Class 10 Science · Chapter 1

Chemical Reactions and Equations

201 Class 10 Science questions on Chemical Reactions and Equations from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 1 of 11

1. Q1 NCERT · Exercises
   Which ​of the statements about ​the reaction below ​are incorrect? $$2\text{PbO}(s)+\text{C}(s)\to 2\text{Pb}(s)+{\text{CO}}_2(g)$$

   1. (a)Lead is getting reduced.
   2. (b)Carbon dioxide ​is getting oxidised.
   3. (c)Carbon ​is getting oxidised.
   4. (d)Lead oxide is getting ​reduced.

   1. (a)(i) (a) and (b)
   2. (b)(ii) (a) and (c)
   3. (c)(iii) (a), (b) and (c)
   4. (d)(iv) all
   Show answer

   (a) (i) (a) and (b)

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2. Q2 NCERT · Intext Questions (Page 6)
   Why should a ​magnesium ​ribbon ​be cleaned before ​burning ​in air?
   Show answer

   Cleaning removes the unreactive magnesium oxide layer that stops oxygen reaching the metal, so the ribbon can burn.

   A magnesium ribbon kept in air becomes coated with a thin, dull layer of magnesium oxide ($\text{MgO}$), formed by its slow reaction with the oxygen in air. This oxide layer is unreactive and stops oxygen from reaching the metal underneath, so an uncleaned ribbon does not catch fire easily or burn properly. Rubbing the ribbon with sandpaper removes the layer and exposes fresh magnesium, which then burns readily with a dazzling white flame: $2\text{Mg}(s)+{\text{O}}_2(g)\to 2\text{MgO}(s)$.

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3. Q3 NCERT · Exercises
   $${\text{Fe}}_2{\text{O}}_3+2\text{Al}\to {\text{Al}}_2{\text{O}}_3+2\text{Fe}$$ The above reaction is an example of ​a

   1. (a)combination reaction.
   2. (b)double displacement reaction.
   3. (c)decomposition reaction.
   4. (d)displacement reaction.
   Show answer

   (d) displacement reaction.

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4. Q4 NCERT · Intext Questions (Page 6)
   Write the ​balanced ​equation for the following ​chemical reactions.

   1. (i)Hydrogen + Chlorine → ​Hydrogen chloride
   2. (ii)Barium chloride + ​Aluminium ​sulphate ​→ Barium sulphate + Aluminium chloride
   3. (iii)Sodium + ​Water → Sodium hydroxide + Hydrogen
   Show answer

   The balanced equations are ${\text{H}}_2+{\text{Cl}}_2\to 2\text{HCl}$, $3{\text{BaCl}}_2+{\text{Al}}_2({\text{SO}}_4{)}_3\to 3{\text{BaSO}}_4+2{\text{AlCl}}_3$ and $2\text{Na}+2{\text{H}}_2\text{O}\to 2\text{NaOH}+{\text{H}}_2$.

   (i) ${\text{H}}_2+{\text{Cl}}_2\to 2\text{HCl}$; (ii) $3{\text{BaCl}}_2+{\text{Al}}_2({\text{SO}}_4{)}_3\to 3{\text{BaSO}}_4+2{\text{AlCl}}_3$; (iii) $2\text{Na}+2{\text{H}}_2\text{O}\to 2\text{NaOH}+{\text{H}}_2$

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5. Q5 NCERT · Exercises
   What happens ​when ​dilute hydrochloric acid is ​added to ​iron fillings? Tick ​the correct answer.

   1. (a)Hydrogen gas and iron chloride are produced.
   2. (b)Chlorine gas and iron hydroxide are produced.
   3. (c)No reaction takes place.
   4. (d)Iron salt and water are produced.
   Show answer

   (a) Hydrogen gas and iron chloride are produced.

   Step-by-step solution →
6. Q6 NCERT · Intext Questions (Page 6)
   Write a balanced chemical equation with state symbols for ​the following reactions.

