Chemical Reactions & Equations CBQs — CBSE Class 10 (2027) with step-wise solutions

Sample Chemical Reactions & Equations CBQs

1. Question 1MCQ1 mark
   Which of the following is a NECESSARY condition for ALL chemical reactions?
   • The reactants should be in the same state.
   • Energy should be supplied to the reactants.
   • The reactants should be at the same temperature.
   • There should be physical contact between the reactants.
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   1. [1 mark] Correct option: (4) There should be physical contact between the reactants. Reactant particles must come into contact in order to collide and react. The other three are not required by every reaction - reactants may be in different states, energy need not be supplied to an exothermic reaction, and equal temperatures are not necessary.

   Answer: There should be physical contact between the reactants.

2. Question 2Assertion-Reason1 mark
   Assertion (A): A sample of pale yellow silver bromide kept in a sealed box in complete darkness for a month does not turn grey. Reason (R): Silver bromide breaks down into silver and bromine when it absorbs light energy.
   • (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
   • (B) Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion.
   • (C) Assertion is true but Reason is false.
   • (D) Assertion is false but Reason is true.
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   1. [1 mark] (A) Both are true and R explains A: the greying is due to silver formed by photolytic decomposition, 2AgBr → 2Ag + Br₂, which needs light; in darkness no light energy is absorbed, so the sample stays pale yellow.

   Answer: (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

3. Question 3Short answer3 marks
   Tina finds a paper covered with a white substance in a chemistry lab. She keeps the paper near the window of the lab and comes back to pick it up after five hours to take it home. She noticed that the white substance had turned grey. (a) What could be the most likely substance on the paper that Tina found? (b) The substance changed from white to grey. Write the chemical equation for this reaction. (c) State ONE application of this property of the substance seen in daily life.
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   1. [1 mark] (a) silver chloride (AgCl) / silver bromide (AgBr)
   2. [1 mark] (b) 2AgCl → 2Ag + Cl₂ OR 2AgBr → 2Ag + Br₂
   3. [1 mark] (c) in black and white photography. (Accept any other valid answer.)

   Answer: (a) silver chloride / silver bromide; (b) 2AgCl → 2Ag + Cl₂; (c) black and white photography

4. Question 4Case study4 marks
   During Diwali in Varanasi, Mohit reads the label on a packet of sparklers. Each sparkler is an iron wire coated with a paste containing fine aluminium powder, a substance that supplies oxygen, and a binder. When the tip is lit, the aluminium burns with a dazzling white light, gives out a large amount of heat and leaves a white, powdery ash of aluminium oxide (Al₂O₃) on the wire. Mohit's science teacher explains that the oxygen-supplying substance only helps the burning; the main light-producing reaction is that of aluminium with oxygen. She reminds the class to hold sparklers at arm's length, wear cotton clothes and drop used wires into a bucket of sand or water. In a class demonstration, she burns 2.7 g of aluminium powder completely in air on a heat-proof tile and collects 5.1 g of white ash. (i) On the basis of the number of products formed, name the type of reaction between aluminium and oxygen. (1 mark) (ii) Write the balanced chemical equation for the burning of aluminium. (1 mark) (iii) The ash weighs more than the aluminium powder that was burnt. Explain why, and calculate the mass of oxygen that combined with the aluminium. (2 marks)
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   1. [1 mark] (i) Aluminium and oxygen form a single product, aluminium oxide, so it is a combination reaction (also exothermic, since heat and light are given out).
   2. [1 mark] (ii) 4Al(s) + 3O₂(g) → 2Al₂O₃(s) (4 Al and 6 O atoms on each side).
   3. [1 mark] (iii) The ash is aluminium oxide; its mass includes the oxygen taken from the air, which combined with the aluminium, so it is heavier than the metal.
   4. [1 mark] (iii) By conservation of mass, oxygen combined = 5.1 g − 2.7 g = 2.4 g.

   Answer: (i) Combination reaction (a single product, Al₂O₃). (ii) 4Al(s) + 3O₂(g) → 2Al₂O₃(s). (iii) Aluminium combines with oxygen from the air, so the oxide contains the mass of the oxygen as well; mass of oxygen = 5.1 g − 2.7 g = 2.4 g.

5. Question 5Long answer5 marks
   You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO₃, ZnCl₂ and H₂O. In which of the above containers these solutions can be kept?
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   1. [1 mark] Copper + dil HCl: Cu is below H in reactivity → no reaction. SAFE to store in Cu container.
   2. [0.5 mark] Copper + dil HNO₃: HNO₃ is an oxidising acid that attacks Cu (Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O). NOT safe in Cu container.
   3. [0.5 mark] Copper + ZnCl₂: Cu is below Zn in reactivity series — cannot displace Zn. SAFE.
   4. [0.5 mark] Copper + H₂O: Cu does not react with water. SAFE.
   5. [0.5 mark] Aluminium + dil HCl: Al above H — reacts (2Al + 6HCl → 2AlCl₃ + 3H₂). NOT safe.
   6. [0.5 mark] Aluminium + dil HNO₃: Al reacts with dilute HNO₃ producing NO/NO₂. NOT safe. (Concentrated HNO₃ passivates Al but the question specifies dilute.)
   7. [0.5 mark] Aluminium + ZnCl₂: Al is above Zn in reactivity → Al displaces Zn: 2Al + 3ZnCl₂ → 2AlCl₃ + 3Zn. NOT safe.
   8. [0.5 mark] Aluminium + H₂O: Al has a protective oxide layer; does not react with water at room temperature. SAFE.
   9. [0.5 mark] Summary table — Cu container: HCl ✓ HNO₃ ✗ ZnCl₂ ✓ H₂O ✓; Al container: HCl ✗ HNO₃ ✗ ZnCl₂ ✗ H₂O ✓.

   Answer: Cu container: stores HCl, ZnCl₂, H₂O (not HNO₃). Al container: stores only H₂O.

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