What is Slaked Lime? Structure, Reactions, and Practical Applications
Slaked lime is the common name that is given to the compound calcium hydroxide which is a chemical compound. It is seen that under standard temperature and pressure this compound is present as a white powdery form which also forms colorless crystals. Some of the other names that are given to Slaked Lime are caustic lime, pickling lime, and slack lime. Students can now know more about Slaked Lime via Vedantu.
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 form one ionic bond with each OH- ion. 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 a hexagonal crystal structure.
Deducing the Formula of Calcium Hydroxide.
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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. An aqueous solution of calcium hydroxide is called limewater. Its solubility decreases with an increase in temperature. Its suspension in water is called milk of lime.
It is insoluble in alcohol.
It is basic or alkaline in nature.
Chemical Properties –
Carbonatation – Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water. The 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. The reaction is given below –
Ca(OH)2 → CaO + H2O
Reaction With Dilute HCl (Acid) – When calcium hydroxide reacts with dil. Hydrochloric acid gives calcium chloride and water. The 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 precipitates calcium sulfate. The 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. The 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. The 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. The reaction is given below –
3Ca(OH)2 + 12S → 2CaS5 + CaS2O3 + 3H2O
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 the Roman empire extensively used lime-based mortars in their architecture and lime is present in the construction material of the Great Wall of China. A few of its modern age uses are listed below –
For whitewashing – Slaked lime is used in whitewashing. 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 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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FAQs on Slaked Lime: Overview, Preparation & Key Properties
1. What is slaked lime and what is its chemical formula?
Slaked lime is the common name for the chemical compound calcium hydroxide. It is an inorganic compound with the chemical formula Ca(OH)₂. It is a white, colourless powder or crystal that is produced when quicklime (calcium oxide) is mixed, or 'slaked,' with water.
2. How is slaked lime prepared from quicklime?
Slaked lime is prepared through a process known as the slaking of lime. This involves adding water to quicklime (calcium oxide, CaO). The reaction is highly exothermic, meaning it releases a significant amount of heat, often causing the water to boil and produce steam. The chemical equation for this combination reaction is:
CaO(s) + H₂O(l) → Ca(OH)₂(s) + Heat
3. What are the main physical and chemical properties of slaked lime?
Slaked lime has several distinct properties:
Physical Properties: It is a soft, white, amorphous powder with a bitter taste. It is sparingly soluble in water, and its solubility decreases as the temperature increases.
Chemical Properties: It is a fairly strong base that reacts with acids to form salt and water (neutralisation). When exposed to air, it slowly reacts with carbon dioxide to form calcium carbonate.
4. What is the difference between quicklime, slaked lime, limewater, and milk of lime?
These terms all relate to calcium compounds but refer to different forms:
Quicklime: This is calcium oxide (CaO), the solid product of heating limestone.
Slaked Lime: This is calcium hydroxide (Ca(OH)₂), the powder formed when water is added to quicklime.
Limewater: This is a clear, saturated aqueous solution of calcium hydroxide. It is used to test for carbon dioxide.
Milk of Lime: This is a suspension of slaked lime in water, appearing milky due to the undissolved particles.
5. How does slaked lime help in whitewashing walls?
When a suspension of slaked lime (Ca(OH)₂) is applied to walls, it reacts slowly with the carbon dioxide (CO₂) present in the atmosphere. This reaction forms a thin, hard, and shiny layer of calcium carbonate (CaCO₃) on the walls, giving them a durable white finish. The reaction is:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). This process usually takes two to three days to complete.
6. Why is slaked lime considered a base?
Slaked lime (calcium hydroxide) is considered a base because when it dissolves in water, it dissociates to release hydroxide ions (OH⁻). According to the Arrhenius theory, any substance that increases the concentration of OH⁻ ions in an aqueous solution is a base. Since it can neutralize two equivalents of an acid, it is also known as a diacidic base.
7. What are the most common uses of slaked lime?
Slaked lime is an important industrial chemical with many applications, including:
Construction: Used in mortar, plaster, and cement.
Agriculture: Used to neutralize acidic soil and improve its fertility.
Water Treatment: Used for softening water and as a flocculant for purifying drinking water.
Food Industry: Used in the refining of sugar.
Chemical Manufacturing: Used as a key ingredient in making other chemicals like bleaching powder.
8. What are the safety precautions to take when handling slaked lime?
Slaked lime is a caustic substance and must be handled with care. Direct contact can cause skin irritation, chemical burns, or eye damage. It is important to:
Wear protective gloves and safety glasses.
Avoid inhaling the dust, as it can irritate the respiratory tract.
Ensure proper ventilation in the area where it is being used.
