Lesson note
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:
H₂gains oxygen, soH₂is oxidised and acts as the reducing agent.CuOloses oxygen, soCuOis 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.
- Find the reactant that gains oxygen or loses hydrogen. It is oxidised, so it is the reducing agent.
- Find the reactant that loses oxygen or gains hydrogen. It is reduced, so it is the oxidising agent.
Pass 2 - Use simple oxidation numbers only when oxygen or hydrogen transfer is not obvious.
- An element by itself has oxidation number
0. - Hydrogen is usually
+1. - Oxygen is usually
-2. - Fluorine is always
-1. - The oxidation numbers in a neutral compound add to
0. - An increase means oxidation. A decrease means reduction.
The substance whose oxidation number increases is the reducing agent. The substance whose oxidation number decreases is the oxidising agent.
[TIP: Do not guess the agent from its name or from the side of the equation. Show the change first, then name the agent.]
Verified previous-year questions
PYQ 1 - Observation, reaction type and equation - 2 marks
Source: CBSE Science 2023, paper 31/2/1, Question 21(a), printed page 13.
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.
Write before checking
PYQ 1 written attempt
PYQ 2 - Photochemical decomposition - 3 marks
Source: CBSE Science 2023, paper 31/2/1, Question 27, printed page 17.
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
PYQ 3 - Linked decomposition and combination reactions - 5 marks
Source: CBSE Science 2024, paper 31/4/1, Question 34(b), printed page 19.
(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
PYQ 4 - Precipitation, ions and whitewashing chemistry - 5 marks
Source: CBSE Science 2024, paper 31/4/2, Question 34(b), printed page 19.
(i) Write the balanced chemical equation for:
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
PYQ 5 - Conservation of mass and unfamiliar balancing - 3 marks
Source: CBSE Science 2025, paper 31/4/1, Question 27(A), printed page 15.
Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation. Balance:
Zn + H₃PO₄ → Zn₃(PO₄)₂ + H₂Write before checking
PYQ 5 written attempt
PYQ 6 - Three observations with equations - 3 marks
Source: CBSE Science 2026, paper 31/1/3, Question 26, printed page 15.
What happens when:
(a) An iron nail is dipped in copper(II) sulphate solution?
(b) Potassium iodide solution is mixed with lead nitrate solution?
(c) Silver chloride is exposed to sunlight?
Write balanced chemical equations to support your answer.
Write before checking
PYQ 6 written attempt
PYQ 7 - Reducing agents in unfamiliar equations - 3 marks
Source: CBSE Science 2023, paper 31/1/1, Question 27, printed page 21.
(a) Identify the reducing agent in the following reactions:
(i) 4NH₃ + 5O₂ → 4NO + 6H₂O
(ii) H₂O + F₂ → HF + HOF
(iii) Fe₂O₃ + 3CO → 2Fe + 3CO₂
(iv) 2H₂ + O₂ → 2H₂O
(b) Define a redox reaction in terms of gain or loss of oxygen.
Write before checking
PYQ 7 written attempt
PYQ 8 - Electrolysis and photodecomposition - 3 marks
Source: CBSE Science 2023, paper 31/4/2, Question 33, printed page 17.
(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
PYQ 9 - Lead nitrate decomposition and gas mass ratio - 3 marks
Source: CBSE Science 2024, paper 31/1/3, Question 33, printed page 17.
(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
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 + FeC₂H₆ + 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
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.
- 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
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
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
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
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. (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)
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.
- Electricity splits water:
2H₂O(l) →[Electricity] 2H₂(g) + O₂(g). Electrical energy is supplied. - Light decomposes silver bromide:
2AgBr(s) →[Sunlight] 2Ag(s) + Br₂(g). Light energy is supplied.
Common mistakes
- Writing only the equation when the question also asks for an observation.
- Calling every solid product a precipitate; a precipitate must form from a solution.
- Naming the oxidised substance as the oxidising agent.
- Forgetting states, heat, sunlight or electricity.
- Confusing the
2:1cathode:anode volume ratio with the1:8cathode:anode mass ratio. - Saying acid is added to water as a reactant instead of explaining that it improves conductivity.
- Answering only one part of an
ORchoice and then switching to the other choice.
Board tips
- Circle every command word before answering.
- For a 3-mark three-part question, give one complete labelled point per part.
- Balance first, then add states and the reaction condition.
- Never claim that the platform marking points are the official marking scheme.
Quick practice
- In
CuO + H₂ → Cu + H₂O, name the oxidising agent and reducing agent. - In
PbO + CO → Pb + CO₂, name both agents and show the oxygen change. - Explain why the cathode:anode volume ratio is
2:1but the mass ratio is1:8. - Write an O-I-E-C answer for iron placed in copper sulphate solution.
- Explain why the silver chloride question can earn marks for both observation and equation.
One-minute revision
- Observation answers describe visible evidence before naming the reaction.
- An oxidising agent is reduced; a reducing agent is oxidised.
- Acidified water conducts better because the added acid provides ions.
- In water electrolysis, cathode:anode is
2:1by volume and1:8by mass. - A balanced equation alone may be incomplete if the question asks for state, condition, type or reason.
- Historical questions guide practice, not predictions.
End-of-lesson comic recap
Comic recap
Quick scene, quick smile, quick memory. Read this once and the idea sticks better.