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 15 of 15

1. Q281 CBSE 2019
   1. (a)In the following reaction : ${\text{MnO}}_2+4\text{HCl}⟶{\text{MnCl}}_2+2{\text{H}}_2\text{O}+{\text{Cl}}_2$ Identify ​the ​oxidant ​and ​reductant.
   2. (b)Give reasons : (i) Antioxidants are added to foods containing fats and oils. (ii) White silver chloride turns ​grey in ​sunlight.
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   ${\text{MnO}}_2$ is the oxidant and $\text{HCl}$ the reductant; antioxidants stop fats going rancid, and light turns white silver chloride grey.

   (a) Oxidant: ${\text{MnO}}_2$, which loses oxygen and is itself reduced to ${\text{MnCl}}_2$. Reductant: $\text{HCl}$, which loses hydrogen and is itself oxidised to ${\text{Cl}}_2$. (b) (i) Fats and oils in food are oxidised by the oxygen of the air and turn rancid, spoiling the smell and taste; antioxidants prevent this oxidation. (ii) In sunlight, white silver chloride decomposes into grey silver and chlorine gas: $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$.

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2. Q282 CBSE 2019
   2 g of silver chloride is ​taken in a ​china dish and the china dish is placed in sunlight for ​sometime. What ​will ​be your ​observation in this case ? Write the chemical reaction involved in the form of ​a balanced chemical equation. Identify the type of chemical reaction.
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   White silver chloride turns grey in sunlight as it decomposes into silver and chlorine; it is a photolytic decomposition reaction.

   Observation: the white silver chloride turns grey in sunlight (silver is formed and chlorine gas is given off). Equation: $2\text{AgCl}(s)\stackrel{\text{sunlight}}{\to }2\text{Ag}(s)+{\text{Cl}}_2(g)$ Type: decomposition reaction brought about by light (photolytic decomposition).

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3. Q283 CBSE 2019
   Identify the type ​of ​reactions taking place in each of the following cases and write the balanced chemical equation for the ​reactions.

   1. (a)Zinc reacts ​with silver nitrate to produce ​zinc nitrate and ​silver.
   2. (b)Potassium iodide reacts with lead nitrate to produce potassium ​nitrate and lead iodide.
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   Zinc and silver nitrate form a displacement reaction; potassium iodide and lead nitrate form a double displacement (precipitation) reaction.

   (a) Displacement reaction (zinc is more reactive than silver): $\text{Zn}(s)+2{\text{AgNO}}_3(aq)⟶\text{Zn}({\text{NO}}_3{)}_2(aq)+2\text{Ag}(s)$ (b) Double displacement (precipitation) reaction, giving a yellow precipitate of lead iodide: $2\text{KI}(aq)+\text{Pb}({\text{NO}}_3{)}_2(aq)⟶{\text{PbI}}_2(s)+2{\text{KNO}}_3(aq)$

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4. Q284 CBSE 2019
   (a) Classify the following reactions into ​different types :

   1. (i)${\text{AgNO}}_3(\text{aq})+\text{NaCl}(\text{aq})⟶\text{AgCl}(\text{s})+{\text{NaNO}}_3(\text{aq})$
   2. (ii)$\text{CaO}(\text{s})+{\text{H}}_2\text{O}(l)⟶{\text{Ca(OH)}}_2(\text{aq})$
   3. (iii)$2{\text{KClO}}_3(\text{s})\stackrel{\Delta }{\to }2\text{KCl}(\text{aq})+3{\text{O}}_2(\text{g})$
   4. (iv)$\text{Zn}+{\text{CuSO}}_4⟶{\text{ZnSO}}_4+\text{Cu}$ (b) Translate ​the following ​statement into ​a ​balanced chemical equation ​: “Barium chloride reacts with ​aluminium sulphate to give aluminium ​chloride and barium sulphate.”
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   The reactions are double displacement, combination, decomposition and displacement; the balanced equation uses $3{\text{BaCl}}_2$, $2{\text{AlCl}}_3$ and $3{\text{BaSO}}_4$.

   (a) (i) Double displacement reaction (also a precipitation reaction). (ii) Combination reaction. (iii) Decomposition reaction (thermal decomposition). (iv) Displacement reaction. (b) $3{\text{BaCl}}_2+{\text{Al}}_2({\text{SO}}_4{)}_3\to 2{\text{AlCl}}_3+3{\text{BaSO}}_4$

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5. Q285 CBSE 2019
   2 g of ​ferrous sulphate crystals are heated ​in a dry boiling tube.

   1. (a)List any two observations.
   2. (b)​Name the ​type ​of chemical reaction ​taking place.
   3. (c)Write balanced chemical ​equation for the reaction and name the products ​formed.
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   Heated ferrous sulphate crystals turn white, leave reddish-brown ferric oxide and give off ${\text{SO}}_2$ and ${\text{SO}}_3$ in a thermal decomposition.

