Chapter 1: Chemical Reactions and Equations

Important questions — the ones the board asked most (20)

1. Q1Long answer5 marksAsked: 2024

   (a) What is a chemical reaction ? Describe one activity each to show that a chemical change has occurred in which (i) change of colour, and (ii) change in temperature has taken place. OR (b) (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.

   

   Medha Ma’am

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   Model answer · chemical reaction, observable changes

   1. 1.(a) A chemical reaction is a process in which the reactants change into products. It involves the breaking and making of bonds between atoms to produce new substances that have properties different from those of the reactants.1 mark
   2. 2.(i) Change of colour: Take about 2 mL of colourless potassium iodide solution in a test tube. Add a few drops of colourless lead nitrate solution to it. A yellow precipitate of lead iodide forms at once. The colour changes from a colourless solution to a yellow precipitate, which shows that a chemical change has taken place. Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) (yellow precipitate) + 2KNO3(aq) (Lead nitrate) + (Potassium iodide) → (Lead iodide) + (Potassium nitrate)2 marks
   3. 3.(ii) Change in temperature: Take a small amount of calcium oxide (quick lime) in a beaker and slowly add water to it. Touch the beaker. It feels very hot. Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide), releasing a large amount of heat. The rise in temperature shows that a chemical change has taken place. CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat (Quick lime) + (Water) → (Slaked lime)2 marks

   • Write the colour change in full: "from a colourless solution to a yellow precipitate".
   • Write "+ Heat" on the product side of CaO(s) + H2O(l) → Ca(OH)2(aq).
   • In the definition, write "breaking and making of bonds between atoms to produce new substances".

   Marks split as in the board’s published marking scheme

2. Q2Short answer3 marksAsked: 2023

   (a) Identify the reducing agent in the following reactions : (i) 4NH_3 + 5O_2 -> 4NO + 6H_2O (ii) H_2O + F_2 -> HF + HOF (iii) Fe_2O_3 + 3CO -> 2Fe + 3CO_2 (iv) 2H_2 + O_2 -> 2H_2O (b) Define a redox reaction in terms of gain or loss of oxygen.

   

   Medha Ma’am

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   Hear it from Medha Ma’am · 85 s

   Model answer · redox reaction, reducing agent

   1. 1.In the reaction 4NH3 + 5O2 → 4NO + 6H2O, the reducing agent is NH3.0.5 marks
   2. 2.In the reaction H2O + F2 → HF + HOF, the reducing agent is H2O.0.5 marks
   3. 3.In the reaction Fe2O3 + 3CO → 2Fe + 3CO2, the reducing agent is CO.0.5 marks
   4. 4.In the reaction 2H2 + O2 → 2H2O, the reducing agent is H2.0.5 marks
   5. 5.Oxidation is the reaction in which there is an increase in oxygen.0.5 marks
   6. 6.Reduction is the reaction in which there is a loss of oxygen.0.5 marks

   • Write the phrase 'reducing agent' with the correct substance for each of the four reactions separately.
   • Give the definition of oxidation using the exact words 'increase in oxygen'.
   • Give the definition of reduction using the exact words 'loss of oxygen'.

   Marks split from a structure derived from past papers (checked twice); not CBSE’s official scheme

3. Q3Short answer3 marksAsked: 2026

   Translate the following statements into chemical equation and then balance them : (a) Water is added to quicklime (b) Burning of natural gas (c) Thermal decomposition of ferrous sulphate

   

   Medha Ma’am

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   Watch it on the board · 25 s

   Model answer · Writing balanced chemical equations

   1. 1.(a) CaO + H₂O → Ca(OH)₂1 mark
   2. 2.(b) CH₄ + 2O₂ → CO₂ + 2H₂O1 mark
   3. 3.(c) 2FeSO₄ → (heat) Fe₂O₃ + SO₂ + SO₃1 mark

   Marks split as in the board’s published marking scheme

4. Q4Short answer3 marksAsked: 2026

   2 g of green coloured crystals of ferrous sulphate are heated in a dry boiling tube. Name the type of chemical reaction taking place. Write the balanced chemical equation for the reaction. Is this an exothermic or an endothermic reaction ?

   How the model answer begins

   1. The reaction is a thermal decomposition reaction. When heated, the green ferrous sulphate crystals first lose water and change colour. Then the single reactant, ferrous sulphate, breaks down into simpler products…

   The full answer from Medha Ma’am →
5. Q5Short answer3 marksAsked: 2026

   Give reasons : (i) Potato chips manufacturers fill the packet of chips with nitrogen gas. (ii) Respiration is an exothermic reaction. (iii) Silver chloride is stored in dark coloured bottles.

