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 6 of 15

1. Q101 CBSE 2026
   Give ​reasons :

   1. (i)Potato chips manufacturers fill the ​packet of ​chips with ​nitrogen gas.
   2. (ii)​Respiration ​is an exothermic reaction.
   3. (iii)Silver ​chloride is stored in dark coloured bottles.
   Show answer

   Nitrogen keeps oxygen away from chips, respiration releases energy so it is exothermic, and dark bottles keep light from decomposing silver chloride.

   (i) Chips contain oils and fats, which are oxidised by the oxygen of the air and turn rancid, changing their smell and taste. Nitrogen is an unreactive gas; filling the packet with it keeps oxygen away, so the oils are not oxidised and rancidity is prevented. (ii) In respiration, glucose combines with oxygen in our cells to give carbon dioxide and water, and energy is released: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$. A reaction that releases energy is exothermic. (iii) Silver chloride decomposes in sunlight into grey silver and chlorine gas: $2\text{AgCl}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$. Dark coloured bottles keep light out, so the silver chloride does not decompose.

   Step-by-step solution →
2. Q102 CBSE 2026
   Define ​the following ​terms ​:

   1. (i)​Corrosion
   2. (ii)​Rancidity
   3. (iii)​Exothermic ​reaction
   Show answer

   Corrosion is attack on a metal by moisture, air or acids; rancidity is oxidation of fats and oils; an exothermic reaction releases heat.

   (i) Corrosion: when a metal is attacked by the substances around it, such as moisture, air and acids, it is said to corrode, and this process is called corrosion; the rusting of iron is the commonest example. (ii) Rancidity: when fats and oils are oxidised, they become rancid and their smell and taste change; this change is called rancidity. (iii) Exothermic reaction: a reaction in which heat is released along with the formation of the products, for example $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2+\text{heat}$.

   Step-by-step solution →
3. Q103 CBSE 2026
   What happens when

   1. (i)lead (II) nitrate is heated ?
   2. (ii)iron (II) sulphate is heated ​?
   3. (iii)silver chloride is ​exposed to sunlight ? Write ​balanced chemical equations ​of the ​reactions ​involved in support of ​your answer.
   Show answer

   Lead nitrate gives brown fumes and lead oxide; iron(II) sulphate gives reddish-brown iron(III) oxide; silver chloride turns grey as it forms silver.

   (i) White lead(II) nitrate decomposes on heating, giving brown fumes of nitrogen dioxide and oxygen gas and leaving a yellow residue of lead(II) oxide: $2\text{Pb}({\text{NO}}_3{)}_2\stackrel{\text{heat}}{\to }2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$ (thermal decomposition). (ii) Green iron(II) sulphate crystals first lose water and turn white, then decompose to a reddish-brown residue of iron(III) oxide with sulphur dioxide and sulphur trioxide, which smell of burning sulphur: $2{\text{FeSO}}_4\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$ (thermal decomposition). (iii) White silver chloride turns grey in sunlight as it decomposes into silver and chlorine: $2\text{AgCl}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$ (photolytic decomposition).

   Step-by-step solution →
4. Q104 CBSE 2026
   What happens when

   1. (i)Calcium ​carbonate ​is ​heated ?
   2. (ii)Silver bromide is exposed to sunlight ​?
   3. (iii)Lead is added to copper (II) chloride solution ​? Write balanced ​chemical equations of ​the reactions involved in support of your answer.
   Show answer

   Calcium carbonate gives quick lime and carbon dioxide; silver bromide turns grey; lead displaces copper from copper(II) chloride solution.

