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 14 of 15

1. Q261 CBSE 2020
   In ​which of ​the following, ​the identity of initial ​substance ​remains unchanged ​?

   1. (a)Curdling of milk
   2. (b)Formation of crystals by process of crystallisation
   3. (c)Fermentation of grapes
   4. (d)Digestion of food
   Show answer

   (b) Formation of crystals by process of crystallisation

   Step-by-step solution →
2. Q262 CBSE 2020
   Strong ​heating ​of ​ferrous sulphate leads to ​the formation of a ​brown solid and two ​gases. This reaction can ​be ​categorised as

   1. (a)displacement and redox.
   2. (b)decomposition and redox.
   3. (c)displacement and endothermic.
   4. (d)decomposition and exothermic.
   Show answer

   (b) decomposition and redox.

   Step-by-step solution →
3. Q263 CBSE 2020
   For question numbers 13 and 14, two statements are given — one labelled as Assertion (A) ​and the ​other ​labelled ​as Reason ​(R). Select the correct answer to these questions from the codes ​(a), (b), (c) and ​(d) as given below ​:

   Assertion (A): Following is a balanced chemical equation for the action of steam on iron : $3\text{Fe}+4{\text{H}}_2\text{O}⟶{\text{Fe}}_3{\text{O}}_4+4{\text{H}}_2$

   Reason (R): The law of conservation of mass holds good for a chemical equation.

   1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
   2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
   3. (c)(A) is true, but (R) is false.
   4. (d)(A) is false, but (R) is true.
   Show answer

   (b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).

   Step-by-step solution →
4. Q264 CBSE 2020
   For question numbers 13 ​and 14, ​two statements are given ​— one ​labelled as Assertion (A) and ​the other ​labelled as ​Reason (R). Select the correct answer to these questions from ​the codes (a), (b), (c) and (d) as given below :

   Assertion (A): The reaction ${\text{MnO}}_2+4\text{HCl}\to {\text{MnCl}}_2+2{\text{H}}_2\text{O}+{\text{Cl}}_2$ is an example of a redox reaction.

   Reason (R): In this reaction, $\text{HCl}$ is reduced to ${\text{Cl}}_2$ whereas ${\text{MnO}}_2$ is oxidised to ${\text{MnCl}}_2$.

   1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
   2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
   3. (c)(A) is true, but (R) is false.
   4. (d)(A) is false, but (R) is true.
   Show answer

   (c) (A) is true, but (R) is false.

   Step-by-step solution →
5. Q265 CBSE 2020
   For question numbers ​13 and 14, ​two statements are given ​— ​one labelled as ​Assertion (A) and the other labelled as Reason ​(R). Select the correct answer to these questions ​from the codes (a), (b), (c) and (d) as given below :

   Assertion (A): A reaction in which a substance is decomposed into two or more simpler products is known as a decomposition reaction.

   Reason (R): The decomposition of a substance is impossible without supplying energy.

   1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
   2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
   3. (c)(A) is true, but (R) is false.
   4. (d)(A) is false, but (R) is true.
   Show answer

   (b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).

   Step-by-step solution →
6. Q266 CBSE 2020
   1 g of copper powder was taken ​in a China dish and heated. What change ​takes place on ​heating? When hydrogen gas is passed over ​this heated substance, a visible change is seen in it. Give the chemical equations of reactions, ​the name and the color of ​the ​products formed in each case.
   Show answer

   Copper turns black as copper(II) oxide on heating; hydrogen turns it brown again, giving copper and colourless water.

   On heating, the surface of the brown copper powder turns black, because copper combines with oxygen of the air: $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ Product: copper(II) oxide, black in colour. When hydrogen is passed over the heated copper(II) oxide, the black coating turns brown again, because hydrogen removes the oxygen: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$ Products: copper, brown in colour, and water, colourless (given off as vapour).

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7. Q267 CBSE 2020
   A compound ‘A’ is ​used in the manufacture of ​cement. When dissolved ​in water, it evolves a large amount of heat and forms compound ‘B’.

   1. (i)Identify A and B.
   2. (ii)​Write chemical equation ​for the reaction of A with water.
   3. (iii)List ​two types of reaction in ​which this reaction ​may be classified.
   Show answer

   A is calcium oxide and B is calcium hydroxide; the reaction is both a combination reaction and an exothermic reaction.

   (i) A is calcium oxide, $\text{CaO}$ (quicklime); B is calcium hydroxide, ${\text{Ca(OH)}}_2$ (slaked lime). (ii) $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)+\text{heat}$ (iii) It is a combination reaction (two reactants form a single product) and an exothermic reaction (heat is released).

