Class 10 Science · Chapter 1

Chemical Reactions and Equations

298 Class 10 Science questions on Chemical Reactions and Equations from the NCERT textbook, NCERT Exemplar and CBSE board papers 2018–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 13 of 15

1. Q241 CBSE 2023
   When ​copper powder ​is heated in a watch glass, ​a black substance is formed.

   1. (i)Why is this black substance formed ? Name ​it.
   2. (ii)How can this ​black substance ​be ​reversed to ​its original form ?
   Show answer

   Copper combines with oxygen of the air to form black copper(II) oxide; passing hydrogen over it gives copper back.

   (i) On heating in the open air the hot copper combines with the oxygen of the air, that is, the copper is oxidised. The black substance is copper(II) oxide, $\text{CuO}$: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$. (ii) Pass hydrogen gas over the heated black copper oxide. The hydrogen removes the oxygen, that is, the copper oxide is reduced, and the reddish brown copper is obtained back: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$.

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2. Q242 CBSE 2023
   Why ​does the colour of ​iron ​sulphate ​solution fade when a piece of zinc is dipped in ​it? Give chemical equation for the ​reaction involved.
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   Zinc is more reactive, so it displaces iron; green ${\text{Fe}}^{2+}$ ions are replaced by colourless ${\text{Zn}}^{2+}$ ions and the colour fades.

   Zinc is more reactive than iron, so it displaces iron from iron(II) sulphate solution. The light green colour of the solution is due to ${\text{Fe}}^{2+}$ ions; as these are displaced and deposited as grey iron on the zinc, they are replaced by colourless ${\text{Zn}}^{2+}$ ions, so the green colour fades. The reaction is $\text{Zn}(s)+{\text{FeSO}}_4(aq)\to {\text{ZnSO}}_4(aq)+\text{Fe}(s)$.

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3. Q243 CBSE 2023
   When ​a zinc ​plate ​is dipped in ​ferrous sulphate solution for some time, it ​is observed that the pale green colour of the solution disappears.

   1. (i)Explain the ​observation.
   2. (ii)Write the chemical equation ​for ​the reaction taking place.
   Show answer

   Zinc is more reactive than iron and displaces it, so pale green ${\text{Fe}}^{2+}$ is replaced by colourless ${\text{Zn}}^{2+}$.

   (i) Zinc is more reactive than iron, so the zinc plate displaces iron from ferrous sulphate solution. The pale green ${\text{Fe}}^{2+}$ ions leave the solution and are deposited on the plate as greyish-black iron, while their place is taken by ${\text{Zn}}^{2+}$ ions, whose solution is colourless. That is why the pale green colour of the solution disappears. It is a displacement reaction. (ii) $\text{Zn}(s)+{\text{FeSO}}_4(aq)⟶{\text{ZnSO}}_4(aq)+\text{Fe}(s)$

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4. Q244 CBSE 2023
   (a) Identify the ​reducing agent in ​the ​following reactions ​:

   1. (i)$4{\text{NH}}_3+5{\text{O}}_2\to 4\text{NO}+6{\text{H}}_2\text{O}$
   2. (ii)${\text{H}}_2\text{O}+{\text{F}}_2\to \text{HF}+\text{HOF}$
   3. (iii)${\text{Fe}}_2{\text{O}}_3+3\text{CO}\to 2\text{Fe}+3{\text{CO}}_2$
   4. (iv)$2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$ (b) Define a ​redox ​reaction ​in terms of gain or loss of oxygen.
   Show answer

   The reducing agents are ${\text{NH}}_3$, ${\text{H}}_2\text{O}$, $\text{CO}$ and ${\text{H}}_2$; in a redox reaction one reactant gains oxygen while another loses it.

   (a) Reducing agents: (i) ${\text{NH}}_3$ (ammonia); (ii) ${\text{H}}_2\text{O}$ (water); (iii) $\text{CO}$ (carbon monoxide); (iv) ${\text{H}}_2$ (hydrogen). (b) A redox reaction is a reaction in which one reactant gains oxygen and is said to be oxidised, while at the same time the other reactant loses oxygen and is said to be reduced. Oxidation and reduction take place together in the same reaction, and the substance that is itself oxidised is the reducing agent.

