LIWA

Chemical Reactions and Equations

CBSE Class 10 · Science · Chapter 1

Past board exam questions

CBSE 2027 sample paper · 1 mark

Chameleons contain crystals that reflect light and produce colour. Adjusting the spacing between these crystals, they can alter the wavelengths of light reflected, thus changing their skin colour. Based on the above information, we can infer that the change in colour by chameleons is a:

  1. chemical change because change in colour takes place.
  2. chemical change because wavelength of reflected light changes.
  3. physical change because the space between crystals is adjusted.
  4. physical change because light is reflected by crystals.
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physical change because the space between crystals is adjusted.

CBSE 2027 sample paper · 1 mark

A food label is given below. E304 and E307b are added to the flavoured biscuits. FLAVOURED BISCUITS Wheat Flour, Vegetable Oil, Sugar, Salt, Malt Extract (From Barley), Baking Powder, Flavour Enhancers (E621, E635), Onion Powder, Natural Flavour, Maltodextrin (Maize), Chicken, Spices, Antioxidants (E304, E307b From Soy), Emulsifier (Soy Lecithin). CONTAINS SOY AND GLUTEN CONTAINING CEREALS. MAY CONTAIN TRACES OF EGG, MILK, PEANUT, SESAME AND TREE NUT. Which of the following is added to food packets for the same reason?

  1. salt
  2. vinegar
  3. nitrogen
  4. vanilla
Show answer
nitrogen

CBSE 2027 sample paper · 5 marks

I. Refer to the dialogue between Alisha, Drishti and Bhushan in the chemistry lab. They are discussing rusting of iron and formation of ammonia gas. Who all do you agree with and why? Alisha: The formation of ammonia by reaction of nitrogen with hydrogen is a reduction reaction. Dristhti: Both rusting of iron and formation of ammonia are redox reactions. Bhushan: Rusting of iron is an oxidation reaction. II. Assess the following chemical equations. Identify the error(s) and rewrite the equation after correction. (i) MgCl2 + AgNO3 → AgCl2 + MgNO3 (ii) Al + H2SO4 → Al2SO4 + H2O

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I. I agree with Drishti (Alisha and Bhushan are only partially correct). Oxidation and reduction go hand in hand. In the formation of ammonia (N₂ + 3H₂ → 2NH₃), nitrogen is reduced (gains hydrogen) and hydrogen is oxidised. In rusting of iron, iron is oxidised while oxygen is reduced. So both are redox reactions; Alisha and Bhushan each describe only one half. II. (i) Errors: the formulae of silver chloride and magnesium nitrate are wrong and the equation is not balanced. Corrected: MgCl₂ + 2AgNO₃ → 2AgCl + Mg(NO₃)₂ (ii) Errors: aluminium sulphate is Al₂(SO₄)₃, the gas evolved is hydrogen (not water), and the equation is not balanced. Corrected: 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂

CBSE 2027 sample paper · 5 marks

I. Refer to the dialogue between Daljeet and Avik in the chemistry lab. They have learnt about decomposition reactions. Who has understood decomposition reactions correctly? Justify your answer. Daljeet: During a decomposition reaction, a substance decomposes to give two simpler compounds. MgCO3 decomposes on heating to magnesium oxide and carbon dioxide. Avik: No, during a decomposition reaction, a substance decomposes to give two elements. Water on electrolysis gives hydrogen and oxygen, both are elements. II. Analyse the observations given in following two cases. Are these observations correct? Give reasons to support your answer. (i) Iron fillings are added to an aqueous solution of magnesium sulphate. Observation: The colourless solution turns greenish due to formation of iron sulphate. (ii) Aluminium metal is added to dilute sulphuric acid Observation: Brisk effervescence are seen due to evolution of a gas.

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I. Both have understood decomposition only partially. A decomposition reaction is one in which a single substance breaks down into simpler substances. The substance that decomposes has to be a compound, while the products can be either elements or compounds. MgCO₃ → MgO + CO₂ on heating is a decomposition even though both products are compounds; and the electrolysis of water, 2H₂O → 2H₂ + O₂, giving two elements, is also a decomposition reaction. Neither 'two compounds' nor 'two elements' is a requirement. II. (i) Incorrect observation. Iron is less reactive than magnesium, so it cannot displace magnesium from magnesium sulphate; no reaction occurs and the solution stays colourless. (ii) …

CBSE 2026 sample paper · 1 mark

Which of the following equations represent redox reactions and what are the values for ‘p’ and ‘q’ in these equations? Equation 1: Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + p Fe(l) + heat Equation 2: 2C₄H₁₀(g) + 13O₂(g) —△→ 8CO₂(g) + q H₂O(g)

  1. Only equation 1 is a redox reaction, p =1 and q=3
  2. Both equations 1 and 2 are redox reactions, p= 2 and q=4
  3. Only equation 2 is a redox reaction, p= 2 and q= 10
  4. Both equations 1 and 2 are redox reactions, p= 2 and q=10
Show answer
Both equations 1 and 2 are redox reactions, p= 2 and q=10

CBSE 2026 board paper · 1 mark

(i) AgNO₃ + NaCl ⟶ NaNO₃ + AgCl (ii) K₂SO₄ + BaCl₂ ⟶ BaSO₄ + 2KCl Which of the following options clearly describes both the reactions ?

  1. (i) is double displacement, (ii) is displacement reaction.
  2. Both, (i) and (ii) are displacement reactions and precipitation reactions.
  3. Both, (i) and (ii) are double displacement reactions and precipitation reactions.
  4. (i) is displacement, (ii) is double displacement reaction.
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Both, (i) and (ii) are double displacement reactions and precipitation reactions.

CBSE 2026 board paper · 1 mark

Which of the following are combination reactions ? (i) 2KClO3 → 2KCl + 3O2 (ii) MgO + H2O → Mg(OH)2 (iii) 4Al + 3O2 → 2Al2O3 (iv) Zn + FeSO4 → ZnSO4 + Fe

  1. (i) and (iii)
  2. (iii) and (iv)
  3. (ii) and (iv)
  4. (ii) and (iii)
Show answer
(ii) and (iii)

CBSE 2026 board paper · 1 mark

Which of the following is an example of a displacement reaction ?

  1. The combustion of natural gas
  2. The reaction of a reactive metal with dilute acid
  3. The electrolysis of water
  4. The reaction of quick lime with water
Show answer
The reaction of a reactive metal with dilute acid