   Step-by-step solution →
8. Q268 CBSE 2020
   Mention with reason the colour ​changes ​observed when ​:

   1. (i)​silver chloride is exposed to sunlight.
   2. (ii)copper powder is strongly heated in the presence of oxygen.
   3. (iii)a ​piece ​of zinc is dropped in ​copper sulphate solution.
   Show answer

   Silver chloride turns from white to grey, copper from reddish brown to black, and copper sulphate solution from blue to colourless.

   (i) White to grey: silver chloride decomposes in sunlight into grey silver and chlorine gas, $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$. (ii) Reddish brown to black: copper combines with oxygen to form black copper(II) oxide, $2\text{Cu}(s)+{\text{O}}_2(g)\stackrel{\text{heat}}{\to }2\text{CuO}(s)$. (iii) Blue to colourless: zinc, being more reactive than copper, displaces it and forms colourless zinc sulphate, $\text{Zn}(s)+{\text{CuSO}}_4(aq)\to {\text{ZnSO}}_4(aq)+\text{Cu}(s)$; a reddish-brown coating of copper forms on the zinc.

   Step-by-step solution →
9. Q269 CBSE 2020
   Identity the type of ​each of the following reactions. Also write ​balanced chemical equation for ​each reaction.

   1. (i)A reaction in which the ​reaction ​mixture becomes warm.
   2. (ii)A reaction ​in which an insoluble substance is formed.
   Show answer

   A warming mixture is an exothermic reaction ($\text{CaO}$ with water); an insoluble product means a precipitation (double displacement) reaction (${\text{Na}}_2{\text{SO}}_4$ with ${\text{BaCl}}_2$).

   (i) Exothermic reaction: heat is released, so the reaction mixture becomes warm. Example: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(aq)+\text{Heat}$. (ii) Precipitation reaction, which is a double displacement reaction: the insoluble product separates as a precipitate. Example: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)⟶{\text{BaSO}}_4(s)+2\text{NaCl}(aq)$, in which barium sulphate is the white precipitate. Any other correct, balanced example of each type is also accepted.

   Step-by-step solution →
10. Q270 CBSE 2020
    A shining metal ‘M’, on ​burning gives a dazzling white flame and changes ​to ​a white powder ‘N’.

    1. (a)Identify ‘M’ ​and ‘N’.
    2. (b)Represent the above reaction in ​the form of a balanced chemical equation.
    3. (c)Does ‘M’ ​undergo oxidation or reduction in ​this ​reaction ? Justify.
    Show answer

    M is magnesium and N is magnesium oxide; $2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$, and magnesium is oxidised because it gains oxygen.

    (a) ‘M’ is magnesium ($\text{Mg}$) and ‘N’ is magnesium oxide ($\text{MgO}$). (b) $2\text{Mg}(s)+{\text{O}}_2(g)⟶2\text{MgO}(s)$ (c) ‘M’ undergoes oxidation, because magnesium gains oxygen to form magnesium oxide (equally, each magnesium atom loses two electrons, $\text{Mg}⟶{\text{Mg}}^{2+}+2e^{-}$).

    Step-by-step solution →
11. Q271 CBSE 2020
    Identify ​the ​type ​of each of ​the following reactions ​stating ​reason for your ​answers :

    1. (a)${\text{Fe}}_2{\text{O}}_3+2\text{Al}⟶{\text{Al}}_2{\text{O}}_3+2\text{Fe}+\text{heat}$
    2. (b)$\text{Pb}({\text{NO}}_3{)}_2+2\text{KI}⟶{\text{PbI}}_2+2{\text{KNO}}_3$
    3. (c)${\text{ZnCO}}_3\stackrel{\text{heat}}{\to }\text{ZnO}+{\text{CO}}_2$
    Show answer

    The reactions are displacement, double displacement (precipitation of yellow lead iodide) and thermal decomposition respectively.

    (a) Displacement reaction: aluminium, being more reactive than iron, displaces iron from iron(III) oxide. (It is also a redox reaction, since aluminium is oxidised and ${\text{Fe}}_2{\text{O}}_3$ is reduced, and an exothermic reaction, since heat is given out; it is the thermit reaction.) (b) Double displacement reaction: the two compounds exchange their ions, ${\text{Pb}}^{2+}$ pairing with ${\text{I}}^{-}$ and ${\text{K}}^{+}$ with ${\text{NO}}_3^{-}$; the insoluble yellow ${\text{PbI}}_2$ precipitates, so it is also a precipitation reaction. (c) Decomposition reaction (thermal decomposition, or calcination): a single reactant, zinc carbonate, breaks down on heating into two simpler products, $\text{ZnO}$ and ${\text{CO}}_2$.