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5. Q245 CBSE 2023
   Silver chloride kept in a china dish ​turns ​grey in sunlight.

   1. (a)Write the colour of silver chloride when it ​was kept ​in the china dish.
   2. (b)Name the type of chemical reaction taking place and ​write ​the chemical equation for the reaction.
   3. (c)State one use of the reaction. ​Name one more chemical which can be used for the same purpose.
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   White silver chloride decomposes in light to grey silver and chlorine; the reaction is used in black and white photography, as is silver bromide.

   (a) Silver chloride is **white** when it is kept in the china dish. (b) It is a decomposition reaction, and since the energy for it comes from light it is called a photolytic (photochemical) decomposition: $2\text{AgCl}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$. The grey colour is that of finely divided silver metal. (c) The reaction is used in black and white photography. Silver bromide, $\text{AgBr}$, is another chemical used for the same purpose.

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6. Q246 CBSE 2023
   1. (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.
   2. (ii)What is ​observed when ​silver chloride is exposed ​to ​sunlight ? Give the type of reaction involved.
   Show answer

   Dilute acid makes water conduct; the anode-to-cathode volume ratio is $1:2$; silver chloride turns grey by photolytic decomposition.

   (i) Pure water ionises only very slightly and is therefore a very poor conductor of electricity, so a few drops of dilute acid are added to supply ions and make the water conducting; without them almost no current would pass. Hydrogen is liberated at the cathode and oxygen at the anode. Since $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$, the volume of gas collected at the anode is half the volume collected at the cathode, that is $V_{\text{anode}}:V_{\text{cathode}}=1:2$. (ii) The white silver chloride turns grey, because it decomposes in sunlight to silver and chlorine: $2\text{AgCl}\to 2\text{Ag}+{\text{Cl}}_2$. This is a photolytic (photochemical) decomposition reaction.

   Step-by-step solution →
7. Q247 CBSE 2023
   1. (a)Define a double displacement ​reaction.
   2. (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.
   Show answer

   A double displacement reaction exchanges ions between two compounds; $\text{NaOH}+\text{HCl}$ is also neutralisation and ${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2$ is also precipitation.

   (a) A double displacement reaction is one in which two compounds exchange their ions (radicals) with each other to form two new compounds: $\text{AB}+\text{CD}⟶\text{AD}+\text{CB}$. (b) (i) Neutralisation: $\text{NaOH}+\text{HCl}⟶\text{NaCl}+{\text{H}}_2\text{O}$. Justification: the sodium and hydrogen ions change partners, so it is a double displacement; and an acid reacts with a base to give a salt and water, so it is also a neutralisation. (ii) Precipitation: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)⟶{\text{BaSO}}_4(s)+2\text{NaCl}(aq)$. Justification: the ions are exchanged, so it is a double displacement; and barium sulphate is insoluble and comes out as a white precipitate, so it is also a precipitation reaction.

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8. Q248 CBSE 2023
   A ​student is heating ferrous sulphate crystals ​in a boiling tube.

   1. (a)State the change in colour he is likely to observe during the process of ​heating.
   2. (b)​Name the gases ​produced during heating.
   3. (c)​Write ​chemical equation for the reaction.
   Show answer

   Green crystals turn white, then reddish-brown, giving sulphur dioxide and sulphur trioxide: $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$.

   (a) The green crystals first turn white as the water of crystallisation is driven off, and on further heating the residue becomes reddish-brown. (b) The gases produced are sulphur dioxide (${\text{SO}}_2$) and sulphur trioxide (${\text{SO}}_3$), which have the smell of burning sulphur. (c) $2{\text{FeSO}}_4\stackrel{\Delta }{\to }{\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$ (the first change being ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}\stackrel{\Delta }{\to }{\text{FeSO}}_4+7{\text{H}}_2\text{O}$).

