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 8 of 15

1. Q141 CBSE 2025
   Balance ​the ​following ​chemical ​equations :

   1. (a)${\text{CO}}_2+{\text{H}}_2\text{O}\underset{\text{Chlorophyll}}{\overset{\text{Sunlight}}{\to }}{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+{\text{O}}_2+{\text{H}}_2\text{O}$
   2. (b)${\text{Pb(NO}}_3{)}_2\stackrel{\text{Heat}}{\to }\text{PbO}+{\text{NO}}_2+{\text{O}}_2$
   Show answer

   The balanced equations are $6{\text{CO}}_2+12{\text{H}}_2\text{O}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2+6{\text{H}}_2\text{O}$ and $2{\text{Pb(NO}}_3{)}_2\to 2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$.

   (a) $6{\text{CO}}_2+12{\text{H}}_2\text{O}\underset{\text{Chlorophyll}}{\overset{\text{Sunlight}}{\to }}{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2+6{\text{H}}_2\text{O}$ (b) $2{\text{Pb(NO}}_3{)}_2\stackrel{\text{Heat}}{\to }2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$

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2. Q142 CBSE 2025
   While burning ​a magnesium ​ribbon in air, list two ​safety measures which ​should be followed. Also state two ​observations ​of ​this ​activity.
   Show answer

   Hold the ribbon with tongs and keep the eyes away from the flame; it burns with a dazzling white flame, leaving white ash.

   Safety measures: 1. Hold the magnesium ribbon with a pair of tongs, not in the fingers. 2. Keep the burning ribbon as far as possible from the eyes and wear suitable eyeglasses; do not look straight at the dazzling flame. Observations: 1. The ribbon burns with a dazzling white flame, giving out heat and light. 2. It changes into a white powder (ash), mainly magnesium oxide, which is collected in a watch-glass: $2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$.

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3. Q143 CBSE 2025
   Name the compound used ​in black ​and ​white photography. State whether the reaction that occurs is ​exothermic ​or endothermic. Give justification ​for your ​answer.
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   Silver bromide is used in black and white photography; its decomposition by light is endothermic as it absorbs energy.

   The compound is silver bromide, $\text{AgBr}$ (silver chloride, $\text{AgCl}$, behaves in the same way). In light it decomposes: $2\text{AgBr}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Br}}_2$. The reaction is endothermic, because in it the silver bromide decomposes by absorbing the energy of the light that falls on it.

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4. Q144 CBSE 2025
   List the ​possible ​sources of energy ​required in ​decomposition ​reactions. Illustrate any one ​with ​a suitable ​example.
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   Heat, electricity or light can supply the energy; for example, electricity splits water into hydrogen and oxygen.

   The energy needed for a decomposition reaction can be supplied as heat, as electricity or as light. Example (electricity): when a current is passed through acidified water, it decomposes into hydrogen at the cathode and oxygen at the anode in the volume ratio $2:1$, $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$.

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5. Q145 CBSE 2025
   What is observed when an aqueous solution of ​sodium ​sulphate ​is added to an aqueous ​solution of barium chloride taken in a ​test tube ​? Write chemical equation for the reaction that occurs and name ​the type of reaction.
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   A white precipitate of barium sulphate forms in a double displacement (precipitation) reaction.

   A white precipitate of barium sulphate appears and settles at the bottom of the test tube. ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)⟶{\text{BaSO}}_4(s)\downarrow +2\text{NaCl}(aq)$. It is a double displacement reaction (a precipitation reaction).

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6. Q146 CBSE 2025
   What ​is observed ​when ​hydrated ferrous sulphate crystals are heated in ​a dry ​boiling tube ? Give ​balanced ​chemical equation(s) of the reaction(s) that occur(s).
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   Green crystals turn white, then reddish brown, giving off gases with the smell of burning sulphur.

   The green crystals first turn white as the water of crystallisation is driven off, and on stronger heating the white powder turns reddish brown while gases with the smell of burning sulphur are given out. ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}\to {\text{FeSO}}_4+7{\text{H}}_2\text{O}$; $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$.

