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Science · 2024 · 5 marks
CBSE 2024 · Region 4 · Set 1 · Q34
What is a chemical reaction ? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place.(b)Define a decomposition reaction. How can we say that(I)electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions ? Mention the type of energy involved in each case.(ii)"The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other." Justify this statement with the help of chemical equations.
What is a chemical reaction ? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place.
(b)
Define a decomposition reaction. How can we say that
(I)
electrolysis of water, and (II) blackening of silver bromide when exposed to sunlight, are decomposition reactions ? Mention the type of energy involved in each case.
(ii)
"The type of reactions in which (I) calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to each other." Justify this statement with the help of chemical equations.
Marking-scheme solution
(a) A chemical reaction involves the breaking and making of bonds between atoms to produce new substances. / when reactant changes to products.
(i) Add lead nitrate solution to potassium iodide solution taken in a test tube. The colour changes from colourless solution to yellow ppt. /
\[\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}+2 \mathrm{KI} \longrightarrow \underset{\text { Yellow }}{\mathrm{PbI}_{2} \downarrow+2 \mathrm{KNO}_{3}}
\]
(or any example)
(ii)
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat. /
\[\begin{aligned}
& \mathrm{CaO}(\mathrm{~s}) \\
& \text { (Quick lime) }
\end{aligned}+\quad+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \underset{\text { (Slaked lime) }}{\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})}+\text { Heat }
\]
(or any example)
(i) •A reactant breaks down to give two or more products. /A reaction which requires energy to split a compound or reactant in two or more simple substances.
(I) Water splits into hydrogen gas and oxygen gas.
Electrical energy
(II) Silver bromide decomposes into silver and bromine
Light energy
(I) Formation of calcium oxide:
\[\mathrm{CaCO}_{3}(\mathrm{~s}) \xrightarrow{\text { Heat }} \mathrm{CaO}(\mathrm{~s})+\mathrm{CO}_{2}(\mathrm{~g})
\]
It is an endothermic reaction/decomposition reaction.
(II) Formation of calcium hydroxide:
\[\mathrm{CaO}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}+\text { Heat }
\]
It is exothermic/combination reaction
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CBSE Class 10 Science past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.