   1. (i)Solutions of ​barium chloride and sodium sulphate in water react to give insoluble barium sulphate ​and the ​solution of sodium chloride.
   2. (ii)Sodium hydroxide solution (in water) ​reacts with hydrochloric acid solution (in water) to produce sodium chloride solution ​and ​water.
   Show answer

   The equations are ${\text{BaCl}}_2(aq)+{\text{Na}}_2{\text{SO}}_4(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$ and $\text{NaOH}(aq)+\text{HCl}(aq)\to \text{NaCl}(aq)+{\text{H}}_2\text{O}(l)$.

   (i) ${\text{BaCl}}_2(aq)+{\text{Na}}_2{\text{SO}}_4(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$; (ii) $\text{NaOH}(aq)+\text{HCl}(aq)\to \text{NaCl}(aq)+{\text{H}}_2\text{O}(l)$

   Step-by-step solution →
7. Q7 NCERT · Exercises
   What ​is ​a balanced ​chemical equation? ​Why should chemical ​equations be balanced?
   Show answer

   A balanced equation has equal atoms of each element on both sides, as required by the law of conservation of mass.

   A balanced chemical equation is one in which the number of atoms of each element is the same on both sides of the arrow. For example $3\text{Fe}(s)+4{\text{H}}_2\text{O}(g)\to {\text{Fe}}_3{\text{O}}_4(s)+4{\text{H}}_2(g)$ is balanced, because iron is $3=3$, oxygen is $4=4$ and hydrogen is $8=8$. Chemical equations must be balanced because of the law of conservation of mass: matter can neither be created nor destroyed in a chemical reaction, so all the atoms present before the reaction must still be present afterwards, only rearranged into new substances. An unbalanced equation would claim that atoms had appeared or vanished, and any quantity worked out from it would be wrong.

   Step-by-step solution →
8. Q8 NCERT · Exercises
   Translate the following statements into chemical equations ​and then balance them.

   1. (a)​Hydrogen gas combines with nitrogen ​to form ammonia.
   2. (b)Hydrogen sulphide gas burns in air to ​give water and sulpur dioxide.
   3. (c)Barium chloride ​reacts with aluminium sulphate to give aluminium chloride ​and a precipitate of barium sulphate.
   4. (d)Potassium metal reacts with water to give potassium ​hydroxide and hydrogen gas.
   Show answer

   Each part is written with correct formulae and balanced by coefficients, e.g. $3{\text{H}}_2+{\text{N}}_2\to 2{\text{NH}}_3$ for (a).

   (a) $3{\text{H}}_2(g)+{\text{N}}_2(g)\to 2{\text{NH}}_3(g)$; (b) $2{\text{H}}_2\text{S}(g)+3{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(l)+2{\text{SO}}_2(g)$; (c) $3{\text{BaCl}}_2(aq)+{\text{Al}}_2({\text{SO}}_4{)}_3(aq)\to 2{\text{AlCl}}_3(aq)+3{\text{BaSO}}_4(s)$; (d) $2\text{K}(s)+2{\text{H}}_2\text{O}(l)\to 2\text{KOH}(aq)+{\text{H}}_2(g)$

   Step-by-step solution →
9. Q9 NCERT · Exercises
   Balance ​the ​following chemical equations.

   1. (a)${\text{HNO}}_3+\text{Ca}(\text{OH}{)}_2\to \text{Ca}({\text{NO}}_3{)}_2+{\text{H}}_2\text{O}$
   2. (b)$\text{NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+{\text{H}}_2\text{O}$
   3. (c)$\text{NaCl}+{\text{AgNO}}_3\to \text{AgCl}+{\text{NaNO}}_3$
   4. (d)${\text{BaCl}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{BaSO}}_4+\text{HCl}$
   Show answer

   Coefficient 2 is needed for ${\text{HNO}}_3$ and ${\text{H}}_2\text{O}$ in (a), $\text{NaOH}$ and ${\text{H}}_2\text{O}$ in (b) and $\text{HCl}$ in (d); (c) is already balanced.

