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 11 of 15

1. Q201 CBSE 2024
   Translate the following statement into a balanced ​chemical equation. “When barium chloride reacts with ​aluminium sulphate, aluminium ​chloride and ​barium sulphate are formed.” State ​the type of this reaction giving ​reason to ​justify your answer.
   Show answer

   The balanced equation is $3{\text{BaCl}}_2+{\text{Al}}_2({\text{SO}}_4{)}_3\to 2{\text{AlCl}}_3+3{\text{BaSO}}_4$, a double displacement reaction.

   Balanced equation: $3{\text{BaCl}}_2(\text{aq})+{\text{Al}}_2({\text{SO}}_4{)}_3(\text{aq})\to 2{\text{AlCl}}_3(\text{aq})+3{\text{BaSO}}_4(\text{s})$. Type: double displacement reaction (also a precipitation reaction). Reason: the two reactants exchange ions, barium ions combining with sulphate ions and aluminium ions with chloride ions, and the insoluble barium sulphate separates out as a white precipitate.

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2. Q202 CBSE 2024
   Translate the following statements into chemical equations ​and then ​balance them :

   1. (i)Solution ​of barium chloride and aluminium sulphate in water react ​to give ​insoluble barium sulphate and the solution of aluminium chloride.
   2. (ii)Aluminium ​metal reacts with steam ​to give aluminium oxide and hydrogen gas.
   Show answer

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

   (i) $3{\text{BaCl}}_2\text{(aq)}+{\text{Al}}_2({\text{SO}}_4{)}_3\text{(aq)}\to 3{\text{BaSO}}_4\text{(s)}+2{\text{AlCl}}_3\text{(aq)}$; (ii) $2\text{Al(s)}+3{\text{H}}_2\text{O(g)}\to {\text{Al}}_2{\text{O}}_3\text{(s)}+3{\text{H}}_2\text{(g)}$

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3. Q203 CBSE 2024
   Write one chemical equation each for the chemical ​reaction in which ​the following ​have ​taken place ​:

   1. (i)Change ​in colour
   2. (ii)Change in temperature
   3. (iii)Formation of precipitate Mention colour change/temperature change ​(rise/fall)/compound precipitated along with equation.
   Show answer

   The examples are iron in copper sulphate (colour change), quick lime in water (temperature rise) and sodium sulphate with barium chloride (white precipitate).

   (i) Change in colour: $\text{Fe}(\text{s})+{\text{CuSO}}_4(\text{aq})\to {\text{FeSO}}_4(\text{aq})+\text{Cu}(\text{s})$. The blue colour of the copper sulphate solution fades to light green, and a brownish coating of copper forms on the iron. (ii) Change in temperature: $\text{CaO}(\text{s})+{\text{H}}_2\text{O}(\text{l})\to {\text{Ca(OH)}}_2(\text{aq})+\text{Heat}$. The temperature rises (the reaction is exothermic). (iii) Formation of precipitate: ${\text{Na}}_2{\text{SO}}_4(\text{aq})+{\text{BaCl}}_2(\text{aq})\to {\text{BaSO}}_4(\text{s})+2\text{NaCl}(\text{aq})$. A white precipitate of barium sulphate, ${\text{BaSO}}_4$, forms.

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4. Q204 CBSE 2024
   Study the experimental set-up ​shown in the diagram ​and write chemical equation for the ​chemical reaction involved. Name and define the type ​of ​reaction. List two other metals ​which can be used ​in place of iron to show the same type of reaction with copper sulphate solution.

   Show answer

   Iron displaces copper from copper sulphate, a displacement reaction; zinc and magnesium (or aluminium), being above copper, can replace iron.

   Equation: $\text{Fe(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu(s)}$; the iron nails become coated with brownish copper and the blue colour of the solution fades. Type: displacement reaction, a reaction in which a more reactive element displaces a less reactive element from its compound (here iron, being more reactive, displaces copper from copper sulphate). Two other metals that can be used in place of iron: zinc and magnesium, both above copper in the reactivity series ($\text{Zn}+{\text{CuSO}}_4\to {\text{ZnSO}}_4+\text{Cu}$; $\text{Mg}+{\text{CuSO}}_4\to {\text{MgSO}}_4+\text{Cu}$). Aluminium also lies above copper and displaces it, so any two of magnesium, aluminium and zinc are correct. Lead lies above copper as well, but the lead sulphate it forms is insoluble and coats the lead, so in copper sulphate solution the reaction soon stops and the copper coating and the fading blue colour do not develop.