   How the model answer begins

   1. (i) Potato chips manufacturers fill packets of chips with nitrogen gas to stop the chips from getting oxidised. When the fats and oils in chips are oxidised, they become rancid, and their smell and taste change.…

   The full answer from Medha Ma’am →
6. Q6Short answer3 marksAsked: 2026

   Account for the following : (a) White silver chloride turns grey when exposed to sunlight. (b) Lead (II) nitrate on heating releases brown fumes. (c) Bags of oil and fat containing food items are flushed with nitrogen gas.

7. Q7Short answer3 marksAsked: 2025

   (A) Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical equation : Zn + H3 PO4 -> Zn3 (PO4)2 + H2 OR (B) Define a precipitation reaction. Give its example and also express the reaction that occurs in the form of a balanced chemical equation.

   How the model answer begins

   1. We balance a chemical equation so that the number of atoms of each element remains the same, before and after a chemical reaction. Atoms do not disappear from the mixture or appear from elsewhere during a reaction, so a…

   The full answer from Medha Ma’am →
8. Q8Short answer3 marksAsked: 2025

   Write balanced chemical equation for the reactions that occur when (a) steam is passed over red hot iron. (b) natural gas is burnt in air. (c) glucose reacts with oxygen in the cells of our body and provides energy.

9. Q9Short answer3 marksAsked: 2024

   Write one chemical equation each for the chemical reaction in which the following have taken place : (i) Change in colour (ii) Change in temperature (iii) Formation of precipitate Mention colour change/temperature change (rise/fall)/compound precipitated along with equation.

10. Q10Short answer3 marksAsked: 2023

    27. (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.

    How the model answer begins

    1. Pure water is a poor conductor of electricity, so a few drops of acid are added to the water before passing the current to increase the electrical conductivity of water.

    The full answer from Medha Ma’am →
11. Q11Short answer3 marksAsked: 2026

    (a) Name the substance oxidised and reduced in the following reaction : ZnO + C -> Zn + CO (b) Balance the following chemical reaction : Pb(NO3)2 + KI -> PbI2 + KNO3 (c) Give one example each of electrolytic decomposition and decomposition by sunlight.

12. Q12Short answer3 marksAsked: 2026

    Seema took a small amount of copper powder in a china dish and heated it. What changes will she observe on heating ? When H_2 gas is passed over this heated substance, what visible changes will be seen in it ? Give the chemical equations of the reactions involved, along with the names and colours of the products in each case.

    How the model answer begins

    1. On heating, the brown copper powder becomes coated with a black substance, because copper combines with oxygen of the air.

    The full answer from Medha Ma’am →
13. Q13Short answer3 marksAsked: 2026

    What happens when (i) lead (II) nitrate is heated ? (ii) iron (II) sulphate is heated ? (iii) silver chloride is exposed to sunlight ? Write balanced chemical equations of the reactions involved in support of your answer.

14. Q14Short answer3 marksAsked: 2025

    (a) (i) Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity. (ii) Decomposition of vegetable matter into compost is considered an exothermic reaction. Why ? OR (b) Why are decomposition reactions called the opposite of combination reactions ? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.

15. Q15Short answer3 marksAsked: 2025

    27. How is a double displacement reaction different from a displacement reaction ? Explain giving example in the form of balanced chemical equations.

    How the model answer begins

    1. In a displacement reaction, a more reactive element displaces a less reactive element from its compound: Fe + CuSO₄ → FeSO₄ + Cu.

    The full answer from Medha Ma’am →
16. Q16Short answer3 marksAsked: 2024

    Study the experimental set-up shown in the diagram and write chemical equation for the chemical reaction involved. Name and define the type of reaction. List two other metals which can be used in place of iron to show the same type of reaction with copper sulphate solution.

17. Q17Short answer3 marksAsked: 2023

    27. 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.

18. Q18Short answer3 marksAsked: 2023

    (a) Define a double displacement reaction. (b) Write the chemical equation of a double displacement reaction which is also a (i) Neutralization reaction and (ii) Precipitation reaction. Give justification for your answer.

    How the model answer begins

    1. (a) Double displacement reactions are reactions in which there is an exchange of ions between the reactants. Two different atoms or groups of atoms (ions) are exchanged.

