NCERT solutions of Class 10 chapter 2

NCERT Intext solutions of chapter-2 Acids, Bases and Salts

Pg-18

1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each tast tube?

Ans: Divide the red litmus paper in three parts and dip each part in three test tubes. The test tube in which red litmus paper turns blue contains basic solution. The blue litmus paper so obtained is dipped into the remaining two solutions. The one which turns blue litmus paper red again is an acidic solution. The third tube in which there is no change of colour contains water.

Pg-22

1. Why should curd and sour substances not be kept in brass and copper vessels?

Ans: Curd and sour substances are acidic and acids react with brass and copper. So, curd and sour substances are not kept in brass and copper vessels.

2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Ans: (i) H2 gas is liberated when an acid reacts with a metal.

        (ii) Illustration: set up an apparatus. Take some zinc granules in the test tube. Add about 5ml dil. HCl slowly. Soon the reaction between zinc and HCl starts and Hydrogen gas is evolved.

                   Zn + 2HCl —–› ZnCl2 + H2

       (iii) Test for H2 gas: H2 gas is not soluble in water. When passed through soap solution, it gets trapped into bubbles which burn with explosion.

3. Metal Compound A reacts with dilute Hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the Reaction if one of the compounds formed is calcium chloride.

Ans: Since the end product is calcium chloride and the gas formed extinguishes a burning candle, it is CO2, the meatal compound A must be calcium carbonate. Hence, the Reaction between calcium carbonate and Hydrochloric acid is as follows:

     CaCO3 + 2HCl —–› CaCl2 + CO2 + H2O

 pg-25

1. Why do HCl, HNO3, etc., Show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans: H+ ions in aqueous solution are responsible for acidic character. HCl, HNO3 etc. generate H+ ions in water while C2H5OH and glucose do not do so, and hence do not show acidic character.

2. Why does an aqueous solution of an acid conduct electricity?

Ans: Acids give ions in aqueous solution and in solution current is carried through ions. Thus aqueous solution of an acid conducts electricity.

3. Why does dry HCl gas not change the colour of the dry litmus paper? 

Ans: Dry HCl gas does not generate H+ ions and hence does not change the colour of dry litmus paper.  

4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? 

Ans: When an acid is mixed with water, there is evolution of a large amount of heat. Therefore, acid is slowly added to water. If on the other hand, water is added to acid, it might spill on your body and clothes due to explosion and evolution of sudden and large amount of heat.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans: When a given amount of an acid is added to water, there are a fixed number of hydronium ions per unit volume of the solution. On dilution the number of hydronium ions per unit volume decreases and concentration of hydronium ion decreases.

6. How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans: The concentration of hydroxide ions will increase when excess base is dissolved in a solution of sodium hydroxide because the amount of hydroxide ions per unit volume increases. This happens only when base added dissolves in water. If the base is not soluble in water, the concentration of hydroxide ions remains constant.

Pg-8

1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more Hydrogen ion concentration? Which of this is acidic and which one is basic?

Ans: Solution A has more Hydrogen ion concentration. Solution A is slightly acidic and solution B is slightly basic.

2. What effect does the concentration of H+(aq) ions have on the nature of the solution?

Ans: More the concentration of H+ ion, higher the acidic nature of the solution.

3. Do basic Solutions also have H+(aq) ions? If yes, then why are these basic?

Ans: Basic solutions have H+(aq) ions. But these are far less in number than OH- ions and their excess of OH- ions counts for their basic nature.

4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Ans: Plants work well in the pH range 6-8. So when the soil becomes acidic, farmers trat the soil fields with bases like quick lime or slaked lime or chalk.

Pg-33

1. What is the common name of the compound CaOCl2?

Ans: Bleaching powder.

2. Name the substance which on treatment with Chlorine yields bleaching powder.

Ans: Lime water.

3. Name the sodium compound which is used for softening hard water. 

Ans: Sodium carbonate.

4. What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.

Ans: Sodium Hydrogen carbonate on heating gives sodium carbonate and CO2 gas is evolved.