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7. Q147 CBSE 2025
   Write the change in colour that occurs ​when crystals of ferrous sulphate are ​heated ​strongly. Give chemical equation for ​the ​reaction ​that occurs and also ​name the type of reaction.
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   Green crystals turn white and then reddish brown; $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$, a thermal decomposition reaction.

   The green crystals first turn white as the water of crystallisation is lost, and on strong heating a reddish brown residue of ferric oxide is left, with gases that smell of burning sulphur. $2{\text{FeSO}}_4\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$. It is a thermal decomposition reaction.

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8. Q148 CBSE 2025
   What happens when ​: ​(write balanced ​chemical equation)

   1. (a)​Lead nitrate ​is thermally decomposed,
   2. (b)Natural gas ​burns in ​oxygen ​(or air) ?
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   Lead nitrate decomposes on heating into lead oxide, nitrogen dioxide and oxygen; natural gas burns to give carbon dioxide, water and heat.

   (a) On heating, lead nitrate decomposes: brown fumes of nitrogen dioxide are given off together with oxygen, and lead(II) oxide is left as a residue. This is a thermal decomposition reaction: $2{\text{Pb(NO}}_3{)}_2(\text{s})\stackrel{\text{heat}}{\to }2\text{PbO(s)}+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$. (b) Natural gas, which is mainly methane, burns with a blue flame to form carbon dioxide and water vapour, giving out heat and light. This is a combustion reaction and is exothermic: ${\text{CH}}_4(\text{g})+2{\text{O}}_2(\text{g})\to {\text{CO}}_2(\text{g})+2{\text{H}}_2\text{O(g)}+\text{heat and light}$.

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9. Q149 CBSE 2025
   Translate the following statements into chemical equations and ​then balance them :

   1. (a)Nitric ​acid reacts ​with calcium ​hydroxide to form ​calcium nitrate and water.
   2. (b)Sodium chloride reacts with silver nitrate to form silver chloride and sodium ​nitrate.
   Show answer

   The balanced equations are $2{\text{HNO}}_3+\text{Ca}(\text{OH}{)}_2\to \text{Ca}({\text{NO}}_3{)}_2+2{\text{H}}_2\text{O}$ and $\text{NaCl}+{\text{AgNO}}_3\to \text{AgCl}+{\text{NaNO}}_3$.

   (a) $2{\text{HNO}}_3+\text{Ca}(\text{OH}{)}_2\to \text{Ca}({\text{NO}}_3{)}_2+2{\text{H}}_2\text{O}$ (b) $\text{NaCl}+{\text{AgNO}}_3\to \text{AgCl}+{\text{NaNO}}_3$, which is balanced as written; with state symbols, $\text{NaCl(aq)}+{\text{AgNO}}_3\text{(aq)}\to \text{AgCl(s)}+{\text{NaNO}}_3\text{(aq)}$, the silver chloride separating as a white precipitate.

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10. Q150 CBSE 2025
    Write chemical ​equation for the ​chemical reaction between the aqueous ​solutions of sodium ​sulphate and barium ​chloride. State two types ​of chemical reactions in ​which this reaction can be placed.
    Show answer

    The reaction is a double displacement reaction and a precipitation reaction, forming white barium sulphate.

    ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$: sodium sulphate and barium chloride give a white precipitate of barium sulphate and a solution of sodium chloride. The reaction is a double displacement reaction, because the two compounds exchange ions, and a precipitation reaction, because an insoluble solid (barium sulphate) is formed.

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11. Q151 CBSE 2025
    A ​copper wire ​on burning in flame, gets coated with a black substance. Write the chemical equation of the reaction that ​takes ​place. How can this ​chemical ​change be ​reversed ?
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    Copper burns to black copper(II) oxide, $2\text{Cu}+{\text{O}}_2\to 2\text{CuO}$; passing hydrogen over the heated wire reverses it.

    The black substance is copper(II) oxide, formed when the hot copper combines with oxygen of the air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ The change is reversed by heating the coated wire and passing hydrogen gas over it. The hydrogen removes the oxygen, so the black copper oxide turns back into reddish-brown copper, and water is formed: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$

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12. Q152 CBSE 2025
    Give ​one ​chemical equation each for the chemical reaction in which the following changes occur (Mention the change in temperature/colour and/or the compound precipated ​with the equation) :

    1. (a)Change in ​temperature
    2. (b)​Change in colour
    3. (c)Formation of precipitate
    Show answer

    Quicklime in water gets hot; iron turns blue copper sulphate pale green; sodium sulphate with barium chloride gives white barium sulphate.

