxide includes many dangerous symptoms, including difficulty breathing, gastrointestinal bleeding, hypotension, and a severe change in blood pH, which is damaging to the internal organs.

ja:水酸化カルシウム de:Kalziumhydroxid fr:Chaux (chimie)