Slaked lime

Slaked lime, chemically known as calcium hydroxide (Ca(OH)₂), is a white, odourless, crystalline powder or soft solid obtained by treating quicklime (calcium oxide) with water. It is an important industrial and construction material widely used in agriculture, water treatment, and chemical processes.

Chemical Composition and Formation

Slaked lime is formed when calcium oxide (CaO), commonly known as quicklime, reacts with water in an exothermic reaction:
CaO + H₂O → Ca(OH)₂ + Heat\text{CaO + H₂O → Ca(OH)₂ + Heat}CaO + H₂O → Ca(OH)₂ + Heat
This process, called slaking, produces a fine white powder or a suspension known as limewater when dissolved in excess water. The reaction releases significant heat, so it must be conducted carefully to avoid spattering or boiling.
Chemically, calcium hydroxide is a strong base, only slightly soluble in water, and forms a clear, colourless solution (limewater) that turns milky when exposed to carbon dioxide due to the formation of calcium carbonate (CaCO₃).

Physical and Chemical Properties

• Chemical Formula: Ca(OH)₂
• Molar Mass: 74.09 g/mol
• Appearance: White crystalline or amorphous solid
• Solubility: Slightly soluble in water; forms limewater
• pH: Strongly basic (approximately 12.4 for saturated solution)
• Decomposition: On heating above 580 °C, decomposes to calcium oxide and water vapour

Key chemical behaviour includes:

• Reaction with acids to form salts and water
• Reaction with carbon dioxide to form calcium carbonate
• Reaction with chlorine to form bleaching powder (calcium oxychloride)

Methods of Preparation

1. From Quicklime: The most common industrial process is the hydration of quicklime (CaO) with controlled addition of water.
2. From Calcium Salts: Precipitation of calcium hydroxide can also occur in aqueous solutions containing calcium salts when reacted with an alkali such as sodium hydroxide.

The resulting product may be marketed as hydrated lime, available in powdered or paste form depending on the intended use.

Applications

1. Construction and Building

• Used in the preparation of mortar, plaster, and whitewash, where it reacts slowly with atmospheric CO₂ to form calcium carbonate, imparting hardness and durability.
• Serves as an ingredient in cement manufacture and stabilisation of soil for road construction.

2. Water Treatment

• Employed for softening hard water and purifying drinking water by precipitating impurities such as carbonates and phosphates.
• Used in sewage treatment plants to neutralise acidic waste and disinfect effluents.

3. Agriculture

• Acts as a soil conditioner to reduce soil acidity (increasing pH) and enhance nutrient availability to crops.
• Provides a source of calcium for plant growth.

4. Industrial Applications

• Used in the manufacture of bleaching powder (Ca(OCl)₂) when treated with chlorine gas.
• Plays a role in the production of ammonia in the Solvay process.
• Acts as a neutralising agent in chemical industries for treating acidic gases and effluents.
• Used in leather tanning, sugar refining, and paper manufacturing.

5. Medicinal and Domestic Uses

• In small quantities, limewater (a dilute solution of slaked lime) is used as an antacid and mild disinfectant.
• Historically used in whitewashing walls and buildings for its antiseptic properties.

Advantages

• Readily available and inexpensive.
• Non-toxic when handled properly.
• Multifunctional: useful in construction, agriculture, environmental protection, and industry.
• Environmentally friendly compared to many synthetic chemicals.

Limitations

• Corrosive: Its strong alkalinity can cause skin and eye irritation.
• Carbonation: Reacts with atmospheric CO₂, leading to gradual hardening or loss of reactivity if stored improperly.
• Limited solubility: Only partially soluble in water, which restricts its use in certain applications requiring uniform aqueous solutions.
• Handling difficulties: The exothermic slaking process requires care to prevent burns or damage to containers.

Safety and Storage

• Should be stored in airtight, moisture-proof containers to prevent reaction with carbon dioxide and moisture from air.
• When handling, protective gloves, goggles, and face masks should be used to avoid contact with skin or eyes.
• Slaked lime should be kept away from acids and incompatible materials.

Environmental and Industrial Significance

Slaked lime is a key reagent in environmental management due to its ability to neutralise acidity, purify water, and reduce pollution. It is also essential in producing construction materials and in numerous chemical processes where alkalinity is required. Its use spans across civil engineering, agriculture, environmental science, and industrial chemistry.