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9. Q249 CBSE 2023
   Consider the types of reaction mentioned below in (i) and (ii) :

   1. (i)$\text{AB}+{\text{C}}_2\text{B}⟶{\text{A(BC)}}_2$
   2. (ii)${\text{ABO}}_3⟶\text{AO}+{\text{BO}}_2$ (a) Identify ​the types of reactions ​mentioned in the chemical ​equations (i) and (ii). (b) Give one example ​for each type ​of reaction in the ​form of word equation or ​chemical equation.
   Show answer

   Equation (i) is a combination reaction and (ii) a decomposition reaction; for example, calcium oxide with water, and calcium carbonate on heating.

   (a) (i) Combination reaction - two substances combine to form a single product. (ii) Decomposition reaction - a single substance breaks down into two products. (b) (i) Combination: calcium oxide + water $\to$ calcium hydroxide, $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)$. (ii) Decomposition: calcium carbonate $\to$ calcium oxide + carbon dioxide, ${\text{CaCO}}_3(s)\stackrel{\text{Heat}}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$.

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10. Q250 CBSE 2023
    Write down the balanced chemical equations ​for the following reactions ​and identify the type of reaction in each case.

    1. (a)Nitrogen gas is treated with ​hydrogen ​gas to form ammonia gas.
    2. (b)​Lead nitrate is ​heated ​strongly to form lead monoxide, nitrogen dioxide and oxygen.
    3. (c)A copper wire is ​dipped in silver nitrate solution and a shining deposit of silver is produced.
    Show answer

    Ammonia formation is combination, heating lead nitrate is thermal decomposition, and copper in silver nitrate is displacement.

    (a) ${\text{N}}_2(\text{g})+3{\text{H}}_2(\text{g})\to 2{\text{NH}}_3(\text{g})$: combination reaction. (b) $2{\text{Pb(NO}}_3{)}_2(\text{s})\stackrel{\text{heat}}{\to }2\text{PbO}(\text{s})+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$: decomposition reaction (thermal decomposition). (c) $\text{Cu}(\text{s})+2{\text{AgNO}}_3(\text{aq})\to {\text{Cu(NO}}_3{)}_2(\text{aq})+2\text{Ag}(\text{s})$: displacement reaction.

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11. Q251 CBSE 2023
    1. (a)Define ​oxidation.
    2. (b)Identify the oxidising agent ​in the following reactions and balance ​the ​chemical equations : (i) ${\text{Pb}}_3{\text{O}}_4+\text{HCl}⟶{\text{PbCl}}_2+{\text{Cl}}_2+{\text{H}}_2\text{O}$ (ii) ${\text{MnO}}_2+\text{Al}⟶\text{Mn}+{\text{Al}}_2{\text{O}}_3$
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    Oxidation is gain of oxygen or loss of hydrogen; ${\text{Pb}}_3{\text{O}}_4$ and ${\text{MnO}}_2$ are the oxidising agents, and both equations are balanced.

    (a) Oxidation is the gain of oxygen by a substance, or the loss of hydrogen from it, during a reaction. (b) (i) The oxidising agent is ${\text{Pb}}_3{\text{O}}_4$: it loses oxygen and is reduced, while $\text{HCl}$ loses hydrogen and is oxidised to chlorine. Balanced: ${\text{Pb}}_3{\text{O}}_4+8\text{HCl}\to 3{\text{PbCl}}_2+{\text{Cl}}_2+4{\text{H}}_2\text{O}$. (ii) The oxidising agent is ${\text{MnO}}_2$: it gives its oxygen to aluminium, which is oxidised to aluminium oxide. Balanced: $3{\text{MnO}}_2+4\text{Al}\to 3\text{Mn}+2{\text{Al}}_2{\text{O}}_3$.

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12. Q252 CBSE 2023
    Observe the given ​diagram and answer ​the following questions :

    1. (a)​Write a balanced chemical equation for the reaction taking place in the beaker.
    2. (b)Name ​the two types of ​reactions in which ​the above reaction can be placed, ​giving justification for each.

    Show answer

    Quicklime reacts with water to form slaked lime and heat; the reaction is both a combination reaction and an exothermic reaction.

