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Science · 2025 · 4 marks
CBSE 2025 · Region 1 · Set 1 · Q37
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…
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)(i)
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)
(ii)
Define water of crystallisation. Give two examples of salts that have water of crystallisation.
Marking-scheme solution
(a)
\(\displaystyle 2 \mathrm{NaCl}+2 \mathrm{H}_{2} \mathrm{O} \xrightarrow{\text { electricity }} 2 \mathrm{NaOH}+\mathrm{H}_{2}+\mathrm{Cl}_{2}\)
(b)
Uses of NaOH : Degreasing metals/ Soaps and Detergents/ paper making/ artificial fibres/ preparation of bleach
Uses of \(\displaystyle \mathrm{H}_{2}\) : As fuel/ Margarine/ In preparation of ammonia for fertilizers/Preparation of HCl
Uses of \(\displaystyle \mathrm{Cl}_{2}\) : Disinfectant/ PVC/ water treatment/ in swimming pools/ CFC's/ preparation of bleach/ preparation of HCl/ pesticides
(c)(i)
\(\displaystyle \mathrm{A}-\mathrm{NaHCO}_{3} /\) Sodium Hydrogen Carbonate/ Baking soda
\[\mathrm{B}-\mathrm{Na}_{2} \mathrm{CO}_{3} / \text { Sodium Carbonate }
\]
\[2 \underset{\mathrm{~A}}{\mathrm{NaHCO}_{3}} \xrightarrow{\text { heat }} \underset{\mathrm{B}}{\mathrm{Na}_{2} \mathrm{CO}_{3}}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}
\] OR
(c)
(ii)
• The fixed number of water molecules present in one formula unit of a salt.
\(\displaystyle \mathrm{CuSO}_{4} .5 \mathrm{H}_{2} \mathrm{O} /\) Copper Sulphate pentahydrate/Blue vitrol
\(\displaystyle \mathrm{CaSO}_{4} .2 \mathrm{H}_{2} \mathrm{O} /\) Gypsum \(\displaystyle /\) Calcium sulphate dihydrate
\(\displaystyle \mathrm{Na}_{2} \mathrm{CO}_{3} .10 \mathrm{H}_{2} \mathrm{O} /\) Washing Soda/Sodium carbonate decahydrate
\(\displaystyle \mathrm{FeSO}_{4} .7 \mathrm{H}_{2} \mathrm{O} /\) Green Vitrol/Ferrous sulphate hepta hydrate
\(\displaystyle \mathrm{CaSO}_{4} . \frac{1}{2} \mathrm{H}_{2} \mathrm{O} /\) Calcium Sulphate hemihydrate /POP
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.