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Science · 2026 · 3 marks
CBSE 2026 · Region 3 · Set 1 · Q27
Attempt either (a) or (b): (a) Give reasons: 3 × 1=3 (i) For dilution of an acid, acid is added into water and not water into acid. (ii) Dry HCl gas…
Attempt either (a) or (b) :
(a)
Give reasons : $\displaystyle 3 \times 1=3$
(i)
For dilution of an acid, acid is added into water and not water into acid.
(ii)
Dry HCl gas does not turn blue litmus red whereas dilute hydrochloric acid does.
(iii)
Blue colour of Copper(II) sulphate crystals turns white on heating.
OR
(b)
Write the preparation of the following compounds with balanced chemical equation : $\displaystyle 3 \times 1=3$
(i)
NaOH by chlor-alkali process
(ii)
Baking Soda
(iii)
Plaster of Paris
Marking-scheme solution
(a) (i) When acid is added to water, it does not cause excessive local heating/ does not splash out and cause burns.
(ii) Because dry HCl does not ionize/produce \(\displaystyle \mathrm{H}^{+}\)ions whereas in dilute \(\displaystyle \mathrm{HCl}, \mathrm{H}^{+}\)ions are present.
(iii) Because of loss of $\displaystyle 5$ molecules of water of crystallisation.
(b)
\(\displaystyle 2 \mathrm{NaCl}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \xrightarrow{\text { electricity }} 2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})\)
(ii) \(\displaystyle \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}+\mathrm{NH}_{3}+\mathrm{CO}_{2} \rightarrow \mathrm{NaHCO}_{3}+\mathrm{NH}_{4} \mathrm{Cl}\)
(iii) \(\displaystyle \mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O} \xrightarrow{373 \mathrm{~K}} \mathrm{CaSO}_{4} \cdot \frac{1}{2} \mathrm{H}_{2} \mathrm{O}+1 \frac{1}{2} \mathrm{H}_{2} \mathrm{O}\)
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CBSE Class 10 Science past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.