    (a) $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{Heat}$. Calcium oxide (quicklime) reacts with water to form calcium hydroxide (slaked lime), and a large amount of heat is released. (b) (i) Combination reaction: two reactants, calcium oxide and water, combine to form a single product, calcium hydroxide. (ii) Exothermic reaction: heat is released along with the formation of the product, which is why the beaker becomes hot to the touch.

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13. Q253 CBSE 2023
    With the help of ​an appropriate example, justify ​that some of the chemical reactions ​are determined by

    1. (a)​Change in temperature,
    2. (b)​Evolution of a ​gas, and
    3. (c)​Change in colour Give chemical equation for the reaction involved in each case.
    Show answer

    Quicklime in water gives heat, zinc in dilute sulphuric acid gives hydrogen bubbles, and an iron nail in copper sulphate changes the colour.

    (a) Change in temperature — quick lime reacts with water and the beaker becomes hot, because the reaction is exothermic: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)+\text{heat}$. (b) Evolution of a gas — zinc granules in dilute sulphuric acid give bubbles of hydrogen: $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)⟶{\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$. (c) Change in colour — an iron nail left in blue copper sulphate solution turns the solution pale green and becomes coated with reddish-brown copper: $\text{Fe}(s)+{\text{CuSO}}_4(aq)⟶{\text{FeSO}}_4(aq)+\text{Cu}(s)$.

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14. Q254 CBSE 2023
    A reddish brown ​metal used in electrical wires when powdered and heated strongly ​turns ​black. When hydrogen gas is passed over this black substance, it regains its original colour. Based on ​this information answer the following ​questions –

    1. (a)Name the metal and the black substance formed.
    2. (b)​Write balanced chemical equations for the two reactions involved in ​the ​above information.
    Show answer

    The metal is copper and the black substance is copper(II) oxide; $2\text{Cu}+{\text{O}}_2\to 2\text{CuO}$ and $\text{CuO}+{\text{H}}_2\to \text{Cu}+{\text{H}}_2\text{O}$.

    (a) The metal is copper, Cu. The black substance is copper(II) oxide, $\text{CuO}$. (b) On heating in air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ (copper is oxidised). When hydrogen is passed over the heated black oxide: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$ (copper oxide is reduced to copper, and hydrogen is oxidised to water).

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15. Q255 CBSE 2023
    State the ​change ​in colour observed in each of the ​following cases mentioning the reason :

    1. (a)​Silver chloride is ​exposed ​to sunlight.
    2. (b)A piece ​of zinc is dipped in ferrous ​sulphate solution.
    3. (c)Copper powder is strongly heated in air.
    Show answer

    Silver chloride goes from white to grey, ferrous sulphate solution from pale green to colourless, and copper powder from reddish-brown to black.

    (a) White silver chloride turns grey. Sunlight decomposes it into silver, which gives the grey colour, and chlorine: $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$. (b) The pale green colour of the ferrous sulphate solution fades and the solution becomes colourless, and a greyish-black coating of iron forms on the zinc. Zinc is more reactive than iron and displaces it: $\text{Zn}(s)+{\text{FeSO}}_4(aq)\to {\text{ZnSO}}_4(aq)+\text{Fe}(s)$; zinc sulphate solution is colourless. (c) The reddish-brown copper powder turns black. Heated copper combines with the oxygen of the air to form black copper(II) oxide: $2\text{Cu}(s)+{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{CuO}(s)$.

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16. Q256 CBSE 2023
    Decomposition is ​a type of reaction in which a single ​compound splits into two or more simple compounds. ​Decomposition reactions ​are generally classified into ​three types, depending upon the type of energy ​required for the reaction. The three ​types are thermal ​decomposition, photolytic decomposition and electrolytic decomposition.

