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Science · 2025 · 3 marks
CBSE 2025 · Region 4 · Set 1 · Q27
(A) Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical…
(A)
Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical equation : \[\mathrm{Zn}+\mathrm{H}_{3} \mathrm{PO}_{4} \rightarrow \mathrm{Zn}_{3}\left(\mathrm{PO}_{4}\right)_{2}+\mathrm{H}_{2} \]
OR
(B)
Define a precipitation reaction. Give its example and also express the reaction that occurs in the form of a balanced chemical equation.
Marking-scheme solution
(A)
The number of atoms of each element remains same before and after a chemical reaction / to satisfy the law of conservation of mass.
Law of conservation of mass.
Mass can neither be created nor destroyed in a chemical reaction.
\(\displaystyle 3 \mathrm{Zn}+2 \mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{Zn}_{3}\left(\mathrm{PO}_{4}\right)_{2}+3 \mathrm{H}_{2}\) OR
(B)
Any reaction in which a precipitate (insoluble substance) is formed is called a precipitation reaction.
Example: when sodium sulphate solution is added to the barium chloride solution a white precipitate of barium sulphate is formed.
\[\mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{BaCl}_{2}(\mathrm{aq}) \longrightarrow \underset{\mathrm{ppt}}{\mathrm{BaSO}_{4}}(\mathrm{~s})+2 \mathrm{NaCl}(\mathrm{aq})
\]
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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.