   (i) Calcium carbonate decomposes on strong heating into calcium oxide, which is quick lime, and carbon dioxide gas; this is a thermal decomposition reaction: ${\text{CaCO}}_3(s)\stackrel{\text{Heat}}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$. (ii) Pale yellow silver bromide is decomposed by sunlight into silver and bromine, so the solid turns grey as silver is set free; this is a photolytic decomposition reaction: $2\text{AgBr}(s)\stackrel{\text{Sunlight}}{\to }2\text{Ag}(s)+{\text{Br}}_2(g)$. (iii) Lead is more reactive than copper, so it displaces copper from copper (II) chloride solution; a reddish-brown deposit of copper forms on the lead and the blue-green colour of the solution fades. This is a displacement reaction: $\text{Pb}(s)+{\text{CuCl}}_2(aq)\to {\text{PbCl}}_2(aq)+\text{Cu}(s)$.

   Step-by-step solution →
5. Q105 CBSE 2026
   What happens when

   1. (i)potassium iodide ​solution is ​added to lead ​(II) nitrate solution ?
   2. (ii)zinc is added to ​copper (II) chloride solution ?
   3. (iii)​hydrogen gas is ​passed over hot copper (II) oxide ? Write balanced ​chemical equations of the reactions involved in ​support of your answer.
   Show answer

   Yellow lead iodide precipitates; zinc displaces copper, fading the blue solution; hot copper(II) oxide is reduced to copper by hydrogen.

   (i) A bright yellow precipitate of lead iodide is formed and the mixture turns yellow; this is a double displacement, or precipitation, reaction: $2\text{KI(aq)}+{\text{Pb(NO}}_3{\text{)}}_2\text{(aq)}\to {\text{PbI}}_2\text{(s)}+2{\text{KNO}}_3\text{(aq)}$. (ii) Zinc, being more reactive than copper, displaces copper from the solution: the blue colour of the copper (II) chloride solution fades to the colourless solution of zinc chloride and a reddish-brown deposit of copper forms on the zinc: $\text{Zn(s)}+{\text{CuCl}}_2\text{(aq)}\to {\text{ZnCl}}_2\text{(aq)}+\text{Cu(s)}$. (iii) The black copper (II) oxide is reduced to reddish-brown copper while the hydrogen is oxidised to water, which condenses at the cool end of the tube: $\text{CuO(s)}+{\text{H}}_2\text{(g)}\stackrel{\text{Heat}}{\to }\text{Cu(s)}+{\text{H}}_2\text{O(l)}$.

   Step-by-step solution →
6. Q106 CBSE 2026
   What happens when :

   1. (a)silver chloride is kept in ​sunlight ?
   2. (b)lead nitrate solution is added to potassium ​iodide solution ?
   3. (c)​food materials containing ​fats and ​oils are left for a long ​time ?
   Show answer

   Silver chloride turns grey in sunlight, lead nitrate and potassium iodide give yellow lead iodide, and fats and oils turn rancid.

   (a) The white silver chloride turns grey. Sunlight decomposes it into silver and chlorine, $2\text{AgCl}\stackrel{\text{sunlight}}{\to }2\text{Ag}+{\text{Cl}}_2$, and the grey colour is the silver metal set free. This is a photochemical decomposition reaction. (b) A bright yellow precipitate of lead iodide is formed at once: ${\text{Pb(NO}}_3{\text{)}}_2+2\text{KI}⟶{\text{PbI}}_2+2{\text{KNO}}_3$. This is a double displacement (precipitation) reaction, the potassium nitrate staying in solution. (c) The food becomes rancid. The fats and oils are slowly oxidised by the oxygen of the air, so the smell and the taste of the food turn unpleasant. Rancidity is delayed by adding antioxidants, by using airtight containers, by flushing the packet with nitrogen and by refrigeration.

   Step-by-step solution →
7. Q107 CBSE 2026
   What happens when ​:

   1. (a)Lead nitrate ​powder is heated ?
   2. (b)​Sodium sulphate ​solution is ​added to barium chloride solution ?
   3. (c)Hydrogen gas ​is passed over hot copper(II) oxide ?
   Show answer

   Lead nitrate gives brown fumes and lead oxide; a white precipitate of barium sulphate forms; black copper(II) oxide turns reddish-brown.

