PYQ on Chemical Reactions and Equations
PYQ on Chemical Reactions and Equations from NCERT Class 10 Science serve as a vital bridge between classroom learning and exam readiness. These questions are not just repetitions of old papers; they are carefully designed reflections of the concepts that matter most. By practicing them, students gain clarity on how chemical reactions are framed in examinations, whether through direct equation balancing, identification of reaction types, or application-based scenarios.
The questions often highlight the importance of the law of conservation of mass, the distinction between combination and decomposition reactions, or the role of redox processes in everyday life. Working through these problems helps learners identify patterns, strengthen their ability to write balanced equations, and sharpen analytical skills. Moreover, previous year questions reveal the examiner’s focus areas, guiding students to prioritize topics that frequently appear.
They also build confidence by simulating the exam environment, reducing anxiety, and improving time management. Ultimately, these questions transform preparation into a strategic process: students not only revise the chapter thoroughly but also learn to apply knowledge in diverse contexts. Thus, engaging with previous year questions ensures that chemical reactions and equations are not just memorized but truly mastered for success.
Section A
Identify an example of a combination reaction from the options given.
- Development of a photographic film
- Combustion of coal (carbon)
- Combustion of methane gas
- Purification of copper metal
Year: 2025
Answer: (B) Combustion of coal (carbon)
Explanation: Carbon combines with oxygen to form a single product, carbon dioxide: C + O₂ → CO₂.
Section B
State whether the assertion and reason are true/false and whether the reason correctly explains the assertion.
- Assertion (A): Conversion of vegetable waste into compost releases heat energy.
- Reason (R): Breaking down reactants in decomposition reactions requires an input of energy.
Year: 2025
Answer
- Assertion (A): True — composting releases heat due to exothermic microbial oxidation of organic matter.
- Reason (R): False — while many thermal decompositions need energy input, composting is not a simple endothermic decomposition; it is largely exothermic biochemical oxidation.
- Explanation link: The reason does not correctly explain the assertion.
Section C
Balance the following chemical equations to represent the reactions described:
(a) Aluminium metal reacts with copper(II) chloride solution to yield aluminium chloride and copper metal.
(b) Zinc reacts with aqueous sodium hydroxide to form sodium zincate and liberates hydrogen gas.
Year: 2025
Answer
(a) Aluminium with copper (II) chloride
- Balanced equation: 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(b) Zinc with aqueous sodium hydroxide
Balanced equation: Zn + 2NaOH → Na₂ZnO₂ + H₂↑
(a) Name the sodium compound (with formula) that is added to food to reduce cooking time. Explain how it is manufactured using sodium chloride as a raw material. Write the chemical equation for the process.
(b) The compound mentioned above is also used in antacids. Justify this application.
Year: 2025
Answer:
(a) Name, formula, manufacturing from NaCl, and equation
- Compound: Sodium hydrogen carbonate (baking soda)
- Formula: NaHCO₃
- Manufacture (from sodium chloride): Prepared via the Solvay process steps, where NaCl is a key raw material. Overall precipitation step:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
(NaHCO₃ is filtered and can be heated to make Na₂CO₃; cooling/CO₂ absorption yields NaHCO₃.)
(b) Use in antacids (justification)
- Reason: NaHCO₃ is a mild base that neutralizes excess stomach acid, giving relief from acidity: HCl + NaHCO₃ → NaCl + H₂O + CO₂.
Define a displacement reaction. Mention one highly exothermic displacement reaction used in welding railway tracks. Provide a description along with the relevant balanced chemical equation.
Year: 2025
Answer:
Definition:
- Displacement reaction: A more reactive element displaces a less reactive element from its compound.
Highly exothermic example (railway track welding):
- Process name: Thermite reaction (aluminothermy)
- Description: Aluminium displaces iron from iron(III) oxide, producing molten iron used to weld tracks.
Balanced equation: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + heat
(a) Outline the method for producing sodium hydroxide from brine, noting that two gases are also released at the electrodes in this process. Name these gases and specify the electrodes where they are formed. One of these gases reacts with dry calcium hydroxide to form a compound used both as an oxidizing agent in industries and for disinfecting drinking water. Identify this compound and write the balanced equation for its formation.
Year: 2025
Answer:
Producing NaOH from brine (electrolysis):
- Products: Sodium hydroxide solution, chlorine gas, hydrogen gas
- Electrodes:
- Anode: Chlorine gas (Cl₂) is released
- Cathode: Hydrogen gas (H₂) is released
Gas reacting with dry Ca(OH)₂ and product identification:
- Gas: Chlorine (Cl₂)
- Product formed: Bleaching powder (calcium oxychloride), CaOCl₂
Balanced formation equation: 2Cl₂ + 2Ca(OH)₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O
When crystals of ferrous sulphate are heated in a dry boiling tube, tiny droplets of water appear inside.
(i) Identify the source of these droplets.
(iI) Mention the colour change observed during heating.
(iII) Provide the balanced chemical equation for this process.
(iV) How many water molecules are present per formula unit in ferrous sulphate crystals?
(v) Write the molecular formula of crystalline copper sulphate and crystalline sodium carbonate.
Year: 2025
Answer:
(i) Source of droplets:
- Answer: Water of crystallization released from FeSO₄·7H₂O.
(iI) Colour change on heating:
- Answer: Pale green crystals turn white on losing water, then become brown due to formation of Fe₂O₃.
(iII) Balanced equation:
- 2FeSO₄·7H₂O → Fe₂O₃ + SO₂ + SO₃ + 14H₂O
(iV) Number of water molecules per formula unit:
- Answer: 7
(v) Molecular formulas (crystalline):
- Copper sulphate: CuSO₄·5H₂O
- Sodium carbonate: Na₂CO₃·10H₂O
Hydrocarbons are compounds containing only carbon and hydrogen. Saturated hydrocarbons are called alkanes, those with double bonds are alkenes, and those with triple bonds are alkynes.
(a) Identify and name the compound: CH_3-C≡CH.
(b) Compare the type of flame produced when saturated hydrocarbons burn versus unsaturated hydrocarbons.
(c) (i) Which category of hydrocarbons undergo addition reactions? Illustrate with an example.
(iI) Define structural isomerism. Draw two structural isomers of butane (C_4H_{10})
Year: 2025
Answer:
(a) Identify CH₃–C≡CH:
- Answer: Propyne (1-propyne)
(b) Flame comparison on burning:
- Answer: Saturated hydrocarbons (alkanes) burn with a clean, blue flame; unsaturated hydrocarbons (alkenes/alkynes) burn with a yellow, sooty flame due to incomplete combustion.
(c) Reactions and isomerism
- (i) Addition reactions:
- Category: Unsaturated hydrocarbons (alkenes and alkynes)
- Example: C₂H₄ + H₂ → C₂H₆ (hydrogenation of ethene in presence of catalyst)
Or, C₂H₄ + Br₂ → C₂H₄Br₂ (decolourisation of bromine water)
- (iI) Structural isomerism and butane isomers:
Definition: Same molecular formula but different structural arrangements of atoms.
Butane (C₄H₁₀) isomers:
- n-Butane: CH₃–CH₂–CH₂–CH₃
- Isobutane (methylpropane): (CH₃)₃CH
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