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Chapter section
Board Exam Practice - Verified Previous-Year Questions
Purpose and source boundary
The first six lessons follow the current NCERT textbook. This lesson is a separate exam-practice supplement. It does not add or renumber any textbook topic.
The previous-year questions below were checked against the original English board-paper pages on 26 July 2026. A question shows what CBSE asked in that historical paper. It does not prove what the current year's paper will contain.
The marking points are platform study guides. They are not official CBSE marking schemes themselves, but the answers for these nine questions were checked against CBSE's published marking-scheme value points for 2023-2026. The explanations sometimes add reasoning so that you understand why a point earns credit.
Your full-marks practice route
Use the chapter in this order. Do not jump straight to the answer check.
Finish the six NCERT lessons and the chapter quiz.
Use the balancing workbench until all four equations are completed without hints.
Write each PYQ answer in its practice box. Reveal the marking points only after writing.
Open every weak-skill repair link, revise that exact section, and start a fresh attempt.
Complete the five current-pattern challenges.
Attempt Diagnostic A under its 30-minute writing limit.
Attempt Diagnostic B as a separate transfer check.
Return for fresh D3 and D7 attempts when the retention card says they are due. A date alone never marks revision complete.
[REMEMBER: A high quiz score proves recognition. Full board confidence also needs complete written answers, balanced equations, timed work and later retention.]
The full-mark answer pattern
For an activity or reaction question, use O-I-E-C:
Observation: What was seen, smelled or felt?
Inference: What substance formed or what change occurred?
Equation: Write the balanced equation with states and conditions when known.
Classification: Name the reaction and justify the pattern.
For a reason question, use cause → process → result. For a multi-part question, copy the same (a), (b), (i) and (ii) labels into the answer.
Board extension: oxidising and reducing agents
This vocabulary appears in board questions and also connects Chapter 1 with later chemistry chapters.
An oxidising agent causes another substance to be oxidised. The oxidising agent itself is reduced.
A reducing agent causes another substance to be reduced. The reducing agent itself is oxidised.
In CuO + H₂ →[Heat] Cu + H₂O:
H₂ gains oxygen, so H₂ is oxidised and acts as the reducing agent.
CuO loses oxygen, so CuO is reduced and acts as the oxidising agent.
[TIP: Track what happens to each reactant first. Name the agent only after identifying which reactant is oxidised and which is reduced.]
A two-pass method for unfamiliar equations
Pass 1 - Use oxygen or hydrogen.
Verified previous-year questions
PYQ 1 - Observation, reaction type and equation - 2 marks
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.
Silver chloride kept in a china dish turns grey in sunlight.
(a) Write the colour of silver chloride when it was kept in the china dish.
(b) Name the type of chemical reaction taking place and write the chemical equation for the reaction.
(c) State one use of the reaction. Name one more chemical which can be used for the same purpose.
Write before checking
PYQ 2 written attempt
3 marks · Attempt 1
Latest official SQP-pattern challenges
As checked on 26 July 2026, CBSE's newest published main-subject Class X SQP is the 2025-26 paper. Its Chapter 1 questions combine more than one skill: balancing with redox reasoning, comparing displacement observations, diagnosing an electrolysis setup, evaluating corrosion in a material choice, and distinguishing double displacement from precipitation in a neutralisation. The five questions below are original platform questions built on those patterns. They are not previous-year questions and are not predictions.
Official references: [CBSE Class X 2025-26 SQP page](https://cbseacademic.nic.in/SQP_CLASSX_2025-26.html), [Science SQP](https://cbseacademic.nic.in/web_material/SQP/ClassX_2025_26/Science-SQP.pdf), and [Science marking scheme](https://cbseacademic.nic.in/web_material/SQP/ClassX_2025_26/Science-MS.pdf).
Current-pattern challenge 1 - Balance, classify and track redox - 2 marks
Balance both equations:
Fe₂O₃ + Al → Al₂O₃ + Fe
C₂H₆ + O₂ → CO₂ + H₂O
State whether each is a redox reaction. For the first equation, name one substance oxidised and one substance reduced.
Write before checking
Current-pattern challenge 1
2 marks · Attempt 1
Current-pattern challenge 2 - Compare two displacement setups - 2 marks
An iron nail is placed in blue copper sulphate solution. A copper strip is placed in pale-green iron sulphate solution.