   (a) Lead nitrate decomposes on heating: brown fumes of nitrogen dioxide are given off along with oxygen, and lead(II) oxide is left behind. $2{\text{Pb(NO}}_3{\text{)}}_2(\text{s})\stackrel{\text{heat}}{\to }2\text{PbO}(\text{s})+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$. This is a thermal decomposition reaction. (b) A white precipitate of barium sulphate is formed, and sodium chloride remains in solution. ${\text{Na}}_2{\text{SO}}_4(\text{aq})+{\text{BaCl}}_2(\text{aq})⟶{\text{BaSO}}_4(\text{s})+2\text{NaCl}(\text{aq})$. This is a double displacement (precipitation) reaction. (c) The black copper(II) oxide turns reddish-brown as copper metal is formed, and water is produced. $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$. This is a redox reaction: copper(II) oxide is reduced and hydrogen is oxidised.

   Step-by-step solution →
8. Q108 CBSE 2026
   Answer the ​following :

   1. (a)Write two observations when ​green ​coloured crystals of ferrous sulphate are heated in a ​boiling tube.
   2. (b)Why brown coloured copper powder turns black on heating ?
   3. (c)​What happens ​when limestone is heated strongly ?
   Show answer

   Ferrous sulphate turns white, then reddish-brown, giving ${\text{SO}}_2$ and ${\text{SO}}_3$; copper forms black copper(II) oxide; limestone gives quicklime and carbon dioxide.

   (a) Two observations. First, the green colour of the crystals is lost: on heating, the crystals of ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}$ give up their water of crystallisation and turn white, and on stronger heating the residue left in the tube turns reddish-brown, since it is ferric oxide, ${\text{Fe}}_2{\text{O}}_3$. Second, gases with the characteristic choking smell of burning sulphur - sulphur dioxide and sulphur trioxide - are given off from the mouth of the boiling tube. The reaction is a thermal decomposition: ${\text{FeSO}}_4\cdot 7{\text{H}}_2\text{O}\stackrel{\Delta }{\to }{\text{FeSO}}_4+7{\text{H}}_2\text{O}$, followed by $2{\text{FeSO}}_4\stackrel{\Delta }{\to }{\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$. (b) Because the hot copper combines with the oxygen of the air and its surface is converted into copper(II) oxide, which is black. Copper is oxidised, and the change is a combination reaction: $2\text{Cu}(s)+{\text{O}}_2(g)\stackrel{\Delta }{\to }2\text{CuO}(s)$, brown copper to black copper(II) oxide. (c) Limestone, which is calcium carbonate, decomposes. It leaves a white residue of calcium oxide, called quicklime, and gives off carbon dioxide gas, which turns lime water milky. This thermal decomposition of a carbonate is called calcination: ${\text{CaCO}}_3(s)\stackrel{\Delta }{\to }\text{CaO}(s)+{\text{CO}}_2(g)$.

   Step-by-step solution →
9. Q109 CBSE 2026
   Translate the following statements into chemical equation and ​then ​balance them :

   1. (a)​Water ​is added to quicklime
   2. (b)Burning of ​natural gas
   3. (c)​Thermal decomposition of ferrous ​sulphate
   Show answer

   Quicklime and water give slaked lime and heat; methane burns to carbon dioxide and water; heated ferrous sulphate gives ferric oxide, ${\text{SO}}_2$ and ${\text{SO}}_3$.

   (a) $\text{CaO(s)}+{\text{H}}_2\text{O(l)}⟶{\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$ (already balanced). (b) ${\text{CH}}_4\text{(g)}+2{\text{O}}_2\text{(g)}⟶{\text{CO}}_2\text{(g)}+2{\text{H}}_2\text{O(g)}+\text{heat}$. (c) $2{\text{FeSO}}_4\text{(s)}\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3\text{(s)}+{\text{SO}}_2\text{(g)}+{\text{SO}}_3\text{(g)}$.

   Step-by-step solution →
10. Q110 CBSE 2026
    Seema took a small amount of copper ​powder in ​a china dish and heated it. What changes will she observe ​on heating ? ​When ${\text{H}}_2$ 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.