    Step-by-step solution →
12. Q272 CBSE 2020
    About 2 ​g ​ferrous sulphate crystals ​are heated in a dry ​boiling tube.

    1. (i)​What is the colour of ferrous sulphate crystals before and after heating ?
    2. (ii)Name the gases that are evolved ​on heating the crystals.
    3. (iii)Name ​the ​type of chemical reaction taking place.
    4. (iv)Write the chemical equation for the reaction.
    Show answer

    Green crystals turn white then reddish-brown; ${\text{SO}}_2$ and ${\text{SO}}_3$ are evolved in a thermal decomposition, $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$.

    (i) Before heating the crystals are green; on heating they first turn white as they lose their water of crystallisation, and finally a reddish-brown solid, ferric oxide (${\text{Fe}}_2{\text{O}}_3$), is left. (ii) Sulphur dioxide (${\text{SO}}_2$) and sulphur trioxide (${\text{SO}}_3$); the smell of burning sulphur comes from the ${\text{SO}}_2$. (iii) Thermal decomposition reaction. (iv) $2{\text{FeSO}}_4(s)\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$

    Step-by-step solution →
13. Q273 CBSE 2020
    A small amount of ​quicklime is added to water in a glass beaker.

    1. (a)​Name and define ​the ​type ​of ​reaction that has taken place.
    2. (b)Write the chemical equation for the above reaction.
    3. (c)List two main ​observations of ​this reaction.
    Show answer

    It is a combination reaction: quicklime and water form calcium hydroxide, releasing heat, so the beaker becomes hot.

    (a) Combination reaction: a reaction in which two or more substances combine to form a single product. Here calcium oxide and water combine to form calcium hydroxide; since heat is released, it is also an exothermic reaction. (b) $\text{CaO(s)}+{\text{H}}_2\text{O(l)}⟶{\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$ (c) The quicklime reacts vigorously with the water, often with a hissing sound, and a large amount of heat is released, so the beaker becomes hot.

    Step-by-step solution →
14. Q274 CBSE 2020
    Lead nitrate solution is ​added ​to a test tube containing potassium iodide solution.

    1. (a)Write ​the name ​and colour of the compound ​precipitated.
    2. (b)Write the ​balanced chemical equation ​for the reaction involved.
    3. (c)Name the type of this reaction justifying ​your answer.
    Show answer

    Yellow lead iodide, ${\text{PbI}}_2$, precipitates in a double displacement (precipitation) reaction between lead nitrate and potassium iodide.

    (a) Lead iodide, ${\text{PbI}}_2$; it is yellow in colour. (b) $\text{Pb}({\text{NO}}_3{)}_2(aq)+2\text{KI}(aq)⟶{\text{PbI}}_2(s)+2{\text{KNO}}_3(aq)$ (c) Double displacement reaction: the two compounds exchange their ions, lead pairing with iodide and potassium with nitrate. Since an insoluble yellow solid forms, it is also a precipitation reaction.

    Step-by-step solution →
15. Q275 CBSE 2020
    When hydrogen sulphide ​gas is ​passed through a blue solution of copper sulphate, the colour of ​the solution fades ​and a black precipitate is obtained.

    1. (a)Name the type of reaction mentioned ​above.
    2. (b)Why does the ​colour of the solution fade ​away ?
    3. (c)Write the chemical name of the black precipitate formed.
    4. (d)Give the balanced chemical equation ​for the reaction involved.
    Show answer

    It is a double displacement (precipitation) reaction: copper leaves as black copper sulphide, and colourless sulphuric acid remains, so the blue colour fades.

    (a) It is a double displacement reaction (it is also a precipitation reaction). (b) The blue colour is due to copper sulphate; its copper is removed as insoluble copper sulphide and colourless sulphuric acid forms in the solution, so the colour fades. (c) Copper sulphide, $\text{CuS}$ (copper(II) sulphide). (d) ${\text{CuSO}}_4(\text{aq})+{\text{H}}_2\text{S}(\text{g})\to \text{CuS}(\text{s})+{\text{H}}_2{\text{SO}}_4(\text{aq})$

    Step-by-step solution →
16. Q276 CBSE 2020
    What happens ​when food ​materials containing fats and oils are ​left for a long time ​? List two observable changes and suggest three ways by ​which ​this phenomenon can be prevented.
    Show answer

    Fats and oils are oxidised and the food turns rancid, with bad smell and taste; antioxidants, airtight containers and nitrogen prevent this.