Board-style format practice
Assertion-Reason 1
Assertion (A): The reaction of quicklime with water is exothermic.
Reason (R): Quicklime reacts with water to form calcium hydroxide and releases heat.
Choose:
Both A and R are true, and R correctly explains A.
Both A and R are true, but R does not correctly explain A.
A is true, but R is false.
A is false, but R is true.
Assertion-Reason 2
Assertion (A): Silver chloride is stored away from sunlight.
Reason (R): Light decomposes silver chloride into silver and chlorine.
Choose the correct relationship using the same four options.
Case-based question - 4 marks
A student performs three reactions:
Test tube A: green ferrous sulphate crystals are heated.
Test tube B: an iron nail is placed in blue copper sulphate solution.
Dish C: white silver chloride is placed in sunlight.
(a) Name the reaction type in test tube A.
🔍 Practice feedback
Answer check
Try first, then reveal the answer
PYQ 1
Platform marking points:
A yellow insoluble precipitate of lead iodide forms.
The reaction is double displacement and also precipitation.
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
PYQ 2
Platform marking points:
(a) Silver chloride was white.
(b) It is photochemical decomposition: 2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g).
The grey colour is due to silver metal.
(c) The reaction was used in black-and-white photography. Silver bromide, AgBr, can be used for the same purpose.
PYQ 3
Platform marking points:
A decomposition reaction is one in which one reactant breaks into two or more products.
(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.
Write before checking
PYQ 3 written attempt
5 marks · Attempt 1
PYQ 4 - Precipitation, ions and whitewashing chemistry - 5 marks
Lead nitrate + Potassium iodide → Lead iodide + Potassium nitrate
Is this a double displacement reaction? Justify your answer. Name the compound precipitated and write the ions present in it.
(ii) Write the method of preparation of Ca(OH)₂. What happens when CO₂ is passed through it? Write the balanced chemical equation for the reaction involved.
Write before checking
PYQ 4 written attempt
5 marks · Attempt 1
PYQ 5 - Conservation of mass and unfamiliar balancing - 3 marks
(i) While electrolysing water before passing the current some drops of an acid are added. Why? Name the gases liberated at cathode and anode. Write the relationship between the volume of gas collected at anode and the volume of gas collected at cathode.
(ii) What is observed when silver chloride is exposed to sunlight? Give the type of reaction involved.
Write before checking
PYQ 8 written attempt
3 marks · Attempt 1
PYQ 9 - Lead nitrate decomposition and gas mass ratio - 3 marks
(i) Define a decomposition reaction. Write chemical equation for the reaction that occurs when lead nitrate is heated strongly in a boiling tube.
(ii) In electrolytic decomposition of water two gases are liberated at the electrodes. Give the mass ratio of the gas liberated at the cathode and at the anode.
Write before checking
PYQ 9 written attempt
3 marks · Attempt 1
State the observation in each test tube.
Write the balanced equation for the reaction that occurs.
Use relative reactivity to explain why the other test tube shows no reaction.
Write before checking
Current-pattern challenge 2
2 marks · Attempt 1
Current-pattern challenge 3 - Diagnose an electrolysis setup - 3 marks
A student connects two electrodes to a battery and dips them in distilled water. The bulb does not glow and no gas is collected.
Explain why the setup does not work.
Name one substance that can be added in a small amount to make the solution conduct.
If the corrected setup works, name the gas expected at each electrode.
Write before checking
Current-pattern challenge 3
3 marks · Attempt 1
Current-pattern challenge 4 - Evaluate a corrosion decision - 3 marks
An old building has thin copper roof sheets that have developed a green coating. The owner proposes replacing them with unpainted iron sheets because the copper has “started corroding.”
Name the green substance formed on copper. (1)
Should unpainted iron replace the copper merely to avoid corrosion? Compare what happens to the two corrosion coatings. (1)
State the property that allows copper to be made into thin shaped roof sheets. (1)
Write before checking
Current-pattern challenge 4
3 marks · Attempt 1
Current-pattern challenge 5 - Classify beyond the obvious - 3 marks
A student mixes aqueous sodium hydroxide with dilute sulphuric acid. Sodium sulphate and water form.
Write the balanced equation. (1)
Give two valid classifications for the reaction. (1)
Explain why it is not a precipitation reaction. (1)
Write before checking
Current-pattern challenge 5
3 marks · Attempt 1
(1)
(b) State one colour observation in test tube B. (1)
(c) Write the balanced equation for the change in dish C. (1)
(d) Name the form of energy responsible for the change in dish C. (1)
. Electrical energy is supplied.
