Formula reading
Ca(OH)₂ contains two oxygens and two hydrogens.
The OH group is a polyatomic ion that must be counted as a unit.
Ionic solid · strong base
Calcium hydroxide
Ca(OH)₂
Calcium hydroxide is Ca(OH)₂, an ionic solid composed of Ca²⁺ and OH⁻ ions. Two hydroxide ions are required to balance each calcium ion.
Material distinction: limewater is an aqueous solution of calcium hydroxide; slaked lime is solid calcium hydroxide material. Neither term changes the underlying compound identity.
Essentials
Ca(OH)₂ is one Ca²⁺ plus two OH⁻ ions.
The parentheses matter: the subscript 2 applies to the entire hydroxide ion, not only to oxygen.
Detail
Calcium hydroxide is only moderately soluble, but the dissolved portion dissociates into ions.
A saturated solution contains solvated Ca²⁺ and OH⁻ in equilibrium with undissolved solid. Strong-base behavior refers to the hydroxide ions present in solution, not to unlimited solubility.
Strong base ≠ highly soluble
A compound can produce strongly basic dissolved ions without dissolving without limit.
Solubility controls how much enters solution; acid–base strength describes the behavior of the dissolved species.
Understand
Hydroxide is the reactive base in neutralization.
In aqueous acid–base chemistry, spectator ions can be omitted so the central change is H₃O⁺ reacting with OH⁻ to make water.
Carbon dioxide link
Calcium hydroxide can also react with CO₂ to form calcium carbonate.
This links the calcium hydroxide and calcium carbonate pages through a familiar precipitation/carbonation process.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | Ca(OH)₂ |
|---|---|
| CAS Registry Number | 1305-62-0 |
| Common material name | slaked lime |
Advanced
Push past the first model without losing the simple picture.
Is Ca(OH)₂ a molecule?
No. The solid is ionic; Ca(OH)₂ is a formula unit.
Why can a sparingly soluble solid still make a basic solution?
The fraction that dissolves supplies OH⁻ ions, and those ions strongly affect pH.
Is limewater pure liquid calcium hydroxide?
No. Limewater is an aqueous solution containing dissolved calcium hydroxide species.
Constituent elements