   (a) $2{\text{HNO}}_3+\text{Ca}(\text{OH}{)}_2\to \text{Ca}({\text{NO}}_3{)}_2+2{\text{H}}_2\text{O}$; (b) $2\text{NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+2{\text{H}}_2\text{O}$; (c) $\text{NaCl}+{\text{AgNO}}_3\to \text{AgCl}+{\text{NaNO}}_3$, which is already balanced as printed; (d) ${\text{BaCl}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{BaSO}}_4+2\text{HCl}$

   Step-by-step solution →
10. Q10 NCERT · Exercises
    Write the balanced ​chemical equations for the following reactions.

    1. (a)Calcium hydroxide ​+ Carbon dioxide → Calcium ​carbonate ​+ Water
    2. (b)Zinc + Silver ​nitrate → Zinc nitrate + Silver
    3. (c)Aluminium ​+ Copper ​chloride → Aluminium chloride + Copper
    4. (d)Barium chloride + Potassium sulphate → Barium sulphate ​+ Potassium chloride
    Show answer

    Balancing needs $2{\text{AgNO}}_3$ in (b), $2\text{Al}+3{\text{CuCl}}_2$ in (c) and $2\text{KCl}$ in (d); (a) is already balanced.

    (a) $\text{Ca}(\text{OH}{)}_2(aq)+{\text{CO}}_2(g)\to {\text{CaCO}}_3(s)+{\text{H}}_2\text{O}(l)$, already balanced; (b) $\text{Zn}(s)+2{\text{AgNO}}_3(aq)\to \text{Zn}({\text{NO}}_3{)}_2(aq)+2\text{Ag}(s)$; (c) $2\text{Al}(s)+3{\text{CuCl}}_2(aq)\to 2{\text{AlCl}}_3(aq)+3\text{Cu}(s)$; (d) ${\text{BaCl}}_2(aq)+{\text{K}}_2{\text{SO}}_4(aq)\to {\text{BaSO}}_4(s)+2\text{KCl}(aq)$

    Step-by-step solution →
11. Q11 NCERT · Exercises
    Write ​the balanced chemical equation for ​the following and identify the ​type of reaction in each case.

    1. (a)Potassium bromide(aq) + Barium iodide(aq) → Potassium ​iodide(aq) + ​Barium bromide(s)
    2. (b)Zinc ​carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
    3. (c)Hydrogen(g) + Chlorine(g) → Hydrogen ​chloride(g)
    4. (d)Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
    Show answer

    The reactions are double displacement (a), decomposition (b), combination (c) and displacement (d), each balanced.

    (a) $2\text{KBr}(aq)+{\text{BaI}}_2(aq)\to 2\text{KI}(aq)+{\text{BaBr}}_2(s)$ — double displacement reaction; (b) ${\text{ZnCO}}_3(s)\to \text{ZnO}(s)+{\text{CO}}_2(g)$ — decomposition reaction; (c) ${\text{H}}_2(g)+{\text{Cl}}_2(g)\to 2\text{HCl}(g)$ — combination reaction; (d) $\text{Mg}(s)+2\text{HCl}(aq)\to {\text{MgCl}}_2(aq)+{\text{H}}_2(g)$ — displacement reaction

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12. Q12 NCERT · Exercises
    What does ​one mean ​by exothermic ​and ​endothermic reactions? ​Give ​examples.
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    Exothermic reactions release energy to the surroundings; endothermic reactions absorb it, e.g. burning of natural gas and decomposition of limestone.