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5. Q205 CBSE 2024
   Answer the following questions in the context of electrolysis of water :

   1. (a)Why is this reaction/process called a ​decomposition reaction ​?
   2. (b)Giving reason ​state whether ​this ​reaction is exothermic or endothermic.
   3. (c)Name the gases ​collected at the anode and ​cathode.
   4. (d)What is the mass ratio of the ​gases collected at the anode and cathode ?
   Show answer

   A decomposition and endothermic reaction giving oxygen at the anode and hydrogen at the cathode, in a mass ratio of $8:1$.

   (a) A single reactant, water, breaks down into two simpler products, hydrogen and oxygen: $2{\text{H}}_2\text{O}(l)\stackrel{\text{electric current}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$. So it is a decomposition reaction. (b) It is endothermic: electrical energy has to be supplied all the time and is absorbed to break the water down; the reaction stops when the current is switched off. (c) Oxygen is collected at the anode and hydrogen at the cathode. (d) Mass of oxygen (anode) : mass of hydrogen (cathode) $=32:4=8:1$.

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6. Q206 CBSE 2024
   1. (a)Write ​a chemical ​equation for the reaction in which change in ​colour is observed when a metal is kept immersed in ​a salt solution of another metal.
   2. (b)When hydrogen gas ​is passed over heated copper(II) oxide, ​copper and steam are formed. Write the balanced chemical equation with physical states for this reaction. State what ​kind of chemical reaction is this.
   Show answer

   Iron displaces copper, turning blue copper sulphate light green; hydrogen reduces hot copper(II) oxide to copper and steam, a redox reaction.

   (a) $\text{Fe(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu(s)}$. Iron is more reactive than copper and displaces it from copper sulphate solution: the blue colour of the solution fades to light green (iron sulphate) and a reddish-brown coating of copper forms on the iron. Zinc in copper sulphate solution, where the blue colour fades, is also correct. (b) $\text{CuO(s)}+{\text{H}}_2\text{(g)}\stackrel{\text{heat}}{\to }\text{Cu(s)}+{\text{H}}_2\text{O(g)}$. It is an oxidation-reduction (redox) reaction: copper(II) oxide loses oxygen and is reduced to copper, while hydrogen gains oxygen and is oxidised to water.

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7. Q207 CBSE 2024
   Write ​one chemical equation each for ​the ​reactions ​in which the following changes occur, ​specifying the change observed :

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

   Iron in copper sulphate turns blue to pale green; quicklime in water gets hot; sodium sulphate and barium chloride give white barium sulphate.

   (a) Change in colour: $\text{Fe}(\text{s})+{\text{CuSO}}_4(\text{aq})\to {\text{FeSO}}_4(\text{aq})+\text{Cu}(\text{s})$; the blue copper sulphate solution turns pale green and a reddish-brown coat of copper forms on the iron. (b) Change in temperature: $\text{CaO}(\text{s})+{\text{H}}_2\text{O}(\text{l})\to {\text{Ca(OH)}}_2(\text{aq})+\text{heat}$; the mixture becomes hot, so the temperature rises. (c) Formation of precipitate: ${\text{Na}}_2{\text{SO}}_4(\text{aq})+{\text{BaCl}}_2(\text{aq})\to {\text{BaSO}}_4(\text{s})+2\text{NaCl}(\text{aq})$; a white precipitate of barium sulphate forms.

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8. Q208 CBSE 2024
   In ​the ​electrolysis of acidulated water (a) Name the ​gas ​collected at the

   1. (i)cathode and
   2. (ii)anode. (b) Why is the volume of one gas collected at one electrode two times the gas collected at ​the ​other electrode ​? (c) Write the method of testing of any one of the gases evolved in ​this case.
   Show answer

   Hydrogen forms at the cathode and oxygen at the anode in a $2:1$ volume ratio; hydrogen burns with a pop sound.

