NCERT Solutions Exercise
Q1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Ans. (d) 10
Q2. 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
Q3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Ans. (d) 16 mL
Q4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans. (c) Antacid
Q5. 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) Zn (s) + H2SO4 (aq) = ZnSO4 (aq) + H2 (g)
(b) Mg (s) + 2HCl (aq) = MgCl2 (aq) + H2 (g)
(c) 2Al (s) + 3H2SO4 (aq) = Al2(SO4)3 (aq) + H2 (g)
(d) Fe(s) + 2HCl (aq) = FeCl2 (aq) + H2 (g)
Q6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Ans. Take a solution of glucose, alcohol, hydrochloric acid, sulfuric acid etc. Put two nails on a cork and place the cork in a 100 mL beaker. Now connect the keys to both the terminals of a 6 volt battery through a bulb and switch. Now pour a little dilute HCl into the beaker and pass an electric current. Repeat the same process with dilute sulfuric acid. Compounds like alcohol and glucose also contain hydrogen but they are not classified as acids because they do not form H+ ions.
Q7. Why does distilled water not conduct electricity, whereas rain water does? Acids, Bases and Salts 35.
Ans. Distilled water is pure. Therefore, they do not conduct electricity because ions are required for the conduction of electricity. Whereas rain water conducts electricity because a small amount of acid is present in it. Which combine with the sulfur-dioxide and nitrogen dioxide present in the air to make it acidic. Being acidic, they produce H+ ions, due to which electricity conducts.
Q8. Why do acids not show acidic behaviour in the absence of water?
Ans. Acids are not acidic in the absence of water because H+ ions dissociate from acids only in the presence of water.
Q9. 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.
| solutions | pH values | tested with universal indicator |
| A | 4 | weakly acidic |
| B | 1 | strongly acidic |
| C | 11 | strongly acidic |
| D | 7 | neutral |
| E | 9 | weakly acidic |
As the concentration of H+ ions increases, the value of pH decreases in the same way.
C < E< D< A < B
Q10. 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. In which test tube will the fizzing occur more vigorously and why?
Ans. There will be more effervescence in test tube 'A' because hydrochloric acid is a stronger acid than acetic acid.
Q11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans. The pH value of fresh milk is 6. Lactic acid is formed in the process of curd formation. Hence the pH of curd will be less than 6.
Q12. 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) Changes the pH of fresh milk from 6 to slightly alkaline because Lactobacillus bacteria present in milk make the milk acidic. Baking soda is added to milk so that the milk remains alkaline for a long time so that it remains for a long time.
(b) It takes more time for this milk to become curd because the lactic acid formed in this process first neutralizes the base (baking soda) found in fresh milk and then converts it into acid due to which curd is formed.
Q13. Plaster of Paris should be stored in a moisture-proof container.
Explain why?
Ans. Plaster of Paris should be kept in a moisture proof container because it absorbs water in the presence of moisture to form the solid material gypsum. Due to which the property of freezing with water is destroyed in it.
Q14. What is a neutralisation reaction? Give two examples.
Ans. The reaction in which base and acid react to form water and salt, this reaction is called neutralization reaction. In this reaction, acid and base neutralize or neutralize each other's effect.
Q15. Give two important uses of washing soda and baking soda.
Ans.
Uses of washing soda:-
(1) Sodium carbonate is used in glass, soap and paper industries.
(2) It is used in the production of sodium compounds such as borax.
(3) Sodium carbonate is used for cleaning in homes.
(4) It is used to remove permanent hardness of water.
Uses of Baking Soda:-
(1) Baking soda is used to make food items soft, spongy and crispy.
(2) Baking soda being alkaline, it is used to bring relief by reducing or neutralizing the excess amount of acid in the stomach.
(3) Sometimes it is also used for quick cooking of food.
(4) It is also used in soda-acid fire extinguishers.