     2NaHCO3 —–› Na2CO3 + CO2 + H2O

5. Write an equation to show the reaction between plaster of paris and water. 

Ans: 

2CaSO4.1/2H2O + H2O  —–› 2CaSO4.2H2O

NCERT Textbook Exercises

(Pg 34-35)

1. A solution turns red litmus blue, its pH is likely to be

  (a) 1       (b) 4        (c) 5         (d) 10

Ans: (d) 10

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

   (a) NaCl                         (b) HCl

   (c) LiCl                           (d) KCl

Ans: (b) HCl

3. 10 ml of a solution of NaOH is found to be completely neutralised by 8ml of a given solution of HCl. If we take 20ml of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be

    (a) 4ml                       (b) 8ml

    (c) 12ml                     (d) 16ml

Ans: (d) 16ml

4. Which one of the following types of medicines is used for treating indigestion?

     (a) Antibiotic          (b) Analgesic

     (c) Antacid              (d) Antiseptic.

Ans: (c) Antacid.

5. Write word equations and then balanced equations for the Reaction taking place when:

   (a) dilute sulphuric acid reacts with zinc granules.

   (b) dilute hydrochloric acid reacts with magnesium ribbon.

   (c) dilute sulphuric acid reacts with Aluminium powder.

   (d) dilute hydrochloric acid reacts with iron filings.

Ans: (a) Zinc + dil sulphuric acid —–› Zinc Sulphate + Hydrogen

           Zn + H2SO4 —–› ZnSO4 + H2

         (b) Magnesium ribbon + dil hydrochloric acid —–› Magnesium chloride + Hydrogen

               Mg + 2HCl —–› MgCl2 + H2

        (c) Aluminium powder + dil sulphuric acid —–› Aluminium sulphate + Hydrogen

             2Al + 3H2SO4 —–› Al2(SO4)3 + 3H2

        (d) Iron filings + dilute hydrochloric acid —–› Iron Chloride + Hydrogen

        2Fe + 6HCl —–› 2FeCl3 + 3H2.

6. Compounds such as alcohols and glucose also contain Hydrogen but are not categorised as acids. Describe an activity to prove it.

Ans: Take solutions of alcohols, glucose in a beaker. Take a cork and fix two nails on the cork upto the end. Keep this cork in the beaker. Connect the nails to two terminals of a 6 volt battery through a bulb and a switch. Switch on the current. You will see that bulb does not glow or the current does not pass through the circuit. This means no ions or H+ ions are present in the solution. This shows that alcohols and glucose are not acids.

7. Why does distilled water not conduct electricity, whereas rain water does?

Ans: Rain water contains ions of acids and so it conducts electricity whereas there are no ions in distilled water.

8. Why do acids not show acidic behaviour in the absence of water?

Ans: Acids in water give hydronium ions which are responsible for their acidic behaviour. But in the absence of water acids do not generate hydronium ions and thus do not show acidic behaviour.

9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9; respectively. Which solution is

(a) Neutral?                   (b) strongly alkaline?

(c) strongly acidic?      (d) weakly acidic?

(e) weakly alkaline?

Arrange the pH in increasing order of Hydrogen-ion concentration.

Ans: (a) D          (b) C             (c) B

         (d) A          (e) E

pH values of in increasing order of Hydrogen-ion concentration:

11 < 9 < 7 < 4 < 1.

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the Acids are same. In which test tube will the fizzing occur more vigorously and why?

Ans: Fizzing will occur more vigorously in test A containing hydrochloric acid. This is because hydrochloric acid is stronger acid than between Mg and acetic acid.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans: pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process.

12. A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Ans: (a) The milkman adds a little baking soda to fresh milk to make it slightly alkaline so that it can be preserved for a longer time. 

         (b) Initially lactic acid formed is used up to neutralize the base i.e., baking soda, and when more lactic acid is formed, the milk sets as curd.

13. Plaster of Paris should be stored in a moisture-proof container. Explain, why?

Ans: Plaster of Paris is calcium sulphate hemihydrate (CaSO4.1/2H2O). On exposure to moisture it absorbs moisture and becomes gypsum (CaSO4.2H2O) which does not have the required setting property.

14. What is a neutralisation Reaction? Give two examples.

Ans: The reaction between an acid and a base to give salt and water is known as a neutralisation Reaction.

         Acid + Base —–› Salt + Water

Examples: 

(i) HCl + NaOH —–› NaCl + H2O   

(ii) CH3COOH +NaOH —–›CH3COONa + H2O

15. Give two important uses of washing soda and baking soda.

Ans: Uses of baking soda:

       (i) It is used in bakery.

       (ii) It is used in soda-acid fire extinguishers.

        Uses of washing soda:

       (i) It is used in the manufacture of glass, soap and paper.

      (ii) It is used for removing permanent hardness of water.