    An **exothermic** reaction is one in which energy is released along with the products, so the surroundings become warmer; the energy is written on the product side, $\text{reactants}\to \text{products}+\text{energy}$. Examples: the burning of natural gas, ${\text{CH}}_4(g)+2{\text{O}}_2(g)\to {\text{CO}}_2(g)+2{\text{H}}_2\text{O}(g)+\text{heat}$; respiration, ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6(aq)+6{\text{O}}_2(g)\to 6{\text{CO}}_2(g)+6{\text{H}}_2\text{O}(l)+\text{energy}$; and the slaking of quicklime, $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to \text{Ca}(\text{OH}{)}_2(aq)+\text{heat}$. An **endothermic** reaction is one in which energy is absorbed from the surroundings, so it goes on only while energy is supplied and the surroundings cool; the energy is written on the reactant side, $\text{reactants}+\text{energy}\to \text{products}$. Examples: the thermal decomposition of limestone, ${\text{CaCO}}_3(s)\to \text{CaO}(s)+{\text{CO}}_2(g)$ on strong heating; the decomposition of silver chloride in sunlight, $2\text{AgCl}(s)\to 2\text{Ag}(s)+{\text{Cl}}_2(g)$; and the electrolysis of water, $2{\text{H}}_2\text{O}(l)\to 2{\text{H}}_2(g)+{\text{O}}_2(g)$.

    Step-by-step solution →
13. Q13 NCERT · Exercises
    Why is respiration ​considered ​an ​exothermic ​reaction? ​Explain.
    Show answer

    Respiration releases energy as glucose reacts with oxygen to form carbon dioxide and water, so it is exothermic.

    Respiration is exothermic because energy is **released** when it takes place. During digestion the food we eat is broken down into simpler substances — the carbohydrates in rice, potatoes and bread give glucose. In the cells this glucose combines with the oxygen taken in during breathing: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6(aq)+6{\text{O}}_2(g)\to 6{\text{CO}}_2(g)+6{\text{H}}_2\text{O}(l)+\text{energy}$. The chemical energy stored in the glucose is greater than that stored in the carbon dioxide and water, so the difference is given out to the body and is used to keep it warm, to move the muscles and to build and repair cells. Since energy appears on the product side of the equation, respiration is an exothermic reaction.

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14. Q14 NCERT · Exercises
    Why ​are decomposition reactions called the ​opposite of ​combination reactions? ​Write ​equations for these reactions.
    Show answer

    Combination joins substances into one product and releases energy; decomposition splits one compound into simpler ones and needs energy, so they are opposites.

    Decomposition is called the opposite of combination because the two run the same change in opposite directions, with opposite energy flows. In a combination reaction two or more substances join to give a single product, and energy is usually released; in a decomposition reaction a single compound breaks up into two or more simpler substances, and energy must be supplied as heat, light or electricity. The same pair of substances can show both: $2{\text{H}}_2(g)+{\text{O}}_2(g)\to 2{\text{H}}_2\text{O}(l)$ is combination, while $2{\text{H}}_2\text{O}(l)\stackrel{\text{electricity}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$ is decomposition. A heat-driven pair is $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)$ and ${\text{CaCO}}_3(s)\stackrel{\text{heat}}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$.

    Step-by-step solution →
15. Q15 NCERT · Exercises
    Write ​one equation each ​for decomposition ​reactions ​where energy is supplied in the form ​of heat, ​light ​or electricity.
    Show answer

    Lead nitrate decomposes on heating, silver chloride in sunlight and water on passing electricity.

    Heat: $2{\text{Pb(NO}}_3{)}_2(s)\stackrel{\text{heat}}{\to }2\text{PbO}(s)+4{\text{NO}}_2(g)+{\text{O}}_2(g)$; Light: $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$; Electricity: $2{\text{H}}_2\text{O}(l)\stackrel{\text{electricity}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$.

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16. Q16 NCERT · Exercises
    What is ​the difference ​between displacement ​and double ​displacement ​reactions? ​Write equations for these reactions.
    Show answer

    Displacement: a more reactive element replaces a less reactive one; double displacement: two compounds exchange ions, with no free element.