    The full answer from Medha Ma’am →
19. Q19Very short answer2 marksAsked: 2024

    (a) "No precipitation reaction can occur without exchange of ions between the two reactants." Justify this statement giving a balanced chemical equation for the reaction. OR (b) Giving one example of each, differentiate between a displacement reaction and a double displacement reaction.

    How the model answer begins

    1. The statement is true because precipitation reactions produce insoluble salts, and an insoluble salt can form only when the ions of the two reactants exchange partners. Reactions in which there is an exchange of ions…

    The full answer from Medha Ma’am →
20. Q20Very short answer2 marksAsked: 2025

    Define oxidation. Identify and name the substance oxidised in the following reaction : CuO + H2 -> Cu + H2O

17 more questions of this chapter, each with its model answer, in the AskMedha app.

Objective questions with the board’s key (10)

1. 1. CBSE 2026

   The gases evolved on heating lead (II) nitrate crystals are :

   • (A) NO and O2
   • (B) N2 and NO2
   • (C) NO2 and H2
   • (D) NO2 and O2 ✓

   Answer: (D)

2. 2. CBSE 2026

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

   • (A) (i) is double displacement, (ii) is displacement reaction.
   • (B) Both, (i) and (ii) are displacement reactions and precipitation reactions.
   • (C) Both, (i) and (ii) are double displacement reactions and precipitation reactions. ✓
   • (D) (i) is displacement, (ii) is double displacement reaction.

   Answer: (C)

3. 3. CBSE 2026

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

   • (A) The combustion of natural gas
   • (B) The reaction of a reactive metal with dilute acid ✓
   • (C) The electrolysis of water
   • (D) The reaction of quick lime with water

   Answer: (B)

4. 4. CBSE 2026

   In the following equation x KI + H2O2 -> I2 + y KOH x and y respectively are :

   • (A) 2, 2 ✓
   • (B) 1, 2
   • (C) 2, 1
   • (D) 1, 1

   Answer: (A)

5. 5. CBSE 2026

   For question number 24, two statements are given - one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to this question from the codes (A), (B), (C) and (D) as given below.

   • (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A). ✓
   • (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
   • (C) Assertion (A) is true, but Reason (R) is false.
   • (D) Assertion (A) is false, but Reason (R) is true.

   Answer: (A)

6. 6. CBSE 2026

   The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is

   • (A) 1 : 1
   • (B) 2 : 1 ✓
   • (C) 4 : 1
   • (D) 1 : 2

   Answer: (B)

7. 7. CBSE 2026

   C_6H_12O_6(aq) + 6O_2(g) -> 6CO_2(g) + 6H_2O(l). The above reaction is an example of -

   • (A) Displacement Reaction
   • (B) Endothermic Reaction
   • (C) Exothermic Reaction ✓
   • (D) Neutralisation Reaction

   Answer: (C)

8. 8. CBSE 2026

   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 Options :

   • (A) (i) and (iii)
   • (B) (iii) and (iv)
   • (C) (ii) and (iv)
   • (D) (ii) and (iii) ✓

   Answer: (D)

9. 9. CBSE 2026

   When ferrous sulphate crystals are heated in a test tube, we observe that :

   • (A) A colourless gas with no smell is evolved.
   • (B) A brown gas is evolved.
   • (C) The green colour of the salt fades and a gas with the smell of burning sulphur is evolved. ✓
   • (D) The green colour of the salt fades and no gas is evolved.

   Answer: (C)

10. 10. CBSE 2026

    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

    • (A) (i) and (iii)
    • (B) (iii) and (iv)
    • (C) (ii) and (iv)
    • (D) (ii) and (iii) ✓

    Answer: (D)

Questions parents and students ask

How many marks does "Chemical Reactions and Equations" carry in CBSE Class 10 Science?

In the CBSE papers of 2023–2026, this chapter carried about 6 marks and about 4 questions a year; by marks it ranks 8 in Science.

Which are the most important (VVI) questions of "Chemical Reactions and Equations"?

The 20 questions above are from the board’s real papers, each with the years it was asked; the most-asked come first with their full model answers. All 37 written questions of this chapter, with answers, are in the AskMedha app.

Do the answers follow the board’s marking scheme?

Yes. CBSE publishes its marking schemes; every model answer here is written point by point to the marks the board’s own scheme gives, and checked by a second, independent model.