CHEMICAL REACTIONS AND EQUATIONS

                                     YASH   CLASSES

                    CHEMICAL REACTIONS AND EQUATIONS

              NCERT  INTEXT  QUESTIONS AND ANSWERS

INTEXT QUESTIONS            [ page 6]

1. Why should 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. when Mg burn in air combines with oxygen

 give magnesium oxide [MgO].

                            2Mg (s)  + O₂ (g) → 2MgO (S)

 2. Write the balanced equation for the following chemical reactions.

  (i)Hydrogen + Chlorine → Hydrogen chloride

               Ans .   H₂ (g)+ Cl₂ (g) → 2HCI (g)

(ii)Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

              Ans-       3 BaCl₂ (s) + Al₂(SO₄)₃ (s) → 3BaSO₄ (s)+ 2AICI₃(s)

 (iii)  Sodium Water Sodium Hydroxide Hydrogen

                Ans . 2 Na(s) +2 H₂O(l) → 2NaOH (l) + H₂ (g)

 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.

Ans. (i)         BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl(aq)

           (ii)         NaOH (aq) + HCl (aq) → NaCl(aq) + H₂O (l)

1. A solution of a substance X is used for white-washing             [ page 10]

        (i)  Name the substance 'X and write its formula.

       (ii)   Write the reaction of the substance X named in above with water.

Ans. X is quick lime, CaO.

       (ii)   CaO(s)      + H₂O (l)         →               Ca(OH)₂ (aq)

            Quick lime                               Slaked lime or Calcium hydroxide

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 H, gas at one electrode and O, 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 2 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.

 

 

 

 

 

  1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?                   [ page13]

   Ans. When an iron mail 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)   →   FeS0₄ (aq) +  Cu (s)

                                    Copper sulphate

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

       and its colour fades.

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

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

         chloride is formed.

                 Pb(NO₃ )₂  (aq) +  2HCl(aq) → PbCl₂ (s )+ 2HNO₃ (aq)

                Lead nitrate                               Lead chloride

     Pb^{2 +} ions from lead nitrate combine 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 for example

      lead nitrate and hydrochloric  acid, and thus this reaction is a double displacement reaction.