Science
Seawater contains many salts dissolved in it. Common salt is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt and is mined like coal. The common salt is an important raw material for chemicals of daily use.
(a) Write balanced chemical equations to show the products formed during electrolysis of brine.
(b) List two uses of any one product obtained during electrolysis of brine.
(c) A mild non-corrosive basic salt ‘A’, used for faster cooking, is strongly heated to produce a compound ‘B’, that is used for removing permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated.
OR
(c) Define water of crystallisation. Give two examples of salts that have water of crystallisation.
Electrolysis
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Answer
(a) Electrolysis of brine (aqueous NaCl) gives chlorine at anode, hydrogen at cathode and sodium hydroxide in solution:
2NaCl + 2H2O ⟶ 2NaOH + H2(↑) + Cl2(↑)
(b) Two uses of chlorine :
- Disinfecting and purifying drinking water.
- Manufacture of bleaching agents and many organic chemicals (e.g. PVC).
(c)
- A = sodium hydrogen carbonate (sodium bicarbonate, NaHCO3).
- B = sodium carbonate (Na2CO3), used to remove permanent hardness of water.
On strong heating of A (NaHCO3) :
OR
(c) Water of crystallisation is the definite number of water molecules chemically incorporated in one formula unit of a crystalline salt.
Examples :
- Copper(II) sulfate pentahydrate (CuSO4⋅5H2O)
- Sodium carbonate decahydrate, Na2CO3⋅10H2O.
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