   (a) Any two: the pale green crystals turn white as they lose their water of crystallisation (droplets of water appear near the mouth of the tube); a reddish-brown solid, ferric oxide (${\text{Fe}}_2{\text{O}}_3$), is left; gases with the smell of burning sulphur (${\text{SO}}_2$ and ${\text{SO}}_3$) are given off. (b) Thermal decomposition reaction. (c) $2{\text{FeSO}}_4(s)\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$. Products: ferric oxide (iron(III) oxide), sulphur dioxide and sulphur trioxide.

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6. Q286 CBSE 2019
   When potassium iodide solution is added to a solution of lead (II) nitrate in a test tube, a ​precipitate ​is formed.

   1. (a)What is the colour of this precipitate ? Name the ​compound precipitated.
   2. (b)Write the balanced chemical ​equation for this reaction.
   3. (c)List two types ​of reactions in which this reaction ​can ​be placed.
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   Yellow lead iodide, ${\text{PbI}}_2$, precipitates; the reaction is both a double displacement reaction and a precipitation reaction.

   (a) The precipitate is yellow; it is lead iodide, ${\text{PbI}}_2$. (b) $\text{Pb}({\text{NO}}_3{)}_2(\text{aq})+2\text{KI}(\text{aq})\to {\text{PbI}}_2(\text{s})+2{\text{KNO}}_3(\text{aq})$ (c) Double displacement reaction and precipitation reaction.

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7. Q287 CBSE 2019
   You might ​have noted that ​when ​copper powder is heated in a china dish, the reddish brown surface of copper powder becomes ​coated with a black substance.

   1. (a)Why ​has this black substance formed ?
   2. (b)What is this black ​substance ​?
   3. (c)Write the chemical equation of the reaction that takes place.
   4. (d)How can the black coating on the surface be turned reddish ​brown ?
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   Heated copper combines with oxygen to form black copper(II) oxide; passing hydrogen over it reduces it back to reddish-brown copper.

   (a) On heating, copper combines with the oxygen of the air: the copper is oxidised, and the oxide formed coats its surface. (b) Copper(II) oxide, $\text{CuO}$. (c) $2\text{Cu}+{\text{O}}_2\stackrel{\text{heat}}{\to }2\text{CuO}$ (d) By passing hydrogen gas over the heated black coating: $\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$. The copper(II) oxide is reduced back to reddish-brown copper.

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8. Q288 CBSE 2019
   (a) What is a double displacement reaction ? Explain ​with an example. (b) A small amount of quick ​lime ​is added to water in a ​beaker.

   1. (i)Name ​and define the type of reaction that has taken place.
   2. (ii)Write balanced chemical equation for the above ​reaction ​and the ​chemical name of the product formed.
   3. (iii)List two main observations of this reaction.
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   Double displacement is an exchange of ions between two compounds; quick lime and water combine to give slaked lime, releasing much heat.

   (a) A double displacement reaction is a reaction in which two compounds exchange their ions to form two new compounds. Example: when solutions of sodium sulphate and barium chloride are mixed, a white precipitate of barium sulphate forms, ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$: the ${\text{Ba}}^{2+}$ ions pair with the ${\text{SO}}_4^{2-}$ ions and the ${\text{Na}}^{+}$ ions with the ${\text{Cl}}^{-}$ ions. (b) (i) Combination reaction: a reaction in which two or more substances (elements or compounds) combine to form a single product. (ii) $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$; the product is calcium hydroxide (slaked lime). (iii) The quick lime reacts vigorously with the water, often with a hissing sound, and a large amount of heat is released, so the beaker becomes hot.

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9. Q289 CBSE 2019
   1. (a)Design an activity ​to demonstrate the ​decomposition reaction of lead nitrate.
   2. (b)​Draw labelled diagram of ​the ​experimental set-up. List two main observations.
   3. (c)Write ​balanced chemical equation for the reaction ​stating the physical ​state of the reactant and the products.
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   Heating lead nitrate in a boiling tube gives brown nitrogen dioxide fumes and a lead oxide residue: $2{\text{Pb(NO}}_3{)}_2\to 2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$.