   (a) (i) Cathode: hydrogen gas (${\text{H}}_2$); (ii) Anode: oxygen gas (${\text{O}}_2$). (b) A molecule of water, ${\text{H}}_2\text{O}$, has two hydrogen atoms for every oxygen atom, so its decomposition $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$ gives two molecules of hydrogen for every molecule of oxygen. Under the same conditions equal numbers of gas molecules occupy equal volumes, so the volume of hydrogen collected at the cathode is twice the volume of oxygen collected at the anode. (c) Test for hydrogen: bring a burning matchstick or candle near the mouth of the test tube; the gas burns with a pop sound. (Or, for oxygen: a glowing splinter brought near the mouth of the test tube bursts into flame, because oxygen supports burning.)

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9. Q209 CBSE 2024
   1. (a)State any two observations when an electric current is passed through ​acidulated water, in a container ​having ​each electrode covered by ​test ​tubes filled with water.
   2. (b)Write ​the ratio of the mass of the ​gas collected at the cathode ​to the mass of the gas collected at the anode.
   Show answer

   Gas bubbles collect at both electrodes, with twice the volume at the cathode; the mass ratio of hydrogen to oxygen is $1:8$.

   (a) Any two of: bubbles of gas form at both electrodes and collect in the test tubes, pushing the water out of them; the volume of gas collected over the cathode is twice the volume collected over the anode; the gas from the cathode burns with a pop sound when a burning candle is brought near (hydrogen), while the gas from the anode makes the candle burn more brightly (oxygen). (b) The gas at the cathode is hydrogen and the gas at the anode is oxygen. From $2{\text{H}}_2\text{O}\to 2{\text{H}}_2+{\text{O}}_2$, mass of hydrogen : mass of oxygen $=2\times 2:32=4:32=1:8$.

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10. Q210 CBSE 2024
    1. (i)Define a decomposition reaction. Write chemical equation for the reaction that ​occurs when lead ​nitrate ​is heated strongly in a boiling tube.
    2. (ii)In electrolytic decomposition of water two gases are liberated at the ​electrodes. Give the mass ratio of ​the gas liberated ​at the cathode and at ​the anode.
    Show answer

    Heated lead nitrate gives lead oxide, nitrogen dioxide and oxygen; the mass ratio of hydrogen (cathode) to oxygen (anode) is $1:8$.

    (i) A decomposition reaction is a reaction in which a single reactant breaks down to give simpler products. When lead nitrate is heated strongly: $2\text{Pb}({\text{NO}}_3{)}_2(\text{s})\stackrel{\text{Heat}}{\to }2\text{PbO}(\text{s})+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$; brown fumes of nitrogen dioxide are given off. (ii) Hydrogen is liberated at the cathode and oxygen at the anode, $2{\text{H}}_2\text{O}(\text{l})\stackrel{\text{Electricity}}{\to }2{\text{H}}_2(\text{g})+{\text{O}}_2(\text{g})$. Mass of hydrogen : mass of oxygen $=2\times 2:32=4:32=1:8$ (by volume the ratio is $2:1$).

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11. Q211 CBSE 2024
    What is a chemical reaction? Describe one activity each to ​show that a chemical change has ​occurred ​in which ​

    1. (i)​change of colour, and ​
    2. (ii)change ​in temperature ​has taken place.
    Show answer

    A chemical reaction forms new substances; iron in copper sulphate shows a colour change, and quick lime in water shows a temperature rise.

    A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with a different composition and different properties. (i) Change of colour: hang a clean iron nail in blue copper sulphate solution for about 20 minutes. The solution turns light green and the nail gets a reddish-brown coating, because iron displaces copper: $\text{Fe}+{\text{CuSO}}_4\to {\text{FeSO}}_4+\text{Cu}$. (ii) Change in temperature: add water slowly to a small amount of quick lime (calcium oxide) in a beaker. The beaker becomes hot, because the reaction releases heat: $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2+\text{Heat}$.

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12. Q212 CBSE 2024
    When lead nitrate ​is heated strongly in ​a boiling tube, two gases are liberated and a solid ​residue is left behind in the test ​tube.

    1. (i)Name the type of chemical reaction and define it.
    2. (ii)Write the ​name and formula of the coloured gas liberated.
    3. (iii)Write ​the balanced chemical equation for the ​reaction.
    4. (iv)Name ​the residue left in the test tube and state the method of testing its nature (acidic/basic).
    Show answer

    Thermal decomposition gives brown nitrogen dioxide, oxygen and a basic residue of lead oxide, $\text{PbO}$.