    (a) Change in temperature — quicklime reacts with water in an exothermic reaction, so the mixture becomes hot: $\text{CaO (s)}+{\text{H}}_2\text{O (l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$ (temperature rises). (b) Change in colour — an iron nail placed in blue copper sulphate solution displaces copper, and the blue changes to the pale green of iron sulphate: $\text{Fe (s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu (s)}$ (blue to pale green, with red-brown copper deposited). (c) Formation of precipitate — sodium sulphate solution mixed with barium chloride solution gives a white precipitate of barium sulphate: ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\downarrow \text{(s)}+2\text{NaCl (aq)}$ (white precipitate of barium sulphate).

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13. Q153 CBSE 2025
    Write balanced chemical ​equation for the reactions that ​occur when

    1. (a)​steam is passed over ​red hot iron. ​
    2. (b)natural gas is burnt in air. ​
    3. (c)glucose reacts with oxygen in the ​cells of ​our body and provides energy.
    Show answer

    Balanced equations: iron and steam give ${\text{Fe}}_3{\text{O}}_4$ and hydrogen, methane burns to ${\text{CO}}_2$ and water, and glucose gives ${\text{CO}}_2$, water and energy.

    (a) $3\text{Fe}(s)+4{\text{H}}_2\text{O}(g)\stackrel{\text{heat}}{\to }{\text{Fe}}_3{\text{O}}_4(s)+4{\text{H}}_2(g)$; (b) ${\text{CH}}_4(g)+2{\text{O}}_2(g)⟶{\text{CO}}_2(g)+2{\text{H}}_2\text{O}(g)$ with heat and light; (c) ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6(aq)+6{\text{O}}_2(g)⟶6{\text{CO}}_2(g)+6{\text{H}}_2\text{O}(l)$ with release of energy.

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14. Q154 CBSE 2025
    How is ​a ​double displacement reaction different from ​a displacement reaction ? Explain giving ​example ​in the form ​of balanced chemical equations.
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    Displacement: a more reactive element replaces another in a compound; double displacement: two compounds exchange ions, often forming a precipitate.

    In a displacement reaction a more reactive element displaces a less reactive element from its compound, so an element and a compound react and one partner changes place: $\text{Fe}(s)+{\text{CuSO}}_4(aq)\to {\text{FeSO}}_4(aq)+\text{Cu}(s)$. In a double displacement reaction two compounds react by exchanging their ions and form two new compounds, often with a precipitate: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$, in which white barium sulphate precipitates.

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15. Q155 CBSE 2025
    State the change that ​is observed when a China dish containing ​copper powder is heated over the flame of a burner. ​Name the phenomenon responsible for the change and write ​balanced ​equation for the chemical reaction that occurs. How is this reaction different from the reaction that occurs when copper wares kept in ​open air slowly lose their shiny brown surface and gain a coat ​? Write chemical name of the coating and state its colour.
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    Copper turns black by oxidation to copper(II) oxide; the slow dulling of copper wares is corrosion, giving a green coat of basic copper carbonate.

    On heating, the brown surface of the copper powder becomes coated with a black substance, copper(II) oxide. The phenomenon is oxidation — copper gains oxygen from the air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$. The dulling of copper wares is different: it happens slowly, at room temperature and without heating, through the action of the moist carbon dioxide and oxygen of the air on the copper — corrosion — rather than by a quick reaction with oxygen alone on heating. The coating formed on copper wares is basic copper carbonate, ${\text{CuCO}}_3\cdot {\text{Cu(OH)}}_2$, which is green.

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16. Q156 CBSE 2025
    Explain why respiration ​is considered ​as an ​exothermic reaction. ​Give ​the chemical equation ​for this ​reaction.
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    Respiration releases energy, partly as heat, when glucose combines with oxygen to form carbon dioxide and water, so it is exothermic.