   (a) Take about $2$ g of lead nitrate powder in a boiling tube, hold the tube with a pair of tongs and heat it over a burner flame, with its mouth pointed away from everyone; lead nitrate decomposes. (b) Labelled diagram: boiling tube containing lead nitrate powder, held with a pair of tongs over a burner flame, with brown fumes of nitrogen dioxide leaving its mouth. Observations: (1) brown fumes of nitrogen dioxide are given off; (2) a residue of lead(II) oxide is left, reddish brown while hot and yellow when cold. (c) $2{\text{Pb(NO}}_3{)}_2\text{(s)}\stackrel{\text{heat}}{\to }2\text{PbO(s)}+4{\text{NO}}_2\text{(g)}+{\text{O}}_2\text{(g)}$: lead nitrate (solid) gives lead(II) oxide (solid), nitrogen dioxide (gas) and oxygen (gas).

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10. Q290 CBSE 2019
    A student ​mixes ​sodium ​sulphate powder in ​barium ​chloride powder. What change would the student observe on mixing the two ​powders ? Justify your answer and explain ​how he can obtain ​the desired change.
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    The dry powders show no change; dissolving each in water and mixing the solutions gives a white precipitate of barium sulphate.

    Observation: no change; the two dry powders simply form a white mixture and no precipitate appears. Justification: the reaction is an exchange of ions, and in the solid (powder) form the ions are held in fixed positions in the crystals, so the ${\text{Ba}}^{2+}$ ions of barium chloride and the ${\text{SO}}_4^{2-}$ ions of sodium sulphate are not free to meet and react. Desired change: dissolve each powder in water separately and mix the two solutions. The ions are then free to move and a white precipitate of barium sulphate forms at once (a double displacement reaction): ${\text{Na}}_2{\text{SO}}_4(\text{aq})+{\text{BaCl}}_2(\text{aq})\to {\text{BaSO}}_4(\text{s})+2\text{NaCl}(\text{aq})$

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11. Q291 CBSE 2019
    A student wants ​to study ​a ​decomposition reaction by taking ferrous ​sulphate ​crystals. Write two ​precautions he ​must observe while performing the experiment.
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    Point the tube's mouth away from people because of the harmful gases, and hold the tube in a slanting position.

    Any two of the following: 1. Point the mouth of the boiling tube away from yourself and others, because harmful, pungent gases (${\text{SO}}_2$ and ${\text{SO}}_3$) come out of it. 2. Hold the boiling tube in a slanting (inclined) position while heating it. 3. Hold the boiling tube with a test-tube holder (tongs), never with the bare hand. Other correct precautions, such as using a dry boiling tube, heating it gently and not inhaling the gases directly, also earn credit.

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12. Q292 CBSE 2019
    State the observation and inference made by a student when he brings

    1. (i)a ​wet ​blue litmus paper and
    2. (ii)a wet red ​litmus paper in ​contact with the ​gas liberated during thermal decomposition of ferrous sulphate.
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    Wet blue litmus turns red and wet red litmus is unchanged, so the gases, ${\text{SO}}_2$ and ${\text{SO}}_3$, are acidic.

    On heating, ferrous sulphate decomposes: $2{\text{FeSO}}_4\to {\text{Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3$. (i) Observation: the wet blue litmus paper turns red. (ii) Observation: the wet red litmus paper shows no change; it stays red. Inference: the gases liberated, sulphur dioxide and sulphur trioxide, are acidic in nature - they dissolve in the water on the wet paper to form acids.

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13. Q293 CBSE 2018
    An ​aqueous solution ​of a substance ‘P’ is used ​in white washing. Name the substance ​‘P’ ​and ​write its chemical ​formula. Also write the chemical equation for the reaction of substance ​‘P’ with water.
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    P is calcium oxide (quick lime), $\text{CaO}$; with water it forms calcium hydroxide and gives out heat.

    P is calcium oxide (quick lime); its chemical formula is $\text{CaO}$. With water it forms calcium hydroxide (slaked lime), whose solution is used for white washing, and a large amount of heat is given out: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{heat}$

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14. Q294 CBSE 2018
    Decomposition reactions require energy either ​in the ​form ​of heat or light ​or electricity for breaking down the reactants. Write one equation each for ​decomposition ​reactions where energy is supplied in the ​form of heat, light and electricity.
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    Heat decomposes ${\text{CaCO}}_3$ into $\text{CaO}$ and ${\text{CO}}_2$, sunlight decomposes $\text{AgCl}$ into $\text{Ag}$ and ${\text{Cl}}_2$, and electricity decomposes water into ${\text{H}}_2$ and ${\text{O}}_2$.

