Ca(OH)₂ — Calcium Hydroxide

Calcium hydroxide (Ca(OH)₂), commonly known as slaked lime or hydrated lime, is an inorganic compound formed by the reaction of calcium oxide (quicklime) with water. It is a soft, white, powdery substance that dissolves slightly in water to form a clear, colorless solution known as limewater. The compound is a strong base and is widely used in various industrial, environmental, and laboratory applications.

Calcium hydroxide is a versatile material employed in construction (mortar, plaster, cement), water treatment, food industry (pickling), and agriculture (soil neutralization). Its basic nature allows it to neutralize acids, making it a crucial component in many neutralization and precipitation processes.

The molecular structure of calcium hydroxide consists of one calcium ion (Ca²⁺) and two hydroxide ions (OH⁻). The calcium ion donates two electrons to form ionic bonds with two hydroxide groups, giving rise to a stable crystal lattice. Because of its ionic nature, Ca(OH)₂ readily dissociates in water:

This dissociation makes calcium hydroxide a strong base. However, its solubility in water is low, and the solution formed is only mildly basic, known as limewater. When exposed to air, calcium hydroxide slowly reacts with carbon dioxide to form calcium carbonate (CaCO₃), a reaction used to test for the presence of CO₂ gas.

Calcium hydroxide is prepared industrially and in laboratories by the slaking of lime—the controlled reaction of calcium oxide (quicklime) with water. The process is highly exothermic and releases a large amount of heat:

This reaction forms a suspension known as milk of lime, which upon settling or filtration yields solid calcium hydroxide. The reaction is important in various industries such as building materials, paper, and sugar manufacturing.

• Solubility: Slightly soluble in water (0.185 g/100 mL at 25°C). The solution is basic due to dissociation of hydroxide ions.
• Thermal Decomposition: On heating to about 580°C, it decomposes to calcium oxide and water vapor.

\(Ca(OH)_2 \xrightarrow{Heat} CaO + H_2O\)

• Reaction with Carbon Dioxide: Limewater reacts with CO₂ to form calcium carbonate, turning the solution milky.

\(Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O\)

• Reaction with Acids: Calcium hydroxide neutralizes acids to form corresponding salts and water.

\(Ca(OH)_2 + 2HCl \rightarrow CaCl_2 + 2H_2O\)

• Construction Industry: Used in making plaster, mortar, and cement where it reacts with sand and CO₂ to form hard, durable materials.
• Water Treatment: Used to remove hardness, neutralize acidic water, and precipitate impurities such as phosphates and heavy metals.
• Food Industry: Used as a food additive (E526) for pickling, fortifying fruit drinks, and clarifying sugarcane juice in sugar refining.
• Agriculture: Used to reduce soil acidity and improve crop yield by neutralizing excess acid content in soil.
• Chemical Industry: Serves as a reagent for producing other calcium compounds such as calcium hypochlorite, calcium stearate, and calcium phosphate.
• Medical and Laboratory Use: Applied as an ingredient in antacid formulations, disinfectants, and for testing the presence of CO₂.

Calcium hydroxide is generally considered safe when handled properly, but due to its strong basic nature, it can cause skin and eye irritation. Prolonged exposure to its dust may cause respiratory discomfort. Direct contact with concentrated forms may lead to burns. Safety measures such as wearing gloves, goggles, and masks should be observed while handling the compound.

Environmentally, Ca(OH)₂ is used beneficially in water treatment to neutralize acidic effluents. However, uncontrolled release can raise the pH of water bodies, which may harm aquatic organisms. Hence, disposal and usage should follow environmental guidelines.