Calcium hydroxide

Ca 1
O 2
H 2

Formula unit Lattice

Choose Lattice to see eight unit cells. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

5 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; calcium hydroxide has no molecular shape.

IUPAC name: calcium dihydroxide
Molar mass: 74.0920(64) g/mol
Atoms: 5
SMILES: [OH-].[OH-].[Ca+2]
InChIKey: AXCZMVOFGPJBDE-UHFFFAOYSA-L
PubChem CID: 6093208

Made of

Ca O H

At 20 °C a litre of saturated solution holds only 1730 mg of dissolved calcium hydroxide, according to the Hazardous Substances Data Bank (HSDB). The low figure follows a trend in the periodic table: on Chem1’s account, metal hydroxides become less soluble moving from Group 1 into Group 2. The saturated solution has a name of its own, limewater. Its pH is 12.4 at 25 °C, on HSDB’s figure.

The compound is also called slaked lime, hydrated lime, caustic lime and calcium hydrate, and its systematic name is calcium dihydroxide. Each calcium ion, Ca²⁺, is balanced by two hydroxide ions, OH⁻. Inside every hydroxide ion, oxygen and hydrogen share a single bond, and the formula unit has a molar mass of 74.09 g/mol. The solid itself forms colourless crystals or a white powder.

Structure and bonding

In the Formula unit drawing, one calcium ion with a charge of +2 sits apart from two hydroxide ions of −1 each, three separate pieces in all. The only lines are two single O–H bonds, one inside each hydroxide ion, and no line joins calcium to oxygen. Together the three ions make one formula unit of an ionic lattice, not a molecule.

In the Lattice view, eight unit cells, each the smallest repeating block of the crystal, hold 1 calcium ion and 2 hydroxide ions apiece, the one-to-two ratio of the formula. Calcium ions appear as spheres and each hydroxide as a small ball-and-stick unit. Every calcium ion has six neighbours, and the ionic attraction is spread across all of them rather than drawn as lines. Turn, Rotate left and Rotate right all swing the block round an upright axis, so each side faces the reader in turn.

The crystals are soft and hexagonal, in HSDB’s description. Calcium hydroxide also crystallises inside setting cement, where its crystals are called portlandite. In cement paste that hardened under normal gravity those hexagonal crystals are plate-like; in paste that hardened in microgravity, in a NASA experiment, they are prismatic.

Where it turns up

Builders’ mortar, plaster, cement and whitewash all take calcium hydroxide, as do other binding and paving materials, according to HSDB. It also treats acidic soils. In food it adjusts pH, acts as a firming agent and serves as a processing aid. The US Food and Drug Administration (FDA) affirmed it as generally recognised as safe in 1984.

Under the name pickling lime, it keeps pickles crisp: vegetables treated with it before pickling firm up as its calcium binds the pectin in their cell walls. Maize boiled and soaked in lime water becomes nixtamal, a name built from the Nahuatl words for ashes and dough. It is then dried as hominy or ground into masa for tortillas. A technical report for the US Department of Agriculture’s organic programme describes both uses.

On HSDB’s list it preserves eggs and helps poultry form shells, and it goes into cattle feed and veterinary medicine. Many dental preparations contain it, root-canal fillings especially.

Why it matters

Hydrated lime is made in a hydrator, where a controlled amount of water meets crushed quicklime and the reaction releases a great deal of heat. Because only water needs adding to calcium oxide, many plants that make the oxide make the hydroxide as well, the US Environmental Protection Agency (EPA) notes. US output of calcium hydroxide for commercial sale came to about 2,690 million kilograms in 2018, while 84 per cent of lime is produced for use as calcium oxide.

At water treatment plants calcium hydroxide raises the pH, and calcium and magnesium then settle out as a solid sludge. Lime softening of this kind serves small and large supply systems alike, and the sludge also traps 80 to 90 per cent of the water’s radium. Industry turns the hydroxide into calcium hypochlorite and bleaching powder, HSDB records, and into lime-sulfur, a spray that fights fungi.

Properties

Properties of calcium hydroxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6093208.
Property	Value	Unit	Source
Formula (Hill order)	CaH₂O₂
Molar mass	74.0920(64) g/mol	g/mol	IUPAC/CIAAW 2024
Calcium atoms	1
Oxygen atoms	2
Hydrogen atoms	2
PubChem molecular weight	74.09 g/mol	g/mol	PubChem

Safety

Calcium hydroxide is corrosive to eyes and skin, and it irritates the respiratory tract. Company reports to the European Chemicals Agency (ECHA) give it the signal word Danger, and 76.9% of those reports list serious eye damage. It can burn and blister skin, and repeated or prolonged contact can cause dermatitis. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide entry, from the US federal institute for workplace safety, carry the full hazard record.

Questions

What is the difference between hydrated lime and slaked lime?

Both are calcium hydroxide; the two names describe the form it is in. Hydrated lime is the dry powder, while slaked lime is the hydroxide as particles suspended in water, in the Hazardous Substances Data Bank's usage. Slaked lime in that sense takes in milk of lime and lime putty, a viscous dispersion of the solid in water. Under US food regulations, slaked lime and calcium hydrate are other names for the same compound.

Why does lime mortar harden?

Mixed with sand, slaked lime sets as it absorbs carbon dioxide from the air, on the account of Los Alamos National Laboratory. The product is calcium carbonate: calcium hydroxide picks up the gas readily from air and turns into it, according to the Hazardous Substances Data Bank. So the lime that binds the sand ends up as carbonate rather than hydroxide.

What happens when calcium hydroxide is heated?

Strong heat drives the water out of it. Calcium hydroxide loses water when ignited and leaves calcium oxide behind, according to the Hazardous Substances Data Bank, and it decomposes at 580 °C on the figure of the International Chemical Safety Card. It does not burn either, being classed on the same card as not combustible.