    In a displacement reaction a more reactive element takes the place of a less reactive one in its compound, so one free element goes in and a different free element comes out: $\text{Fe}(s)+{\text{CuSO}}_4(aq)\to {\text{FeSO}}_4(aq)+\text{Cu}(s)$. In a double displacement reaction two compounds exchange their ions and two new compounds are formed, with no free element anywhere: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$. A displacement is decided by the reactivity series; a double displacement is driven by one product leaving the solution as a precipitate, a gas or water.

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17. Q17 NCERT · Exercises
    In ​the refining ​of silver, the ​recovery ​of silver ​from silver nitrate solution involved displacement ​by copper metal. Write down the reaction involved.
    Show answer

    Copper displaces silver from silver nitrate solution, forming copper(II) nitrate and silver metal.

    $\text{Cu}(s)+2{\text{AgNO}}_3(aq)\to {\text{Cu(NO}}_3{)}_2(aq)+2\text{Ag}(s)$. Copper, being more reactive than silver, displaces it from silver nitrate solution: the silver is set free as the metal and settles out, while the copper passes into solution as blue copper(II) nitrate.

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18. Q18 NCERT · Exercises
    What ​do you mean ​by a ​precipitation reaction? ​Explain by ​giving ​examples.
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    A precipitation reaction forms an insoluble solid when two solutions are mixed, e.g. lead iodide from lead nitrate and potassium iodide.

    A precipitation reaction is one in which an insoluble solid — the precipitate — is formed when two solutions are mixed. The dissolved ions change partners, and the pairing that cannot stay in solution separates out as a solid. For example, lead nitrate solution added to potassium iodide solution gives a yellow precipitate of lead iodide: ${\text{Pb(NO}}_3{)}_2(aq)+2\text{KI}(aq)\to {\text{PbI}}_2(s)+2{\text{KNO}}_3(aq)$. Similarly, sodium sulphate solution added to barium chloride solution gives a white precipitate of barium sulphate: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$. In each case the other product stays dissolved, and both reactions are also double displacement reactions.

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19. Q19 NCERT · Exercises
    Explain ​the following ​in ​terms of ​gain ​or loss of ​oxygen with ​two examples each.

    1. (a)Oxidation
    2. (b)Reduction
    Show answer

    Oxidation is gain of oxygen, e.g. copper to copper oxide; reduction is loss of oxygen, e.g. copper oxide to copper.

    (a) Oxidation is the gain of oxygen by a substance during a reaction. Examples: copper heated in air gains oxygen, $2\text{Cu}(s)+{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{CuO}(s)$; and methane burning gains oxygen, ${\text{CH}}_4(g)+2{\text{O}}_2(g)\to {\text{CO}}_2(g)+2{\text{H}}_2\text{O}(l)$. (b) Reduction is the loss of oxygen by a substance during a reaction. Examples: copper oxide heated in hydrogen loses its oxygen, $\text{CuO}(s)+{\text{H}}_2(g)\stackrel{\text{heat}}{\to }\text{Cu}(s)+{\text{H}}_2\text{O}(l)$; and zinc oxide heated with carbon loses its oxygen, $\text{ZnO}(s)+\text{C}(s)\stackrel{\text{heat}}{\to }\text{Zn}(s)+\text{CO}(g)$. In the last two the oxygen is passed from one substance to another, so each equation is an oxidation and a reduction at once — a redox reaction.

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20. Q20 NCERT · Exercises
    A shiny brown ​coloured ​element ‘X’ on heating in air ​becomes black ​in colour. Name ​the element ​‘X’ and the black ​coloured ​compound formed.
    Show answer

    X is copper; the black compound is copper(II) oxide, $\text{CuO}$, formed when copper combines with oxygen on heating.

    The element $X$ is copper (Cu), and the black compound formed is copper(II) oxide, $\text{CuO}$. On heating in air the shiny brown copper combines with atmospheric oxygen and its surface becomes coated with black copper oxide: $2\text{Cu}(s)+{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{CuO}(s)$. Copper is oxidised in the process, since it gains oxygen.

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