Intext Questions

Q.1       Why should a magnesium ribbon be cleaned before burning in air?
Sol.

Magnesium forms magnesium oxide when reacts with oxygen present in air. On the magnesium ribbon a stable layer of magnesium oxide is formed when it is stored, which prevents the further reaction of magnesium ribbon. Hence, to clean up the layer of magnesium oxide, magnesium ribbon is cleaned before burning.

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Q.2       Write the balanced equation for the following chemical reactions.
                 (i) Hydrogen + Chlorine → Hydrogen chloride
                 (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
                 (iii) Sodium + Water → Sodium hydroxide + Hydrogen
Sol.

(i) H2+CL2→2HCl
(ii) 3BaCl2+Al2(SO4)3→3BaSO4+2AlCl3
(iii) 2Na+2H2O→2NaOH+H2

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Q.3       Write a balanced chemical equation with state symbols for the following reactions.
                (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
               (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Sol.

(i) BaCl2(aq)+NaSO4→BaSO4(S)+2NaCl(aq)
(ii) NaOH(aq)+HCl(aq)→Nacl(aq)+H2O(l)

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NCERT Book Page Number – 10

Q.1       A solution of a substance ‘X’ is used for white washing.
                (i) Name the substance ‘X’ and write its formula.
                (ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Sol.

(i) The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime.
The chemical formula of Calcium Oxide is CaO.
(ii)When calcium oxide reacts with water it forms calcium hydroxide. Calcium hydroxide is also known as slaked lime.

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Q.2       Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Sol.

(Hydrolysis of water has been performed in the activity 1.7 in the text book.)
When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1.

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NCERT Book – Page Number – 13

Q.1       Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Sol.

The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution.
The colour of ferrous sulphate is green. That’s why the solution becomes green.

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Q.2       Give an example of a double displacement reaction other than the one given in Activity 1.10.
Sol.

Pb(NO3)2(aq)+Kl(aq)→KNO3+Pbl2
When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction.

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Q.3        Identify the substances that are oxidised and the substances that are reduced in the following reactions.
                 (i) 4Na(S)+O2(g)→2Na2O(S)
                 (ii) CuO(S)+H2(g)→Cu(S)+H2O(l)
Sol.

(i) In this reaction sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently oxygen is reduced.
(ii) In this reaction Hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand copper loses oxygen and hence, reduced.

Exercise

Q.1       Which of the statements about the reaction below are incorrect?
              2PbO(s)+C(s)→2Pb(s)+CO2(g)
              (a) Lead is getting reduced.
              (b) Carbon dioxide is getting oxidized.
              (c) Carbon is getting oxidized.
              (d) Lead oxide is getting reduced.
              (i) (a) and (b)        (ii) (a) and (c)      (iii) (a), (b) and (c)         (iv) all
Sol.        (i) (a) and (b)

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Q.2       2.Fe2O3+2Al→Al2O3+2Fe
              The above reaction is an example of a
              (a) combination reaction.
              (b) double displacement reaction.
              (c) decomposition reaction.
              (d) displacement reaction.
Sol.        (d) displacement reaction.

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Q.3       What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
            (a) Hydrogen gas and iron chloride are produced.
             (b) Chlorine gas and iron hydroxide are produced.
             (c) No reaction takes place.
             (d) Iron salt and water are produced.
Sol.       (a) Hydrogen gas and iron chloride are produced.

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Q.4      What is a balanced chemical equation? Why should chemical equations be balanced?
Sol.       When the number of atoms of reactants is equal to the number of atoms of the products, the reaction is called a balanced chemical equation. According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always.

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Q.5      Translate the following statements into chemical equations and then balance them.
            (a) Hydrogen gas combines with nitrogen to form ammonia.
            (b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
            (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
            (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Sol.      (a) 3H2+N2→2NH3
(b) 2H2S+3O2→2H2O+2SO2
(c) 3BaCl2+Al2(SO4)3→2AlCl3+3BaSO4
(d) 2K+2H2O→2KOH+H2

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Q.6     Balance the following chemical equations:
           (a) HNO3+Ca(OH)2→Ca(NO3)2+H2O
           (b) NaOH+H2SO4→Na2SO4+H2O
           (c) NaCl+AgNO3→AgCl+NaNO3
           (d) BaCl2+H2SO4→BaSO4+HCl
Sol.      (a) 2HNO3+Ca(OH)2→Ca(NO3)2+2H2O
(b) 2NaOH+H2SO4→Na2SO4+2H2O
(c) NaCl+AgNO3→AgCl+NaNO3
(d) BaCl2+H2SO4→BaSO4+2HCl

