Calcium Hydroxide

Last Updated: April 15, 2024

Calcium Hydroxide

Calcium hydroxide is also known as slaked lime and it is a compound with the chemical formula Ca(OH)₂. It is commonly found in both laboratories and everyday products such as mortar and plaster. In chemistry, it is categorized as a base because it reacts with acids to form salts and water. This white powdery substance is highly versatile, used in water purification, food preparation, and even in the dental field to maintain healthy teeth. Its ability to adjust acidity makes it invaluable in various industrial and environmental applications, showcasing the practical uses of bases in our daily lives.

What is Calcium Hydroxide?

Calcium hydroxide is also known as slaked lime and it is a chemical compound with the formula Ca(OH)₂. It is a white powdery substance that is created by mixing calcium oxide (lime) with water. This reaction produces heat and forms calcium hydroxide, which is slightly soluble in water and forms a basic solution known as lime water. Commonly used in construction for making mortar and plaster, it also has applications in the food industry for processing water and in agriculture to adjust soil pH levels.

Chemical Names and Formulas

Hill FormulaH₂CaO₂
NameCalcium hydroxide
IUPAC NameCalcium dihydroxide
Alternate NamesCalcium dihydroxide, Calcium hydrate, Hydrated lime, Slaked lime

Structure of Calcium Hydroxide


Calcium hydroxide is composed of one calcium atom bonded to two hydroxide groups. The chemical structure can be represented as Ca(OH)₂. In this structure, the calcium atom is in the center, linked to two hydroxide ions, each consisting of one oxygen and one hydrogen atom. These bonds form a layered crystalline structure that is slightly soluble in water. This arrangement allows calcium hydroxide to readily react with carbon dioxide in the air to form calcium carbonate, a process important in many environmental and industrial applications.

Preparation of Calcium Hydroxide

Calcium hydroxide is prepared by adding water to calcium oxide, a process known as slaking. The chemical reaction involved is quite straightforward and can be represented by the equation:

CaO + H₂O → Ca(OH)₂

In this reaction, calcium oxide (CaO), commonly called quicklime, reacts vigorously with water (H₂O), producing calcium hydroxide (Ca(OH)₂) and releasing a significant amount of heat. This exothermic reaction results in a fine dry powder of calcium hydroxide, often referred to as slaked lime. This substance is used in various applications, such as in construction for making plasters and mortars and in agriculture to neutralize acidic soils.

Physical Properties of Calcium Hydroxide

AppearanceWhite powder or colorless crystals
Density2.211 g/cm³ (solid)
Melting PointApproximately 580°C (decomposes)
SolubilitySlightly soluble in water; solubility decreases as temperature increases
pHHighly basic (about 12.4 when in a saturated solution)

Chemical Properties of Calcium Hydroxide

Reactivity with Carbon Dioxide

  • Calcium hydroxide reacts with carbon dioxide to form calcium carbonate, a common reaction used in the construction industry to cure lime mortar and plaster. This process is also critical in the natural carbonation of lime-based materials.
  • Equation: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Reaction with Acids

  • Calcium hydroxide is a strong base and reacts readily with acids to form salts and water. This type of reaction are known as neutralization.
  • Equation: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Formation of Precipitates

  • When calcium hydroxide is mixed with solutions containing carbonate or phosphate ions, insoluble precipitates are formed. This property is utilized in water treatment processes to remove impurities.
  • Equation: Ca(OH)₂ + CO₃²⁻ → CaCO₃ ↓ + 2OH⁻

Heat Generation

  • The preparation of calcium hydroxide from calcium oxide and water is highly exothermic, releasing a considerable amount of heat. This reaction, known as slaking, is crucial in many applications where heat plays a role in chemical processing or material transformation.
  • Equation: CaO + H₂O → Ca(OH)₂ (Heat is released during this reaction)

Calcium Hydroxide (Ca(OH)₂) Chemical Compound Information

Chemical Identifiers

CAS Registry Number1305-62-0
PubChem Compound ID14777
PubChem Substance ID24868536
SMILES Identifier[OH-].[OH-].[Ca+2]
InChI IdentifierInChI=1/Ca.2H2O/h;21H2/q+2;;/p-2/fCa.2HO/h;21h/qm;2*-1
RTECS NumberEW2800000
MDL NumberMFCD00010901

NFPA Label

NFPA Health Rating3
NFPA Fire Rating0
NFPA Reactivity Rating0

Uses of Calcium Hydroxide


In Construction

Calcium hydroxide is widely used in the construction industry for making mortar and plaster. It helps improve the workability of mortar and increases its durability once set. It also aids in the curing process by reacting with carbon dioxide in the air to form a hard, durable surface layer of calcium carbonate.

In Water Treatment

It plays a crucial role in water treatment processes. By adjusting the pH of water, calcium hydroxide helps in purifying drinking water and treating wastewater. It is also used to remove impurities by precipitating unwanted ions through the formation of insoluble compounds.


Farmers use calcium hydroxide to adjust soil acidity, an essential step in improving soil health and crop yields. It neutralizes acidic soils, allowing plants to better absorb nutrients.

Food Industry

This compound is used as a food additive known as E526. It is used in the processing of various foods, such as corn tortillas and pickles, to improve texture and maintain freshness. Additionally, it is used in sugar refining to purify raw sugar and in the clarification of juices.

Dental Care

Calcium hydroxide is used in dentistry for its antibacterial properties. It is a common ingredient in root canal treatments, helping to reduce infections and promote the formation of secondary tooth material.

Side Effects of Calcium Hydroxide

  • Skin Irritation: Direct contact with calcium hydroxide can cause skin irritation, leading to redness and burning sensations. It’s important to wear protective gloves and clothing when handling this chemical.
  • Eye Damage: Exposure to calcium hydroxide can be very harmful to the eyes. It can cause severe irritation and even chemical burns that may lead to permanent eye damage if not washed out immediately and thoroughly.
  • Respiratory Issues: Inhaling calcium hydroxide dust can irritate the respiratory tract, causing coughing and shortness of breath. In severe cases, it can lead to lung injury.
  • Digestive Problems: If ingested, calcium hydroxide is corrosive and can cause severe pain, vomiting, and damage to the mouth, throat, and stomach.


Is Calcium Hydroxide a Base or Acid?

Calcium hydroxide is a strong base, commonly known as slaked lime, with a formula of Ca(OH)₂.

Is Calcium Hydroxide a pH?

Calcium hydroxide has a high pH, typically around 12.4, indicating strong basicity when dissolved in water.

Is Calcium Hydroxide Safe for Skin?

Calcium hydroxide can cause skin irritation and burns; it is not considered safe for direct skin contact.

Is Calcium Hydroxide Toxic?

While not highly toxic, calcium hydroxide can be harmful if inhaled or contacted with skin and eyes, requiring careful handling.

AI Generator

Text prompt

Add Tone

10 Examples of Public speaking

20 Examples of Gas lighting