    1. (a)Why are decomposition reactions known as endothermic reactions ?[1]
    2. (b)Give an example of photolytic decomposition.[1]
    3. (c)(i)Name the gas liberated at the (i) cathode, and (ii) anode when water is electrolysed. Write the molar ratio of hydrogen and oxygen (Hydrogen : Oxygen) liberated during electrolysis.[2]
    4. (c)(ii)OR : What happens when lead nitrate powder is heated in a boiling tube ? Write chemical equation for the reaction.[2]
    Show answer

    Decomposition reactions absorb energy, so they are endothermic; silver chloride decomposes in sunlight; electrolysis of water gives hydrogen and oxygen in the ratio $2:1$.

    (a) Because decomposition reactions need energy to be supplied from outside to break the single compound into simpler substances - heat in thermal decomposition, light in photolytic decomposition and electricity in electrolytic decomposition. A reaction that absorbs energy from its surroundings is endothermic, so decomposition reactions are endothermic. (b) Silver chloride kept in sunlight turns grey because it decomposes into silver and chlorine: $2\text{AgCl}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$. (Silver bromide, $2\text{AgBr}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Br}}_2$, is an equally good example.) (c)(i) Hydrogen is liberated at the cathode and oxygen at the anode. From $2{\text{H}}_2\text{O}\stackrel{\text{electricity}}{\to }2{\text{H}}_2+{\text{O}}_2$, the molar ratio of hydrogen to oxygen is $2:1$ - which is why the volume of hydrogen collected is twice that of oxygen. OR (c)(ii) The white lead nitrate powder decomposes: a yellow residue of lead oxide is left in the boiling tube, thick brown fumes of nitrogen dioxide are given off, and oxygen is also released. It is a thermal decomposition reaction: $2{\text{Pb(NO}}_3{)}_2\stackrel{\text{heat}}{\to }2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$.

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17. Q257 CBSE 2020
    Calcium oxide ​reacts vigorously with water ​to produce slaked ​lime. $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}$ This reaction can ​be ​classified as : (A) Combination reaction (B) Exothermic reaction (C) Endothermic reaction (D) Oxidation ​reaction Which of the ​following is a correct option ?

    1. (a)(A) and (C)
    2. (b)(C) and (D)
    3. (c)(A), (C) and (D)
    4. (d)(A) and (B)
    Show answer

    (d) (A) and (B)

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18. Q258 CBSE 2020
    When hydrogen sulphide gas is passed through a blue solution ​of copper ​sulphate, a ​black ​precipitate of copper sulphide is ​obtained and the sulphuric acid so formed remains in the solution. The reaction ​is an ​example of a :

    1. (a)Combination reaction
    2. (b)Displacement reaction
    3. (c)Decomposition reaction
    4. (d)Double displacement reaction
    Show answer

    (d) Double displacement reaction

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19. Q259 CBSE 2020
    In ​a double ​displacement reaction such ​as the reaction ​between sodium sulphate solution ​and barium chloride ​solution : (A) exchange of atoms takes place (B) exchange of ions takes place (C) a ​precipitate is produced (D) an ​insoluble salt is produced The correct option is :

    1. (a)(B) and (D)
    2. (b)(A) and (C)
    3. (c)only (B)
    4. (d)(B), (C) and (D)
    Show answer

    (d) (B), (C) and (D)

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20. Q260 CBSE 2020
    Identify ‘$x$’, ​‘$y$’ and ​‘$z$’ ​in the following ​reaction : $2{\text{KClO}}_3(x)\stackrel{(y)}{\to }2\text{KCl}(x)+{\text{O}}_2(z)$

    1. (a)$x$ = gas ; $y$ = reaction condition, $z$ = gas
    2. (b)$x$ = solid ; $y$ = liquid; $z$ = gas
    3. (c)$x$ = number of moles of ${\text{KClO}}_3$ ; $y$ = reaction condition; $z$ = no. of molecules of oxygen.
    4. (d)$x$ = physical state of ${\text{KClO}}_3$ and $\text{KCl}$; $y$ = reaction condition; $z$ = physical state of ${\text{O}}_2$.
    Show answer

    (d) $x$ = physical state of ${\text{KClO}}_3$ and $\text{KCl}$; $y$ = reaction condition; $z$ = physical state of ${\text{O}}_2$.

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