    Show answer

    Copper turns black as copper(II) oxide on heating; hydrogen reduces it back to reddish-brown copper and colourless water.

    On heating: the surface of the reddish-brown copper powder becomes coated with a black substance, copper(II) oxide, as copper combines with the oxygen of the air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$. Product: copper(II) oxide (cupric oxide), black. When ${\text{H}}_2$ is passed over the heated black substance: the black coating turns reddish-brown again, because copper is formed back, and water is formed: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$. Products: copper, reddish-brown; water, colourless (given off as vapour). The first reaction is the oxidation of copper; the second is a redox reaction in which copper(II) oxide is reduced and hydrogen is oxidised.

    Step-by-step solution →
11. Q111 CBSE 2026
    Define ​the ​following terms ​:

    1. (a)Corrosion
    2. (b)​Rancidity
    3. (c)​Double ​displacement ​reaction
    Show answer

    Corrosion is attack on a metal by moisture and acids; rancidity is oxidation of fats and oils; double displacement is exchange of ions.

    (a) Corrosion: When a metal is attacked by substances around it, such as moisture and acids, it is said to corrode, and this process is called corrosion. Example: rusting of iron. (b) Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change. This is called rancidity. (c) Double displacement reaction: A reaction in which there is an exchange of ions between the reactants is called a double displacement reaction. Example: ${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{BaSO}}_4+2\text{NaCl}$.

    Step-by-step solution →
12. Q112 CBSE 2026
    1. (a)Identify the ​type ​of reaction ​taking ​place and mention why does ​the ​beaker ​become hot.
    2. (b)Write the balanced ​chemical equation.

    Show answer

    It is a combination reaction in which quick lime and water form slaked lime; the beaker gets hot because the reaction is exothermic.

    (a) The reaction is a combination reaction — calcium oxide (quick lime) and water combine to form a single product, calcium hydroxide (slaked lime). The beaker becomes hot because this reaction is exothermic: a large amount of heat energy is released as the reaction takes place, and that heat raises the temperature of the water and of the beaker. (b) $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)+\text{Heat}$

    Step-by-step solution →
13. Q113 CBSE 2026
    Write chemical equations for the ​following when

    1. (a)Silver bromide is exposed to sunlight.
    2. (b)A piece of lead metal is dropped into copper (II) chloride solution.
    3. (c)Hydrogen gas ​is ​passed over heated ​copper oxide.
    4. (d)Methane gas ​is ​burnt in air.
    5. (e)Quick lime is ​mixed ​with water.
    Show answer

    Silver bromide decomposes in sunlight, lead displaces copper, hydrogen reduces copper oxide, methane burns, and quick lime combines with water.

    (a) $2\text{AgBr}(s)\stackrel{\text{Sunlight}}{\to }2\text{Ag}(s)+{\text{Br}}_2(g)$ — photolytic decomposition. (b) $\text{Pb}(s)+{\text{CuCl}}_2(aq)⟶{\text{PbCl}}_2(aq)+\text{Cu}(s)$ — displacement. (c) $\text{CuO}(s)+{\text{H}}_2(g)\stackrel{\text{Heat}}{\to }\text{Cu}(s)+{\text{H}}_2\text{O}(g)$ — reduction of copper oxide. (d) ${\text{CH}}_4(g)+2{\text{O}}_2(g)⟶{\text{CO}}_2(g)+2{\text{H}}_2\text{O}(g)+\text{Heat and light}$ — combustion. (e) $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)+\text{Heat}$ — combination.

    Step-by-step solution →
14. Q114 CBSE 2026
    1. (a)Study the experimental setup shown in the diagram and ​write chemical ​equation for the same. Name and define the type of reaction. Mention the ​colour of ​the products formed.
    2. (b)​What type of reaction takes place when aqueous solution ​of lead (II) ​nitrate and potassium iodide are mixed together ? Write chemical equation also.

