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Science · 2023 · 3 marks
CBSE 2023 · Region 5 · Set 1 · Q28
(a) Sometimes the pH of our mouth gets lower than 5 · 5. Why? (b) A basic salt 'X' is obtained by heating baking soda followed by crystallisation.…
(a)
Sometimes the pH of our mouth gets lower than $\displaystyle 5 \cdot 5$. Why ?
(b)
A basic salt 'X' is obtained by heating baking soda followed by crystallisation. Identify 'X' and state its two industrial uses.
(c)
Why do copper sulphate crystals turn white on heating ?
Marking-scheme solution
(a)
Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating.
(b)
$\displaystyle \mathrm{X}: \mathrm{Na}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O} /$ Washing soda /
Sodium carbonate decahydrate
Uses :
Glass, soap and paper industries
Manufacture of borax
Cleaning agent
Removing permanent hardness of water
(c)
The water of crystallisation is removed./
\[\mathrm{CuSO}_{4.5 \mathrm{H}_{2} \mathrm{O}} \xrightarrow{\text { heat }} \underset{\text { (white) }}{\mathrm{CuSO}_{4}}+5 \mathrm{H}_{2} \mathrm{O} \uparrow
\]
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CBSE Class 10 Science past-paper question from the 2023 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.