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Calcium Hydroxide (CaOH2) - Structure, Formula, Preparation, Physical and Chemical Properties, Uses, Practice Problems and FAQ
Calcium hydroxide is a very respectable and ubiquitous chemical. Calcium hydroxide is all around us, from acting as an important filler for root canals by dentists to creating pre-historic walls of the Egyptian Pyramids.
It is much easier to count where it is not used than where it is. This is the multifaceted utility of calcium hydroxide. Let us travel to the world of calcium hydroxide.
TABLE OF CONTENTS
- Formula and Structure
- Physical Properties
- Chemical Properties
- Uses
- Practice Problems
- Frequently Asked Questions - FAQ
Formula and Structure
The chemical formula of calcium hydroxide, also known as slaked lime, is Ca(OH)2. It is a white, powdery solid inorganic compound, slightly flaky in solid form. However, Ca(OH)2 has a colourless appearance in its crystalline form. In crystalline form Calcium hydroxide is colourless.
Calcium hydroxide is also known as hydrated lime, slack lime, pickling lime, and caustic lime. Calcium hydroxide is prepared by mixing water and calcium oxide (quick lime).
The reaction of sodium hydroxide with aqueous calcium chloride also yields this compound. The structure of a Ca(OH)2 molecule is illustrated below.
Calcium hydroxide is made of ionic bonds between the calcium ion (Ca2+) and two hydroxide ions (OH-). Calcium hydroxide occurs naturally in some mineral form as portlandite, found in some volcanic and metamorphic rocks.
Preparation of Calcium hydroxide
Calcium hydroxide takes up a polymeric structure, as do all metal hydroxides. Calcium hydroxide is produced commercially by treating quicklime with water.
CaO (s) + H2O (l)Ca(OH)2(aq)
When combined with water, a relatively small proportion of quicklime dissolves, resulting in the formation of a solution known as limewater. The remainder of the quicklime remains in a suspension, which is generally referred to as lime milk.
Physical Properties
- Calcium hydroxide has a hexagonal crystal structure.
- It is not very soluble in water and its solubility decreases with an increase in temperature. For example, its solubility at 273 K is 1.89 g L-1 and its solubility is 1.73 g L-1 at 293 K.
- Its solubility decreases with rise in temperature in accordance with Le Chatelier’s principle as dissolution of calcium hydroxide in water is exothermic.
- At temperatures approaching its melting point, it tends to lose water and decompose.
- The solubility product (Ksp) of calcium hydroxide is 5.5 10-6.
- Calcium hydroxide tends to lose water and decompose at temperatures close to its melting point.
- It is very easily soluble in glycerol and acids, but only sparingly soluble in water. It forms a solution that behaves as a mild base when dissolved in water to saturation.
- The molar mass of calcium hydroxide is 74.093 g mol-1.
- Density is 2.211 g cm-3.
- Melting point of calcium hydroxide is 853 K.
Chemical Properties
- The aqueous solution of calcium hydroxide is called lime water. And a suspension of slaked lime in water is the milk of lime.
- When CO2 is passed through lime water, it turns milky due to the formation of calcium carbonate.
Ca(OH)2(aq)+CO2(g)CaCO3(s) +H2O(aq)
- On passing excess CO2, the milkiness disappears as calcium hydrogen carbonate is formed.
CaCO3(s) +H2O(aq)+ CO2(g) Ca(HCO3)2
- Milk of lime reacts with chlorine gas to form calcium hypochlorite, an active ingredient of bleaching powder.
2Ca(OH)2(aq)+ 2Cl2(g) → Ca(OCl)2(s)+CaCl2(s)+ 2H2O(l)
- Calcium hydroxide is a basic hydroxide as it can dissociate to form hydroxide ions. But it is a weak base and does not undergo complete dissociation.
- When lime water reacts with acids, it produces salts, as it is basic in nature.