    Show answer

    Heated lead nitrate decomposes into yellow lead oxide, brown nitrogen dioxide and oxygen; lead nitrate and potassium iodide solutions give a double displacement precipitate.

    (a) $2{\text{Pb(NO}}_3{)}_2(s)\stackrel{\text{Heat}}{\to }2\text{PbO}(s)+4{\text{NO}}_2(g)+{\text{O}}_2(g)$. It is a thermal decomposition reaction - a reaction in which a single compound breaks down on heating into two or more simpler substances. Colours of the products: lead oxide is a yellow solid, nitrogen dioxide is a reddish brown gas and oxygen is colourless. (b) A double displacement reaction, and since an insoluble solid separates out it is a precipitation reaction: ${\text{Pb(NO}}_3{)}_2(aq)+2\text{KI}(aq)⟶{\text{PbI}}_2(s)+2{\text{KNO}}_3(aq)$, the precipitate being yellow lead iodide.

    Step-by-step solution →
15. Q115 CBSE 2025
    Electrolysis ​of ​water is ​a decomposition ​reaction. The mass ratio ($M_{\text{H}}:M_{\text{O}}$) of ​hydrogen and oxygen gases liberated at ​the electrodes during electrolysis ​of water is ​:

    1. (a)$8:1$
    2. (b)$2:1$
    3. (c)$1:2$
    4. (d)$1:8$
    Show answer

    (d) $1:8$

    Step-by-step solution →
16. Q116 CBSE 2025
    The correct ​balanced chemical ​equation showing ​exothermic reaction ​in ​which natural gas ​burns in air is ​:

    1. (a)${\text{CH}}_4+{\text{O}}_2⟶{\text{CO}}_2+2{\text{H}}_2\text{O}$
    2. (b)${\text{CH}}_4+2{\text{O}}_2⟶2{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{Energy}$
    3. (c)${\text{CH}}_4+2{\text{O}}_2⟶{\text{CO}}_2+2{\text{H}}_2\text{O}$
    4. (d)${\text{CH}}_4+2{\text{O}}_2⟶{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{Energy}$
    Show answer

    (d) ${\text{CH}}_4+2{\text{O}}_2⟶{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{Energy}$

    Step-by-step solution →
17. Q117 CBSE 2025
    Consider the following ​chemical equation ​: $p\text{Al}+q{\text{H}}_2\text{O}⟶r{\text{Al}}_2{\text{O}}_3+s{\text{H}}_2$ To balance ​this ​chemical equation, the values ​of 'p', 'q', 'r' and 's' ​must ​be respectively :

    1. (a)$3,2,2,1$
    2. (b)$2,3,3,1$
    3. (c)$2,3,1,3$
    4. (d)$3,1,2,2$
    Show answer

    (c) $2,3,1,3$

    Step-by-step solution →
18. Q118 CBSE 2025
    The chemical reaction ​of solid ​calcium ​oxide ​with water ​is :

    1. (a)Exothermic and the pH of the solution formed is less than $7$.
    2. (b)Endothermic and the pH of the solution formed is less than $7$.
    3. (c)Endothermic and the pH of the solution formed is more than $7$.
    4. (d)Exothermic and the pH of the solution formed is more than $7$.
    Show answer

    (d) Exothermic and the pH of the solution formed is more than $7$.

    Step-by-step solution →
19. Q119 CBSE 2025
    From the ​following, select a process ​in ​which ​combination ​reaction ​occurs ​:

    1. (a)Black and white photography
    2. (b)Burning of carbon (coal)
    3. (c)Burning of methane
    4. (d)Refining of copper
    Show answer

    (b) Burning of carbon (coal)

    Step-by-step solution →
20. Q120 CBSE 2025
    In which one ​of ​the ​following situations a chemical ​reaction ​does not ​occur?

    1. (a)Milk is left open at room temperature during summer
    2. (b)Grapes get fermented
    3. (c)An iron nail is left exposed to humid atmosphere
    4. (d)Melting of glaciers
    Show answer

    (d) Melting of glaciers

    Step-by-step solution →