    Heat (thermal decomposition): ${\text{CaCO}}_3(\text{s})\stackrel{\text{heat}}{\to }\text{CaO}(\text{s})+{\text{CO}}_2(\text{g})$ Light (photolytic decomposition): $2\text{AgCl}(\text{s})\stackrel{\text{sunlight}}{\to }2\text{Ag}(\text{s})+{\text{Cl}}_2(\text{g})$ Electricity (electrolytic decomposition): $2{\text{H}}_2\text{O}(\text{l})\stackrel{\text{electricity}}{\to }2{\text{H}}_2(\text{g})+{\text{O}}_2(\text{g})$ Other correct equations also earn the marks, for example $2{\text{FeSO}}_4(\text{s})\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(\text{s})+{\text{SO}}_2(\text{g})+{\text{SO}}_3(\text{g})$ for heat and $2\text{AgBr}(\text{s})\stackrel{\text{sunlight}}{\to }2\text{Ag}(\text{s})+{\text{Br}}_2(\text{g})$ for light.

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15. Q295 CBSE 2018
    In ​the ​reaction : ${\text{MnO}}_2+4\text{HCl}\to {\text{MnCl}}_2+2{\text{H}}_2\text{O}+{\text{Cl}}_2$ (a) Name the compound

    1. (i)oxidised, ​
    2. (ii)reduced. (b) Define oxidation ​and ​reduction on ​its basis.
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    $\text{HCl}$ is oxidised (loses hydrogen) and ${\text{MnO}}_2$ is reduced (loses oxygen); oxidation is gain of oxygen or loss of hydrogen, reduction the reverse.

    (a) (i) Compound oxidised: hydrogen chloride (hydrochloric acid), $\text{HCl}$; it loses hydrogen and is converted to chlorine, ${\text{Cl}}_2$. (ii) Compound reduced: manganese dioxide, ${\text{MnO}}_2$; it loses oxygen and is converted to ${\text{MnCl}}_2$. (b) Oxidation is the gain of oxygen or the loss of hydrogen by a substance; in this reaction $\text{HCl}$ loses hydrogen, so it is oxidised. Reduction is the loss of oxygen or the gain of hydrogen by a substance; in this reaction ${\text{MnO}}_2$ loses oxygen, so it is reduced.

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16. Q296 CBSE 2018
    What is ​the difference between displacement reaction and ​double displacement reaction ? Give two ​examples each ​of ​these reactions.
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    Displacement: a more reactive element displaces a less reactive one from its compound; double displacement: two compounds exchange ions, often giving a precipitate.

    Difference: in a displacement reaction, a more reactive element displaces a less reactive element from its compound in solution, so an element and a compound react to give a new element and a new compound. In a double displacement reaction, two compounds react by exchanging their ions to form two new compounds; one of the products is often an insoluble precipitate. Displacement reactions: (i) $\text{Fe(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu(s)}$ (ii) $\text{Zn(s)}+{\text{FeSO}}_4\text{(aq)}\to {\text{ZnSO}}_4\text{(aq)}+\text{Fe(s)}$ Double displacement reactions: (i) ${\text{AgNO}}_3\text{(aq)}+\text{NaCl(aq)}\to \text{AgCl(s)}+{\text{NaNO}}_3\text{(aq)}$ (ii) ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$ Other correct examples also earn credit.

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17. Q297 CBSE 2018
    (a) What is ​a balanced ​chemical equation ? Why should chemical equations ​be balanced ? (b) Write the chemical equation of the reaction in which the following changes have taken place ​:

    1. (i)​Change ​in colour.
    2. (ii)Change in temperature.
    3. (iii)​Formation of precipitate.
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    A balanced equation has equal atoms of each element on both sides because mass is conserved; three equations show colour change, temperature change and precipitation.

    (a) A balanced chemical equation is one in which the number of atoms of each element is the same on both sides, the reactants and the products. Example: $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$ ($4$ H and $2$ O atoms on each side). Chemical equations must be balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction. The atoms are only rearranged, so the total mass of the products must equal the total mass of the reactants, which is true only when every atom of the reactants appears in the products. A balanced equation also shows the correct ratio in which the substances react. (b) (i) Change in colour (the blue copper sulphate solution turns pale green): $\text{Fe(s)}+{\text{CuSO}}_4\text{(aq)}\to {\text{FeSO}}_4\text{(aq)}+\text{Cu(s)}$ (ii) Change in temperature (the mixture becomes hot): $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}+\text{heat}$ (iii) Formation of precipitate (a white precipitate of barium sulphate): ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq)}$ (Any other correct balanced equation that shows each change also earns credit.)

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18. Q298 CBSE 2018
    What is observed when ​a solution ​of sodium sulphate is added to ​a solution of barium chloride taken in ​a test tube ​? Write equation for the chemical reaction involved and ​name ​the type of reaction in this case.
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    A white precipitate of barium sulphate forms; the reaction is a double displacement (precipitation) reaction.

    Observation: a white precipitate (of barium sulphate) is formed. Equation: ${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)\to {\text{BaSO}}_4(s)+2\text{NaCl}(aq)$ Type of reaction: double displacement reaction (also a precipitation reaction), since the two compounds exchange their ions.

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