Ca(OH)2(aq)+ HCl(aq)CaCl2(s) + H2O (l)
- Metals such as aluminium, also react with a saturated solution of calcium hydroxide in water.
2Al (s)+ Ca(OH)2(aq) + 2H2O (aq) 3H2(g) + Ca(AlO2)2
Uses
- Calcium hydroxide is a major flocculant used in sewage treatment plants.
- Ca(OH)2 is used in the paper-pulp industry especially during the Kraft process of converting wood into wood pulp.
- Calcium hydroxide is also used in the preparation of ammonia.
- It is used in the preparation of mortar.
- It is also used in whitewashing, due to its disinfectant properties.
- It is used in glassmaking, tanning industry and manufacturing of sugar.
- This compound is also used as a pH modifier due to its basicity.
- The pickling of cucumbers is generally done with the help of Ca(OH)2.
- The production of many plastics involves the use of calcium hydroxide as an ingredient.
- It is also used in pesticides, hair care products, and the manufacture of ebonite.
- In root canal procedures, this compound is used to fill the cavities in the human teeth.
- Sugar beets and sugarcane are processed via carbonation, which involves the use of Ca(OH)2.
- Calcium hydroxide Ca(OH)2 is used in the leather and tanning industry to separate the fur/hair from the animal hide.
Practice Problems
1. What is the suspension of slaked lime known as?
1. Lime water
2. Quick lime
3. Milk of lime
4. Aqueous solution of slaked lime
Answer: Solid calcium hydroxide in the form of flakes is called slaked lime. Its suspension in water is an opaque milky solution and so, the suspension of slaked lime is called milk of lime.
Quick lime is CaO.
Limewater is a Ca(OH)2 solution that reacts with carbon dioxide to produce a white precipitate of calcium carbonate.
So, option C) is the correct answer.
2. What is the colour of calcium hydroxide?
1. White
2. Grey
3. Yellow
4. Colourless
Answer: It is white in colour, be it in solid state or in solution or suspension form. So, option A) is the correct answer.
3. What is the product obtained from heating calcium hydroxide?
1. Calcium carbonate
2. Calcium sulphate
3. Calcium bicarbonate
4. Calcium oxide
Answer: Calcium hydroxide on heating produces calcium oxide according to the reaction given below.
Ca(OH)2(aq)CaO (s) + H2O (l)
So, option D) is the correct answer.
4. Calcium hydroxide reacts with hydrochloric acid to produce4
1. Hydrogengas
2. Water
3. Carbon dioxide
4. Carbon monoxide
Answer: Calcium hydroxide reacts with hydrochloric acids to produce calcium chloride and water. Since calcium hydroxide is a base, it reacts with acids to produce salt and water.
Ca(OH)2(aq)+ HCl(aq)CaCl2(s) + H2O (l)
So, option B) is the correct answer.
Frequently Asked Questions - FAQ
Q 1. What happens when calcium hydroxide is coated on walls during white-washing?
Answer: During white-washing of walls, first calcium oxide (CaO) is made to react with water to form calcium hydroxide, Ca(OH)2. The calcium hydroxide is then applied to the walls, which slowly reacts with the carbon dioxide gas present in air to form a thin shining layer on the walls.
Q 2. Why is calcium hydroxide feebly soluble in water?
Answer: As we go down the group (calcium is an alkaline earth metal), the solubility of hydroxides decreases due to a decrease in the lattice energy. Hence, Ca(OH)2is less soluble.
Q 3. What is the role of calcium hydroxide in sewage treatment plants?
Answer: Calcium hydroxide, Ca(OH)2, is majorly used as a flocculating agent or as a clarifying agent in the sewage treatment process.
Q 4. What is the use of calcium hydroxide in dentistry?
Answer: It is used to fill cavities in human teeth, especially during root canal treatment.
Related Topics
|
Calcium Carbonate |
Alkali Metals |
|
Sodium Chloride |
Potassium |
|
Sodium Carbonate |
Calcium oxide |
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