Light decomposes silver bromide: 2AgBr(s) →[Sunlight] 2Ag(s) + Br₂(g). Light energy is supplied.
Calcium oxide is formed by an endothermic decomposition reaction: CaCO₃(s) →[Heat] CaO(s) + CO₂(g).
Calcium hydroxide is formed by an exothermic combination reaction: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
Decomposition is one-to-many; combination is many-to-one. Their patterns are opposite.
PYQ 4
Platform marking points:
(i) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
It is double displacement because the two compounds exchange ions.
Yellow lead iodide is precipitated. Its ions are Pb²⁺ and I⁻.
(ii) Calcium hydroxide is prepared by adding water to quicklime: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
When carbon dioxide is passed through calcium hydroxide, the solution turns milky white because calcium carbonate forms: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
PYQ 5
Platform marking points:
A chemical equation is balanced because atoms are neither created nor destroyed during a reaction.
The law is the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction.
3Zn + 2H₃PO₄ → Zn₃(PO₄)₂ + 3H₂
Check: Zn = 3, P = 2, O = 8 and H = 6 on each side.
PYQ 6
Platform marking points:
(a) A reddish-brown copper coating forms on the nail and the blue solution turns pale green: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
(b) A yellow lead iodide precipitate forms: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
(a)(i) NH₃ is the reducing agent. Nitrogen changes from -3 in NH₃ to +2 in NO, so ammonia is oxidised.
(a)(ii) H₂O is the reducing agent. Oxygen changes from -2 in H₂O to 0 in HOF, so water is oxidised.
(a)(iii) CO is the reducing agent. It gains oxygen to form CO₂, so it is oxidised.
(a)(iv) H₂ is the reducing agent. It gains oxygen to form water, so it is oxidised.
(b) A redox reaction is one in which oxidation and reduction occur together. Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen.
PYQ 8
Platform marking points:
(i) Dilute acid supplies ions and increases the electrical conductivity of water.
Hydrogen gas is liberated at the cathode. Oxygen gas is liberated at the anode.
The anode:cathode volume ratio is 1:2.
(ii) White silver chloride turns grey because silver forms.
The reaction is photochemical or photolytic decomposition: 2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g).
PYQ 9
Platform marking points:
(i) In a decomposition reaction, one reactant breaks into two or more products.
2Pb(NO₃)₂(s) →[Heat] 2PbO(s) + 4NO₂(g) + O₂(g)
Brown nitrogen dioxide fumes are released.
(ii) Hydrogen forms at the cathode and oxygen at the anode.
Cathode:anode mass ratio H₂:O₂ = 4:32 = 1:8.
Current-pattern challenge 1
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Both are redox reactions because oxidation and reduction occur together.
In the first reaction, aluminium is oxidised and iron(III) oxide is reduced.
Current-pattern challenge 2
The iron nail gets a reddish-brown copper coating and the blue copper sulphate solution turns pale green.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
The copper strip in iron sulphate solution shows no reaction.
Iron is more reactive than copper, so iron displaces copper; copper cannot displace iron.
Current-pattern challenge 3
Distilled water contains too few ions and is a poor conductor, so current does not pass effectively.
A few drops of dilute sulphuric acid can be added to supply ions. It is used as the electrolyte, not as the gas-forming reactant.
Hydrogen is collected at the cathode and oxygen at the anode.
Current-pattern challenge 4
The green coating is basic copper carbonate.
Do not replace copper with unpainted iron merely to avoid corrosion. Copper's coating remains attached and protects the metal below, whereas iron rust is loose and flaky and keeps exposing fresh iron.
Copper is malleable, so it can be made into thin sheets and shaped for the roof.
Current-pattern challenge 5
2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
It is neutralisation because an acid and a base form salt and water. It is also double displacement because the ions exchange partners.
It is not precipitation because sodium sulphate is soluble and no insoluble solid forms.
Assertion-Reason answers
Option 1. Both statements are true, and the release of heat explains why the quicklime reaction is exothermic.
Option 1. Both statements are true, and photochemical decomposition explains why silver chloride must be protected from light.
Case-based answer
(a) Thermal decomposition.
(b) The blue solution fades and becomes pale green, or a reddish-brown copper coating forms on the nail.
