NCERT solutions of class 10 science ch-1(chemical reactions and equations)

NCERT Intext question and Answers of ch-1 chemical reactions and equations. 

Pg:6

1. Why should a Magnesium ribbon be cleaned before burning in air ?

Ans:  Magnesium ribbon should be cleaned before burning in air so that unwanted impurities are removed and we burn only pure magnesium metal.

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

Ans:       (i) H2 + Cl2 –—› HCl

                    H2 + Cl2 –—› 2HCl

   (ii) BaCl2 + Al2(SO4)3 —–› BaSO4 + AlCl3

  3BaCl2 + Al2(SO)3 –—› 3BaSO4 + 2AlCl3

         (iii) Na + H2O –—› NaOH + H2

              2Na + 2H2O —–› 2NaOH + H2 

 

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.

Ans: BaCl2(aq) + Na2SO4 (aq) –—› BaSO4 (s) + 2NaCl(aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans: NaOH(aq) + HCl(aq) –—› NaCl(aq) + H2O

Pg:10

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' name in (i) above with water. 

Ans: (i) 'X' is a quick lime, CaO.

        (ii) CaO(s) +H2O(l) –—› Ca(OH)2(aq) 

             Here,  ~ Ca(OH)2—slaked lime

              

2. Why is the amount of gas collected in one of the test tubes in activity 1.7 (textbook) double of the amount collected in the other? Name this gas.

Ans:  In activity (1.7) water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode, according to the given chemical reaction:

          2H2O(l) –—› 2H2(g) + O2(g)

Thus, two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of Oxygen gas or the amount of hydrogen gas collected would be double than that of Oxygen gas.

Pg-13

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

Ans: When an iron nail is dipped in copper sulphate solution, it becomes brownish in colour and colour of copper sulphate fades due to the following reaction:

     Fe(s) +CuSO4(aq) –—› FeSO4(aq) + Cu(s)

In this reaction iron has replaced copper from its solution. Thus amount of copper sulphate decreases and its colour fades.

2. Give an example of a Double Displacement reaction other than the one given in activity 1.10.

Ans: When a solution of hydrochloric acid is added to a solution of lead nitrate, a white precipitate of lead chloride is formed.

 Pb(NO3)2(aq) + 2HCl(aq) –—› pbCl2(s) + 2HNO3(aq)

Pb++ ions from lead nitrate combines with  2CL-  ions from hydrochloric acid and a precipitate of lead chloride is formed. Thus, there is an exchange of ions between the two reactants that is lead nitrate and hydrochloric acid, and thus this reaction is a double displacement reaction.

3. Identify the substances that are oxidised and the substances that are reduced in the  following reactions.

(i) 4Na(s) + O2(g) –—› 2Na2O(s)

Ans: Substance Oxidised: Na(s),

         Substance Reduced: O2(g).

(ii) CuO(s) + H2(g) –—› Cu(s) +H2O(l)

Ans: Substance Oxidised: H2(g)

         Substance Reduced: CuO(s).

NCERT Exercise Solutions: pg 14-16

 

1. Which of these 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 Oxidised.

(c) Carbon is getting oxidised.

(d) lead oxide is getting reduced.

     (i)    (a) and (b)

    (ii)    (a) and (c)   

    (iii)   (a), (b) and (c)

    (iv)    all

Ans: (i) (a) and (b)  { are Incorrect}

2. Fe2O3 + 2Al —–› Al2O3+2Fe

   The above reaction is an example of

(a) Combination reaction.

(b) Double displacement reaction.

(c) Decomposition reaction. 

(d) Displacement reaction.

Ans: (d) Displacement reaction.

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.

Ans: (a) Hydrogen gas and iron chloride are produced.

4. What is balanced chemical equation? Why should chemical equations be balanced?

Ans: In a balanced equation, the number of atoms of each element on both sides of the  equation are equal. A balanced equation satisfies mass balance as well as charge balance.

           The chemical equation must be balanced with respect to mass because matter can neither be created nor destroyed. The equation must be balanced with respect to charge because the electrons lost by atoms to form cations are equal to the electrons gained by other atoms to form anions.

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 sulphur 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.

Ans: (a) 3H2 + N2(g) –—› 2NH3(g)

         (b) 2H2S(g) + 3O2(g) –—› 2SO2(g) + 2H2O(l) 

         (c) 3BaCl2(aq) + Al2(SO4)3(aq)–—›2AlCl3(aq) + 3BaSO4(s)

         (d) 2K(s) + 2H2O(l) –—›2KOH(aq) + H2(g).

 

6. Balance the following chemical equation: 

(a) HNO3 + Ca(OH)2 –—› Ca(NO3)2 + H2O

 Ans: 2HNO3 + Ca(OH)2 –—› Ca(NO3)2 + 2H2O

(b) NaOH + H2SO4 –—› Na2SO4 + H2O

Ans: 2NaOH + H2SO4  –—› Na2SO4 + 2H2O

(c) NaCl + AgNO3 –—› AgCl +NaNO3

Ans: NaCl + AgNO3 –—› AgCl + NaNO3

(d) BaCl2 + H2SO4 –—› BaSO4 + HCl

Ans: BaCl2 + H2SO4 –—› BaSO4 + 2HCl.

7. write the balanced chemical equations for the following reactions:

(a) calcium hydroxide + carbon dioxide –—› calcium carbonate + water

Ans: Ca(OH)2 + CO2 —–› CaCO3 + H2O

(b) zinc + silver nitrate —–› zinc nitrate + silver

Ans: Zn + 2AgNO3 –—› Zn(NO3)2 + 2Ag

(c) aluminium + copper chloride –—› aluminium chloride + copper

Ans: 2Al + 3CuCl2 –—› 2AlCl3 + 3Cu

(d) Barium chloride + potassium sulphate –—› Barium sulphate + potassium chloride.