    The fats and oils are slowly oxidised by the oxygen of the air and the food becomes rancid; this change is called rancidity. Two observable changes: (1) the food gives out an unpleasant smell; (2) its taste changes and becomes unpleasant. Three ways to prevent it: (1) add antioxidants, substances that prevent oxidation, to the food; (2) keep the food in airtight containers; (3) flush packets of food such as chips with nitrogen gas.

    Step-by-step solution →
17. Q277 CBSE 2020
    Study the figure ​given below and answer ​the following questions :

    1. (a)Name the process depicted in ​the ​diagram.
    2. (b)Write the composition of the anode and the cathode.
    3. (c)Write the balanced chemical equation ​of the reaction taking place ​in this case.
    4. (d)The reaction does not take place if a few drops of dilute sulphuric ​acid are not added to water. Why ?

    Show answer

    The process is electrolysis of water on carbon electrodes; dilute sulphuric acid is added because pure water is a very poor conductor.

    (a) Electrolysis of water (electrolytic decomposition of water). (b) Both the anode and the cathode are carbon electrodes (graphite rods). (c) $2{\text{H}}_2\text{O}(\text{l})\stackrel{\text{electric current}}{\to }2{\text{H}}_2(\text{g})+{\text{O}}_2(\text{g})$ (d) Pure water is a very poor conductor of electricity; the dilute sulphuric acid supplies ions that carry the current through the water, so the water can be decomposed.

    Step-by-step solution →
18. Q278 CBSE 2020
    In the electrolysis of water

    1. (a)Name the gases liberated at anode and cathode.
    2. (b)​Why ​is it that ​the volume ​of gas ​collected on ​one electrode is ​two times that on the other electrode ?
    3. (c)What ​would happen if dil. ${\text{H}}_2{\text{SO}}_4$ is not added to water ?
    Show answer

    Hydrogen forms at the cathode and oxygen at the anode, with twice the volume of hydrogen; without acid, practically no electrolysis occurs.

    (a) Oxygen is liberated at the anode (positive electrode) and hydrogen at the cathode (negative electrode). (b) One molecule of water contains two atoms of hydrogen and one atom of oxygen, so its decomposition gives two molecules of hydrogen for every molecule of oxygen, $2{\text{H}}_2\text{O}⟶2{\text{H}}_2+{\text{O}}_2$. Collected under the same conditions, the hydrogen therefore occupies twice the volume of the oxygen. (c) The electrolysis of water would not take place (practically no gas would be collected), because pure water is a very poor conductor of electricity; dilute ${\text{H}}_2{\text{SO}}_4$ provides the ions that let the current flow.

    Step-by-step solution →
19. Q279 CBSE 2020
    “Respiration is an ​exothermic process.” Justify ​this statement giving ​the chemical ​equation for the ​reaction ​involved. ​How is ​respiration different from breathing ?
    Show answer

    Respiration releases energy from glucose, ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$, so it is exothermic; breathing is only a physical exchange of gases.

    During respiration, glucose obtained from food combines with oxygen in the cells and is oxidised to carbon dioxide and water, and energy is released. Since energy is given out, respiration is an exothermic process: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2⟶6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$ Breathing is a physical process by which an organism takes in oxygen from the surroundings and gives out carbon dioxide, whereas respiration is a biochemical process in the cells in which food is oxidised to release energy.

    Step-by-step solution →
20. Q280 CBSE 2019
    On heating blue coloured powder of copper (II) nitrate in a boiling tube, black copper oxide, ${\text{O}}_2$ and a brown gas ​X is formed.

    1. (a)Identify ​the type of reaction and ​the gas X.
    2. (b)Write balanced chemical equation ​of the reaction.
    3. (c)Write the pH ​range of aqueous solution ​of ​the gas ​X.
    Show answer

    It is a thermal decomposition; gas X is nitrogen dioxide, whose aqueous solution is acidic with pH below 7.

    (a) It is a thermal decomposition reaction (a decomposition reaction brought about by heat). The brown gas X is nitrogen dioxide, ${\text{NO}}_2$. (b) $2{\text{Cu(NO}}_3{\text{)}}_2(s)\stackrel{\text{heat}}{\to }2\text{CuO}(s)+4{\text{NO}}_2(g)+{\text{O}}_2(g)$ (c) ${\text{NO}}_2$ is a non-metal oxide and dissolves in water to give an acidic solution, so its pH is less than $7$: it lies in the acidic range, from $0$ up to but not including $7$.

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