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Q.7     Write the balanced chemical equations for the following reactions.
           (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
           (b) Zinc + Silver nitrate → Zinc nitrate + Silver
           (c) Aluminium + Copper chloride → Aluminium chloride + Copper
           (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Sol.     (a) Ca(OH)2+CO2→CaCO3+H2O
(b) Zn+2AgNO3→Zn(NO3)2+2Ag
(c) 2Al+3CuCl2→2AlCl3+3Cu
(d) BaCl2+K2SO4→BaSO4+2KCl

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Q.8     Write the balanced chemical equation for the following and identify the type of reaction in each case.
           (a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
           (b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
           (c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
           (d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Sol.     (a) 2KBr(aq)+BaCl2→2Kl(aq)+BaBr3(s)
It is a double displacement reaction.
(b) ZnCO3(s)→ZnO(s)+CO2(g)
It is a decomposition reaction.
(c) H2(g)+Cl2→2HCl(g)
It is a combination reaction.
(d) Mg(s)+2HCl(aq)→MgCl2+H2(g)
It is a displacement reaction.

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Q.9     What does one mean by exothermic and endothermic reactions? Give examples.
Sol.      Exothermic reaction – A reaction in which heat is released is called the exothermic reaction. Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.

Endothermic reaction – A reaction in which heat is absorbed is called endothermic reaction.
Example – When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.

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Q.10     Why is respiration considered an exothermic reaction? Explain.
Sol.         In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.
C6H12O6(aq)+6O2→6CO2(aq)+6H2O(I)+Energy

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Q.11      Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Sol.         In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.

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Q.12      Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Sol.

In this reaction energy is supplied in the form of heat.

In this reaction energy is supplied in the form of light,i.e.sunlight

In this reaction energy is supplied in the form of electricity.

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Q.13      What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Sol.        When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction. In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place.
Displacement reaction

Double displacement reaction

 

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Q.14     In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Sol. 

 

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Q.15     What do you mean by a precipitation reaction? Explain by giving examples.
Sol.        Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.

In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

 

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Q.16     Explain the following in terms of gain or loss of oxygen with two examples each.
              (a) Oxidation                  (b) Reduction
Sol.        Oxidation – When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation.                 

In this reaction copper gains oxygen and hence is oxidized.

In the above reaction hydrogen gains oxygen, and hence oxidized.
Reduction – If a substance loses oxygen or gains hydrogen during a reaction, it is that the                        substance is reduced. This phenomenon is called reduction.
ZnO + C

Zn + CO
In this reaction zinc loses oxygen, and hence called is reduced.

In this reaction Mangnese dioxide loses oxygen and hence is called reduced.

 

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Q.17     A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Sol.       Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.

Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.

 

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Q.18    Why do we apply paint on iron articles?
Sol.       When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.

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Q.19    Oil and fat containing food items are flushed with nitrogen. Why?
Sol.       When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.

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Q.20     Explain the following terms with one example each.
              (a) Corrosion            (b) Rancidity
Sol.        (a) Corrosion – Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
(b) Rancidity – When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.

Exemplar

Multiple Choice Questions :

Q.1 Which of the following is not a physical change?

(a) Boiling of water to give water vapour

(b) Melting of ice to give water

(c) Dissolution of salt in water

(d) Combustion of Liquefied Petroleum Gas (LPG)

Sol. (d)

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Q.2 The following reaction is an example of

4NH₃ (g) + 5O₂ (g) → 4NO + 6H₂O

(i) Displacement reaction

(ii) Combination reaction.

(iii) Redox reaction.

(iv) Neutralisation reaction

(a) (i) and (iv) (b) (ii) and (iii)

(c) (i) and (iii) (d) (iii) and (iv)

Sol. (d)

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Q.3 Which of the following statements about the given reaction are correct?

3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g)

(i) Iron metal is getting oxidised

(ii) Water is getting reduced

(iii) Water is acting as reducing agent

(iv) Water is acting as oxidising agent

(a) (i), (ii) and (iii)

(b) (iii) and (iv)

(c) (i), (ii) and (iv)

(d) (ii) and (iv)

Sol. (c)

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Q.4 Which of the following are exothermic processes?

(i) Reaction of water with quick lime

(ii) Dilution of an acid

(iii) Evaporation of water

(iv) Sublimation of camphor (crystals)

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (iii) and (iv)

Sol. (a)

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Q.5 Three beakers labelled as A, B and C each containing 25 mL of water was taken. A small amount of NaOH, anhydrous CuSO₄and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is (are) correct?