    (i) Thermal decomposition reaction. A reaction in which a single reactant breaks down to give simpler products is a decomposition reaction; when the decomposition is brought about by heating, it is called thermal decomposition. (ii) The coloured (brown) gas is nitrogen dioxide, ${\text{NO}}_2$. (iii) $2{\text{Pb(NO}}_3{\text{)}}_2(\text{s})\stackrel{\text{heat}}{\to }2\text{PbO}(\text{s})+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$. (iv) The residue is lead oxide (lead(II) oxide), $\text{PbO}$. To test its nature, shake a little of it with water and test with red and blue litmus paper, since a basic oxide turns red litmus blue. Because lead oxide dissolves only slightly in water, confirm by warming it with dilute nitric acid: it dissolves to give lead nitrate and water, as a base does. Lead oxide is therefore basic in nature.

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13. Q213 CBSE 2024
    1. (i)​Define a decomposition reaction. How can we say that (I) electrolysis of water, and ​(II) blackening of ​silver bromide when exposed to ​sunlight, are decomposition reactions? Mention the type of energy involved in each case.
    2. (ii)“The type of reactions in which (I) ​calcium oxide is formed, and (II) calcium hydroxide is formed are opposite reactions to ​each other.” Justify this statement with the help ​of chemical equations.
    Show answer

    Decomposition breaks one reactant into simpler products (electrical and light energy here); calcium oxide forms by decomposition and calcium hydroxide by combination, opposite types.

    (i) A decomposition reaction is one in which a single reactant breaks down to give two or more simpler products. (I) In electrolysis of water, one reactant, water, breaks into hydrogen and oxygen: $2{\text{H}}_2\text{O}(l)\stackrel{\text{Electric current}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$; the energy involved is electrical energy. (II) In sunlight, silver bromide breaks into silver, which makes it grey-black, and bromine: $2\text{AgBr}(s)\stackrel{\text{Sunlight}}{\to }2\text{Ag}(s)+{\text{Br}}_2(g)$; the energy involved is light energy. So both are decomposition reactions. (ii) (I) Calcium oxide is formed by decomposition, one reactant giving two products: ${\text{CaCO}}_3(s)\stackrel{\text{Heat}}{\to }\text{CaO}(s)+{\text{CO}}_2(g)$. (II) Calcium hydroxide is formed by combination, two reactants giving one product: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{Heat}$. Decomposition splits one substance into simpler ones while combination joins substances into one, so the two types are opposite reactions.

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14. Q214 CBSE 2024
    1. (i)Write balanced ​chemical equation for the ​following word equation. Lead nitrate + Potassium iodide → Lead ​iodide + Potassium nitrate Is this a double displacement reaction ? ​Justify your answer. Name the compound precipitated and write the ions ​present in it.
    2. (ii)​Write the method ​of preparation of ${\text{Ca(OH)}}_2$. What happens when ${\text{CO}}_2$ is passed through it ? Write ​balanced chemical equation for the reaction involved.
    Show answer

    Lead nitrate and potassium iodide give yellow lead iodide by double displacement; slaked lime solution turns milky with carbon dioxide, forming calcium carbonate.

    (i) ${\text{Pb(NO}}_3{\text{)}}_2(\text{aq})+2\text{KI}(\text{aq})\to {\text{PbI}}_2(\text{s})+2{\text{KNO}}_3(\text{aq})$. Yes, it is a double displacement reaction, because the reactants exchange their ions: lead, first combined with nitrate, combines with iodide, and potassium, first combined with iodide, combines with nitrate. The compound precipitated is lead iodide, ${\text{PbI}}_2$, a yellow precipitate; it contains lead ions, ${\text{Pb}}^{2+}$, and iodide ions, ${\text{I}}^{-}$ (two ${\text{I}}^{-}$ for each ${\text{Pb}}^{2+}$). (ii) Calcium hydroxide (slaked lime) is prepared by adding water to calcium oxide (quicklime); the reaction is vigorous and gives out a large amount of heat: $\text{CaO}(\text{s})+{\text{H}}_2\text{O}(\text{l})\to {\text{Ca(OH)}}_2(\text{aq})+\text{heat}$. When carbon dioxide is passed through its solution (lime water), the solution turns milky because insoluble calcium carbonate is formed: ${\text{Ca(OH)}}_2(\text{aq})+{\text{CO}}_2(\text{g})\to {\text{CaCO}}_3(\text{s})+{\text{H}}_2\text{O}(\text{l})$.

