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Science · 2023 · 2 marks
CBSE 2023 · Region 2 · Set 1 · Q21
(a) What is observed when aqueous solutions of potassium iodide and lead nitrate are mixed together? Name the type of reaction and write the chemical…
(a)
What is observed when aqueous solutions of potassium iodide and lead nitrate are mixed together ? Name the type of reaction and write the chemical equation for the reaction that occurs.
OR
(b)
When copper powder is heated in a watch glass, a black substance is formed.
(i)
Why is this black substance formed ? Name it.
(ii)
How can this black substance be reversed to its original form ?
Marking-scheme solution
(a) • Yellow precipitate of lead iodide is formed.
Double displacement reaction / Precipitation reaction
\[\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{KI} \longrightarrow \mathrm{PbI}_{2}+2 \mathrm{KNO}_{3}
\]
(b) (i) • Oxygen is added to copper / Copper is oxidised
copper oxide / CuO
(ii) By passing hydrogen gas over it Alternative answer
(i) \(\displaystyle 2 \mathrm{Cu}+\mathrm{O}_{2} \xrightarrow{\Delta} 2 \mathrm{CuO}\)
(ii) \(\displaystyle \mathrm{CuO}+\mathrm{H}_{2} \longrightarrow \mathrm{Cu}+\mathrm{H}_{2} \mathrm{O}\)
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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.