Chapter 1 · Chemical Reactions and Equations

NCERT Class 10 Science Chapter 1 Exercises

All 20 questions of NCERT Class 10 Science Chapter 1 Exercises (Chemical Reactions and Equations), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   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
   $${\text{Fe}}_2{\text{O}}_3+2\text{Al}\to {\text{Al}}_2{\text{O}}_3+2\text{Fe}$$ 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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3. Q3
   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.

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4. Q4
   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.

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5. Q5
   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 →
6. Q6
   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) ${\text{BaCl}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{BaSO}}_4+2\text{HCl}$

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7. Q7
   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 →
8. Q8
   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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9. Q9
   What does ​one mean ​by exothermic ​and ​endothermic reactions? ​Give ​examples.
   Show answer

   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, $2{\text{H}}_2\text{O}(l)\to 2{\text{H}}_2(g)+{\text{O}}_2(g)$.

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10. Q10
    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.

    Step-by-step solution →
11. Q11
    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 ${\text{CaCO}}_3(s)\stackrel{\text{heat}}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$.

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12. Q12
    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: $2{\text{H}}_2\text{O}(l)\stackrel{\text{electricity}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$.

    Step-by-step solution →
13. Q13
    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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14. Q14
    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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15. Q15
    What ​do you mean ​by a ​precipitation reaction? ​Explain by ​giving ​examples.
    Show answer

    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.

    Step-by-step solution →
16. Q16
    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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17. Q17
    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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18. Q18
    Why ​do ​we apply paint on iron ​articles?
    Show answer

    Paint forms a continuous film that cuts off air and moisture, so the iron cannot rust.

    Paint is applied to iron articles to prevent rusting, which is the corrosion of iron. Iron rusts only when oxygen of the air and moisture reach its surface together, as the equation $4\text{Fe}(s)+3{\text{O}}_2(g)+2x{\text{H}}_2\text{O}(l)\to 2{\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}(s)$ shows. A coat of paint forms a continuous impervious film over the metal and cuts off both air and moisture at once, so the reaction cannot begin. This matters because rust is flaky and porous and does not protect the iron beneath it: once rusting starts it eats steadily deeper and weakens the article, so a large amount of iron would otherwise be lost. The protection is purely a physical barrier, which is why rusting begins at any point where the paint is scratched or chipped.

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19. Q19
    Oil ​and ​fat ​containing ​food ​items are flushed ​with nitrogen. Why?
    Show answer

    Nitrogen is unreactive and drives out oxygen, so the fats and oils cannot be oxidised and turn rancid.

    Oils and fats are slowly oxidised by the oxygen of the air, and the oxidation products give the food an unpleasant smell and taste — the food is said to have become rancid. Nitrogen is an unreactive gas that does not attack fats and oils, so flushing the packet with it drives out the air and leaves almost no oxygen in contact with the food. With the reactant removed, the oxidation cannot take place, the food is kept from turning rancid, and its shelf life is extended. Nitrogen is not itself a preservative — it protects only by occupying the space that oxygen would otherwise fill.

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20. Q20
    Explain the ​following ​terms ​with one example ​each.

    1. (a)​Corrosion
    2. (b)​Rancidity
    Show answer

    Corrosion, e.g. rusting of iron, slowly eats away a metal; rancidity, e.g. in stale butter, is oxidation of fats and oils.

    (a) Corrosion is the slow eating away of a metal surface by the action of the air, moisture or chemicals around it, in which the metal at the surface is converted into a compound. Example: the rusting of iron, in which an iron article exposed to damp air becomes coated with flaky reddish-brown rust, $4\text{Fe}(s)+3{\text{O}}_2(g)+2x{\text{H}}_2\text{O}(l)\to 2{\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}(s)$. (b) Rancidity is the spoiling of fats and oils in a food by oxidation on standing in air, which changes their smell and taste and makes them unpleasant. Example: butter or fried snacks left for a long time in an open container develop a bad smell and taste. Both are slow oxidation by atmospheric oxygen, and both are prevented by keeping the air away — by painting or oiling the metal, and by airtight packing, antioxidants or flushing with nitrogen for the food.

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