Preparation and Properties of Calcium Hydroxide (Ca(OH)2)

Calcium hydroxide, also known as slaked lime, is a compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is obtained when calcium oxide (called lime or quicklime) is mixed, or "slaked" with water. It has many uses in industry and is also known as hydrated lime or slaked lime.

Preparation of Calcium Hydroxide

Calcium hydroxide is prepared by adding water to calcium oxide (quicklime) in a process known as slaking. The chemical reaction for the preparation is as follows:

[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 ]

This is an exothermic reaction, meaning it releases heat. The reaction can be carried out in a number of ways:

1. Dry Slaking (Dry Hydration):

   • Calcium oxide is mixed with a limited amount of water. This process is controlled to avoid forming excess water that would dissolve the Ca(OH)2.

2. Wet Slaking:

   • Calcium oxide is mixed with an excess amount of water to form a calcium hydroxide suspension or milk of lime.

Industrial Preparation

On an industrial scale, calcium hydroxide is also produced by mixing calcium oxide with water in a hydrator. The reaction conditions are carefully controlled to obtain the desired quality of the product.

Properties of Calcium Hydroxide

Calcium hydroxide exhibits several physical and chemical properties:

Physical Properties

• Appearance: White powder or colorless crystals
• Solubility: Slightly soluble in water (forming a solution known as lime water), but more soluble in pure water than in impure water. Solubility decreases with temperature.
• Density: About 2.211 g/cm³
• Molar Mass: 74.093 g/mol
• Melting Point: 580 °C (decomposes)

Chemical Properties

• Basicity: It is a strong base and can neutralize acids.
• Reactivity with Carbon Dioxide: It reacts with carbon dioxide to form calcium carbonate, a process called carbonatation.

[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} ]

• Thermal Stability: It decomposes into calcium oxide and water when heated to 580 °C or above.
• Reactivity with Acids: It reacts with acids to form the corresponding calcium salts and water.

Differences and Important Points

Property	Calcium Oxide (CaO)	Calcium Hydroxide (Ca(OH)2)
Common Name	Quicklime	Slaked Lime or Hydrated Lime
Solubility in Water	Reacts violently with water	Slightly soluble in water
pH	Strongly basic (when dissolved)	Strongly basic
Reaction with CO2	Reacts to form Ca(OH)2	Reacts to form CaCO3
Uses	Cement production, soil stabilization	Neutralizing acidic soils, water treatment

Examples to Explain Important Points

1. Neutralization of Acidic Soil:

   • Farmers use calcium hydroxide to neutralize acidic soils. The hydroxide ions (OH-) from Ca(OH)2 neutralize the hydrogen ions (H+) in the soil, which are responsible for the acidity.

2. Water Treatment:

   • In water treatment plants, calcium hydroxide is used to adjust the pH of water, to precipitate heavy metals, and to remove impurities.

3. Carbonatation in Sugar Refining:

   • The sugar industry uses calcium hydroxide to remove impurities from raw juice during sugar refining. The Ca(OH)2 reacts with carbon dioxide to form insoluble calcium carbonate, which precipitates out, carrying impurities with it.

4. Preparation of Mortars and Plasters:

   • Calcium hydroxide is an important ingredient in the preparation of mortars and plasters. It reacts with carbon dioxide in the air to form a hard, solid mass of calcium carbonate, which provides strength to the mortar or plaster.

5. Food Industry:

   • It is used in the food industry for processing water for alcoholic beverages and soft drinks, and as a flocculant in the clarification of juice.

Understanding the preparation and properties of calcium hydroxide is essential for its application in various industrial processes, environmental management, and construction. Its ability to neutralize acids and react with carbon dioxide makes it a valuable compound in many fields.