✓ Board-verified
Science · 2024 · 5 marks
CBSE 2024 · Region 4 · Set 1 · Q34
(a) 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)…
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.
(i)
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
All Chemical Reactions and Equations questions2024 question paperNCERT solutions: Chemical Reactions and EquationsPractice Chemical Reactions and Equations →
More from Chemical Reactions and Equations
- Copper powder is taken in a china dish and heated over a burner. Name the product formed and state its colour. Write the chemical equation for the…2024
- Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical…2025
- Zn+2 CH_3 COOH →(CH_3 COO)_2 Zn+H_2 The above reaction is a:2024
- Which of the following is not a thermal decomposition reaction?2024
- Example of thermal decomposition reaction are (i) 2 AgCl → 2 Ag+Cl_2 (ii) CaCO_3 → CaO+CO_2 (iii) 2 H_2 O → 2 H_2+O_2 (iv) 2 KClO_3 → 2 KCl+3 O_22025
- Which of the following is an example of a displacement reaction?2026
- Define a double displacement reaction. Write the chemical equation of a double displacement reaction which is also a (i) Neutralization reaction and…2023
- Answer the following questions in the context of electrolysis of water: Why is this reaction/process called a decomposition reaction? Giving reason…2024
CBSE Class 10 Science past-paper question from the 2024 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.