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15. Q215 CBSE 2024
    Consider the ​unbalanced ​chemical equations ​of ​a ​the ​chemical reactions listed below ​:

    1. (1)$\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$
    2. (2)$\text{Pb}({\text{NO}}_3{)}_2\to \text{PbO}+{\text{NO}}_2+{\text{O}}_2$
    3. (3)${\text{CuSO}}_4+\text{Fe}\to {\text{FeSO}}_4+\text{Cu}$
    4. (4)${\text{Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{BaSO}}_4+\text{NaCl}$

    1. (i)Select combination reaction from the reactions listed above. Is this reaction endothermic or exothermic ?[1]
    2. (ii)Which one of the these reactions represents a displacement reaction ? State the colour change observed in this reaction.[1]
    3. (iii)(a)Identify from the above list a reaction in which thermal decomposition occurs. Write the balanced chemical equation to represent thermal decomposition of ferrous sulphate.[2]
    4. (iii)(b)OR : Write the word equation for the double displacement reaction given in the above list. Giving reason, state whether this reaction is a redox reaction or not.[2]
    Show answer

    Reaction (1) is combination and exothermic; (3) is displacement (blue to pale green); (2) is thermal decomposition; the double displacement reaction is not redox.

    (i) Reaction (1), $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$, is the combination reaction. It is exothermic: a large amount of heat is released. (ii) Reaction (3), ${\text{CuSO}}_4+\text{Fe}\to {\text{FeSO}}_4+\text{Cu}$, is the displacement reaction. The blue colour of the copper sulphate solution fades to pale green, and a reddish-brown coating of copper forms on the iron. (iii)(a) Reaction (2), heating lead nitrate, is a thermal decomposition. For ferrous sulphate: $2{\text{FeSO}}_4(s)\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$. (iii)(b) Sodium sulphate + barium chloride $\to$ barium sulphate + sodium chloride. It is not a redox reaction: the ions only exchange partners (a white precipitate of barium sulphate forms), and no substance gains or loses oxygen or hydrogen, so nothing is oxidised or reduced.

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16. Q216 CBSE 2023
    Which ​of the following ​is an ​example ​of ​endothermic process ?

    1. (a)Formation of slaked lime
    2. (b)Decomposition of vegetable matter into compost
    3. (c)Dissolution of ammonium chloride in water
    4. (d)Digestion of food in our body
    Show answer

    (c) Dissolution of ammonium chloride in water

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17. Q217 CBSE 2023
    In order to ​balance ​the ​following chemical equation, the ​values of $x$ and $y$ ​respectively ​are ​: $3\text{Fe}+x{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+y{\text{H}}_2$

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

    (d) 4, 4

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18. Q218 CBSE 2023
    In the electrolysis of water, if ​the ​mass ​of the gas collected at the ​anode is $m_a$ and ​the mass of the gas ​collected at ​the cathode is $m_c$, the value of $(m_c/m_a)$ is :

    1. (a)$8$
    2. (b)$16$
    3. (c)$\frac{1}{16}$
    4. (d)$\frac{1}{8}$
    Show answer

    (d) $\frac{1}{8}$

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19. Q219 CBSE 2023
    During ​electrolysis of water, ​if the ​volumes of oxygen ​and ​hydrogen evolved at the electrodes ​are $V_O$ and $V_H$ respectively, then $V_O/V_H$ is

    1. (a)$4$
    2. (b)$2$
    3. (c)$\frac{1}{2}$
    4. (d)$\frac{1}{4}$
    Show answer

    (c) $\frac{1}{2}$

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20. Q220 CBSE 2023
    To balance the following chemical equation ​the ​values of $x$ ​and $y$ ​should respectively ​be : $$2\text{NaOH}+x{\text{Al}}_2{\text{O}}_3⟶y{\text{NaAlO}}_2+{\text{H}}_2\text{O}$$

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

    (b) $1,2$

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