(i) In beakers A and B, exothermic process has occurred.

(ii) In beakers A and B, endothermic process has occurred.

(iii) In beaker C exothermic process has occurred.

(iv) In beaker C endothermic process has occurred.

(a) (i) only

(b) (ii) only

(c) (i) and (iv)

(d) (ii) and (iii)

Sol. (c)

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Q.6 A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

(a) KMnO₄ is an oxidising agent, it oxidises FeSO₄

(b) FeSO₄ acts as an oxidising agent and oxidises KMnO₄

(c) The colour disappears due to dilution; no reaction is involved

(d) KMnO₄ is an unstable compound and decomposes in presence of FeSO₄ to a colourless compound.

Sol. (d)

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Q. 7 Which among the following is (are) double displacement reaction(s)?

Pb + CuCl₂→ PbCl₂ + Cu

Na₂SO₄+ NaCl₂ →  BaSO₄ + 2NaCl

C + O₂→ CO₂

CH₄+ 2O₂  → CO₂ + 2H₂O

(i) and (iv)

(ii) only

(i) and (iii)

(iii) and (iv)

Sol. (b)

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Q.8 Which among the following statement(s) is (are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to

(i) The formation of silver by decomposition of silver chloride

(ii) Sublimation of silver chloride

(iii) Decomposition of chlorine gas from silver chloride

(iv) Oxidation of silver chloride

(a) (i) only (b) (i) and (iii)

(c) (ii) and (iii) (d) (iv) only

Sol.  (b)

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Q.9 Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves readily in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed?

(i) It is an endothermic reaction

(ii) It is an exothermic reaction

(iii) The pH of the resulting solution will be more than seven

(iv) The pH of the resulting solution will be less than seven

(a) (i) and (ii) (b) (ii) and (iii)

(c) (i) and (iv) (d) (iii) and (iv)

Sol. (b)

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Q.10 Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?

(i) Displacement reaction

(ii) Precipitation reaction

(iii) Combination reaction

(iv) Double displacement reaction

(a) (i) only (b) (ii) only

(c) (iv) only (d) (ii) and (iv)

Sol. (d)

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Q.11 Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is

(a) 1:1

(b) 2:1

(c) 4:1

(d) 1:2

Sol. (b)

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Q.12 Which of the following is (are) an endothermic process (es)?

(i) Dilution of sulphuric acid

(ii) Sublimation of dry ice

(iii) Condensation of water vapours

(iv) Evaporation of water

(a) (i) and (iii) (b) (ii) only

(c) (iii) only (d) (ii) and (iv)

Sol. (d)

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Q.13 In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?

(a) Lead sulphate (insoluble)

(b) Lead acetate

(c) Ammonium nitrate

(d) Potassium sulphate

Sol. (b)

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Q.14 Which of the following gases can be used for storage of fresh sample of an oil for a long time?

(a) Carbon dioxide or oxygen

(b) Nitrogen or oxygen

(c) Carbon dioxide or helium

(d) Helium or nitrogen

Sol. (d)

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Q.15 The following reaction is used for the preparation of oxygen gas in the laboratory

2KClO₃ (s) + heat + catalyst →  2KCl + 3O₂

Which of the following statement(s) is (are) correct about the reaction?

(a) It is a decomposition reaction and endothermic in nature

(b) It is a combination reaction

(c) It is a decomposition reaction and accompanied by release of heat

(d) It is a photochemical decomposition reaction and exothermic in nature

Sol. (a)

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Q.16 Which one of the following processes involves chemical reactions?

(a) Storing of oxygen gas under pressure in a gas cylinder

(b) Liquefaction of air

(c) Keeping petrol in a china dish in the open

(d) Heating copper wire in presence of air at high temperature

Sol. (d)

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Q.17 In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?

(a) 2H₂ (l) + O₂ (l) → 2H₂O (g)

(b) 2H₂ (g) + O₂ (l) → 2H₂O (g)

(c) 2H₂ (g) + O₂ (g) → 2H₂O (l)

(d) 2H₂ (g) + O₂ (g) → 2H₂O (g)

Sol. (c)

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Q.18 Which of the following are combination reactions?

(i) 2KClO₃ → 2KCl + 3O₂

(ii) MgO + H₂O →  Mg(OH)₂

(iii) 4Al + 3O₂ → 2Al₂O₃

(iv) Zn + FeSO₄ → ZnSO₄ + Fe

(a) (i) and (iii) (b) (iii) and (iv)

(c) (ii) and (iv) (d) (ii) and (iii)

Sol. (d)