    Respiration is exothermic because energy is released as it takes place: in the cells of our body, glucose obtained from the digestion of food combines with oxygen to form carbon dioxide and water, and this reaction gives out energy, part of it as heat that helps keep the body warm. Equation: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6(\text{aq})+6{\text{O}}_2(\text{aq})\to 6{\text{CO}}_2(\text{aq})+6{\text{H}}_2\text{O}(\text{l})+\text{energy}$

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17. Q157 CBSE 2025
    1. (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.
    2. (ii)Decomposition of ​vegetable matter into compost is ​considered an exothermic reaction. Why ?
    Show answer

    A decomposition reaction breaks one reactant into simpler products; composting releases heat to the surroundings, so it is exothermic.

    (i) A decomposition reaction is one in which a single reactant breaks down into two or more simpler products. Heat: ${\text{CaCO}}_3\stackrel{\text{Heat}}{\to }\text{CaO}+{\text{CO}}_2$; Light: $2\text{AgCl}\stackrel{\text{Sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$; Electricity: $2{\text{H}}_2\text{O}\stackrel{\text{Electricity}}{\to }2{\text{H}}_2+{\text{O}}_2$. (ii) Micro-organisms decompose the vegetable matter and energy is released to the surroundings as heat, so the compost heap warms up; a reaction that gives out energy is exothermic.

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18. Q158 CBSE 2025
    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 : $\text{Zn}+{\text{H}}_3{\text{PO}}_4\to {\text{Zn}}_3({\text{PO}}_4{)}_2+{\text{H}}_2$
    Show answer

    Equations are balanced to obey the law of conservation of mass; the balanced equation is $3\text{Zn}+2{\text{H}}_3{\text{PO}}_4\to {\text{Zn}}_3({\text{PO}}_4{)}_2+3{\text{H}}_2$.

    A chemical equation is balanced because a reaction only rearranges atoms, so the number of atoms of every element (and hence the total mass) must be the same on both sides. The law behind it is the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction, so the total mass of the products equals the total mass of the reactants. Balanced equation: $3\text{Zn}+2{\text{H}}_3{\text{PO}}_4\to {\text{Zn}}_3({\text{PO}}_4{)}_2+3{\text{H}}_2$

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19. Q159 CBSE 2025
    In the process of electrolysis of water ​(i) Name the gas collected at

    1. (1)cathode and ​
    2. (2)anode. (ii) ​Why ​is the ​volume of the ​two gases not same ? (iii) What would happen if dil. $\text{HCl}$ is ​not added to water ?
    Show answer

    Hydrogen is collected at the cathode and oxygen at the anode, in a $2:1$ volume ratio; the acid makes water conduct.

    (i) Hydrogen is collected at the cathode and oxygen at the anode. (ii) Water contains two atoms of hydrogen for every atom of oxygen, so $2{\text{H}}_2\text{O}⟶2{\text{H}}_2+{\text{O}}_2$ liberates twice as many molecules of hydrogen as of oxygen; equal volumes of gases contain equal numbers of molecules, so the volume of hydrogen is twice that of oxygen. (iii) Pure water has very few ions and is a poor conductor, so without the dil. $\text{HCl}$ hardly any current would flow and electrolysis would be extremely slow or would not occur at all; the acid supplies ions and makes the water conducting.

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20. Q160 CBSE 2025
    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.
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    Decomposition reverses combination: calcium oxide and water combine to give calcium hydroxide, whereas calcium carbonate breaks into calcium oxide and carbon dioxide.

    In a combination reaction two or more substances join to form a single product, whereas in a decomposition reaction a single substance breaks up into two or more products; combination usually gives out energy while decomposition needs energy to be supplied. Each change is therefore the reverse of the other, which is why decomposition is called the opposite of combination. Combination: $\text{CaO}+{\text{H}}_2\text{O}⟶{\text{Ca(OH)}}_2$, that is calcium oxide (quick lime) and water giving calcium hydroxide (slaked lime). Decomposition: ${\text{CaCO}}_3\stackrel{\text{Heat}}{\to }\text{CaO}+{\text{CO}}_2$, that is calcium carbonate (limestone) giving calcium oxide (quick lime) and carbon dioxide.

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