Ans: BaCl2 + K2SO4 –—›BaSO4 + 2KCl.

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)

Ans: 2KBr(aq) + BaI2(aq) –—› 2KI(aq) + BaBr2(s)

        Type– Double Displacement reaction.

(b) zinc carbonate(s) –—› zinc oxide(s) + carbon dioxide(g)

Ans: ZnCO3(s) –—› ZnO(s) + CO2(g)

        Type– Decomposition reaction

(c) hydrogen(g) + chlorine(g)–—› hydrogen chloride(g) 

Ans: H2(g) + Cl2(g) –—› 2HCl(g)

        Type: Combination reaction

(d) magnesium(s) + hydrochloric acid(aq) –—› Magnesium chloride (aq) + hydrogen(g)

Ans: Mg(s) + 2HCl(aq) –—› MgCl2(aq) + H2(g)

       Type: Displacement reaction.

9. What does one mean by exothermic and endothermic reactions give examples.

Ans: (i) one knows that constituent elements in a compound are bond by chemical bonds. During a chemical change,when bonds between the constituents of the reactants are broken, energy is consumed to wear as energy is given out when the bonds are formed as in the case of products. Thus, depending on whether overall energy is given out or consumed, the reaction is exothermic or endothermic.

      Exothermic reaction: chemical reactions in which energy is evolved (are given out) are known as exothermic reactions. for example,

    C + O2 –—› CO2 +heat (∆)

       Endothermic reactions: Reactions in which energy is absorbed are called endothermic reactions. for example, 

   N2 + O2 –—› 2NO2 - heat (∆)

10. Why is respiration considered an exothermic reaction? Explain.

Ans: During digestion, food is broken down into simpler substances. Food like rice, potato and bread are made up of carbohydrates. These carbohydrates are further broken down to glucose. Glucose during respiration (inhalation of oxygen) is oxidised with the liberation of energy as shown below:

C6H12O6(aq) + 6O2(aq) –—› 6CO2(aq) + 6H2O + energy

 Thus, respiration is an exothermic process.

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Ans:  The decomposition reaction is the opposite of combination reaction.

in a combination reaction, two or more substances simply combine to form a new substance example magnesium combines with oxygen to give magnesium oxide.

    2Mg + O2 –—› 2MgO

         In Decomposition reaction, a single compound breaks down to produce two or more simpler substances, example.., mercuric oxide on heating decomposes to Mercury and Oxygen.

     2HgO –—› 2Hg + O2

12.write an equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Ans: 

   Heat:  2MgO(s) –—› 2Mg(s) + O2(g)

   Light:  AgBr(s) –—› Ag(s) + Br2(g)

  Electricity:  2H2O(l) –—› 2H2(g) + O2(g).

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans: In displacement reactions, a more reactive metal displaces a less reactive metal from its solution why in double displacement reactions, two reactants in solution exchange their ions. For example,

 (i) Fe(s) + CuSO4(aq) –—› Cu(s) + FeSO4(aq)

This displacement reaction where Iron displaces Copper from its solution.

(ii) AgNO3(aq) + NaCl(aq) –—› AgCl(s) + NaNO3(aq)

This is a Double Displacement reaction where silver nitrate and Sodium chloride exchange Cl- and NO3- ions between them.

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

Ans: 2AgNO3(aq) + Cu(s) –—› Cu(NO3)2(aq) + 2Ag(s).

15. What do you mean by a precipitation reaction? Explain by giving examples.

Ans: When two reactant in solution react and one or more of the products is insoluble or forms a precipitate, the reaction is called a precipitation reaction. For example, when a solution of iron chloride and ammonium hydroxide are mixed, a brown precipitate of iron hydroxide is formed.

              FeCl3(aq) + 3NH4OH(aq) –—›Fe(OH)3(s) + 3NH4Cl(aq)

 

16. Explain the following in terms of gain or loss of oxygen with two examples each:

(a) Oxidation

Ans: A chemical reaction in which a substance gains oxygen is called oxidation.

E.g., 2Cu+O2 –—›2CuO,Cu is Oxidised to Cuo.

 2Mg + O2 –—› 2MgO,Mg is Oxidised to MgO.

 (b)Reduction

Ans: A chemical reaction in which a substance loses oxygen is called reduction.

E.g., ZnO + C –—› Zn +Co,ZnO is reduced.

  CuO +H2 –—› Cu + H2O, CuO is reduced.

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.

Ans: Element 'X' = copper(Cu)

        Compound = copper oxide (CuO)

    2Cu(s) + O2 –—› 2CuO

18. Why do we apply paint on Iron articles? 

Ans: Paint acts as antioxidant and saves iron articles from damaged due to corrosion. It acts as buffer between iron artical and moist air.

19. Oil and fat containing food items are flushed with nitrogen why?

Ans: To keep food items fresh and safe these form oxidizing effect of oxygen, food items are flushed with nitrogen.

20. Explain the following terms with one example each:

(a) Corrosion:

Ans: Due the effect of moisture and oxygen, metals get corroded that is matters particularly iron forms a porous layer on the surface and thus gets damaged. This effect is called corrosion.

        corrosion causes damages to metal articles like car bodies, bridges, iron railings, ships and other substances of daily use.

(b) Rancidity:

Ans: fats and oils in food kept for long time get oxidised and become rancid and taste of food changes and causes infection on eating. This is called rancidity.

       to prevent rancidity antioxidants (which prevent oxidation) are added to foods containing fats and oils. rancidity can also be prevented by flushing out oxygen with an inert gas like nitrogen. For example, packets of food items like chips are flushed with nitrogen so that these can be used even after long duration.