Slaked Lime

Slaked lime is an inorganic compound whose chemical formula is Ca(OH)2. Its IUPAC name is calcium hydroxide. As it is produced by slaking quicklime (calcium oxide CaO) with water, that’s why it is called slaked lime. It is known by many common names such as hydrated lime, caustic lime, builder’s lime, slack lime, cal, milk of lime and pickling lime etc. We have been using calcium hydroxide since ancient times. It was found that it has been used in Egyptian pyramids and architecture of Roman Empire. Presently it is used in the food industry, sugar industry, medicinal field, and various other fields.

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Structure of Calcium Hydroxide

As stated above the chemical formula of calcium hydroxide is Ca(OH)2. It is an ionic compound. In which calcium loses its two electrons to polyatomic hydroxide ions. Thus, in slaked lime Ca+2 ions forms one ionic bond with each OH- ions. Although oxygen and hydrogen atoms of polyatomic hydroxide anion possess covalent bonds between them. Strong hydrogen bonds exist between the layers of calcium hydroxides. That’s why it has hexagonal crystal structure.

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Preparation of Calcium Hydroxide 

From Calcium Oxide - Commercial production of calcium hydroxide is done by treating quicklime with water. Calcium oxide is commonly known as quicklime. The process of production of calcium hydroxide from lime is called slaking of lime as when a limited quantity of water is mixed with quicklime it disintegrates and crumbles. The reaction involved is given below –

CaO + H2O 🡪 Ca(OH)2

From Calcium Chloride – In laboratories slaked lime is prepared by mixing aqueous solutions of calcium chloride and caustic soda. Caustic soda is a common name for sodium hydroxide. 

CaCl2 + 2NaOH 🡪 Ca(OH)2 + 2NaCl

Properties of Calcium Hydroxide 

Physical Properties – 

  • Its molar mass is 74.09 g/mol. 

  • It appears as colorless crystals or white powder.

  • It is odorless.

  • Its melting point is 580

  • It is soluble in water, glycerol, and acids. Aqueous solution of calcium hydroxide is called limewater. Its solubility decreases with increase in temperature. Its suspension in water is called milk of lime. 

  • It is insoluble in alcohol.

  • It is basic or alkaline is nature. 

Chemical properties – 

  • Carbonatation – Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water. Reaction is given below –

Ca(OH)2 + CO2 → CaCO3 + H2O

  • Decomposition – When calcium hydroxide is heated to 512, it decomposes into calcium oxide (quicklime) and water. Reaction is given below –

Ca(OH)2 → CaO + H2O

  • Reaction With Dilute HCl (Acid) – When calcium hydroxide reacts with dil. Hydrochloric acid, it gives calcium chloride and water. Reaction is given below-

Ca(OH)2 + 2HCl(diluted)  🡪 CaCl2 + 2H2O

  • Reaction With Concentrated Acid – When calcium hydroxide reacts with concentrated sulfuric acid, it forms water and precipitate of calcium sulphate. Reaction is given below –

Ca(OH)2 + H2SO4(conc.) 🡪 CaSO4↓ + 2H2O

  • Reaction With Hydrogen Sulfide – When calcium hydroxide reacts with hydrogen sulfide, it gives calcium hydrosulfide and water. Reaction is given below –

Ca(OH)2 + 2H2S 🡪 Ca(HS)2 + 2H2O

  • Preparation of Ammonia – On reacting with ammonium chloride, calcium hydroxide gives ammonia, calcium chloride and water. Reaction is given below-

Ca(OH)2 + 2NH4Cl → 2NH3 + CaCl2 + 2H2O

  • Reaction With Flowers of Sulfur – When these are boiled with calcium hydroxide, they form lime sulfur. Reaction is given below –

3Ca(OH)2 ​+ 12S → 2CaS5 ​+ CaS2​O3 ​+ 3H2​O

Uses of Calcium Hydroxide 

We are using calcium hydroxide from ancient warfare, Roman Empire time to modern industry. you will be amazed to know that Roman empire extensively used lime – based mortars in their architecture and lime is present in the construction material of the great wall of china. Few of its modern age uses are listed below –

  • For white washing – Slaked lime is used in white washing. It reacts with carbon dioxide slowly and forms a layer of calcium carbonate on the wall which gives shine to the wall.

  • For manufacturing of sugar – Slaked lime is used in manufacturing of sugar from sugar cane. It makes sugarcane juice alkaline and precipitates its impurities. 

  • It is used in sewage treatment. It is used as a flocculant in water. 

  • It is used in the paper industry. 

  • It is used in pickles.

  • It is used to process water for various beverages. 

  • It is used to remove the cellulose hull of maize kernels. 

  • It is used as a digestive aid. 

  • It is used in making papadum.

  • It is also used in paan and tobacco.


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