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1. A solution turns red litmus blue, its pH is likely to be : (a) 1 (b) 4 (c) 5 (d) 10 Ans. The solution which turns red litmus blue is basic. Hence its pH should be

greater than 7. Therefore, (d) is the correct option. 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. Egg shell contain calcium carbonate which reacts with HCl to give out CO2

which turns lime water milky. Hence, the correct option is (b). 3. 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, theamountHClsolution(thesamesolutionasbefore)requiredtoneutraliseit will be :

(a) 4 ml (b) 8 ml (c) 12 ml (d) 16 ml Ans. If we take double the amount of the same NaOH solution, the amount of

thesameHClsolutionrequiredtoneutralizeit,isalsodouble.Hence,thecorrect option is (d).

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

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic Ans. The indigestion is due to excess of acid produced in the stomach. The

medicine used to neutralize it is called antacid. Hence, the correct option is (c).

5. Writewordequationsandthenbalancedequationsforthereactiontakingplace 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) Dilutehydrochloricacidreactswithironfilings. Ans. (a) Thewordequationis Zinc + Sulphuric → Zinc + Hydrogen acid sulphate

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Thebalancedequationwillbe Zn + H2SO4 → ZnSO4 + H2 (b) Thewordequationis Magnesium + Hydrochloric → Magnesium + Hydrogen acid chloride Thebalancedequationwillbe Mg + 2HCl → MgCl2 + H2 (c) Thewordequationis Aluminium + Sulphuric → Aluminium + Hydrogen acid sulphate Thebalancedequationwillbe 2Al + 3H2SO4 → Al2(SO4)3 + 3H2 (d) Thewordequationis Iron + Hydrochloric → Iron(II) + Hydrogen acid chloride Thebalancedequationwillbe Fe + 2HCl → FeCl2 + H2 6. Compounds such as alcohols and glucose also contain hydrogen but are not

categorised as acids. Describe an activity to prove it. Ans. Alcohols and glucose though contain hydrogen but do not ionise in the

solution to produce H+ ions. This is proved by the fact that there solutions do not conduct electricity.

Twonailsarefittedonacorkandarekeptitina100mlbeaker.Thenailsare then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.

Fig. 6.7

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Observations : It will be observed that the bulb glows in the HCl solution and does not glow in the glucose solution.

Result : HCl dissociates into H+ and Cl– ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.

Conclusion : From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.

That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

7. Why does distilled water not conduct electricity, whereas rain water does? Ans. Rain water contains dissolved gases like carbon dioxide (CO2), sulphur

dioxide (SO2), etc. These gases on dissolving in water form acids like carbonic acid (H2CO3), sulphurous acid (H2SO3) etc. which dissociate into ions. Hence, rain water conducts electricity. On the other hand, distilled water contains no dissolved salts or gases. Hence, it does not contain any ions and therefore, does not conduct electricity.

8. Why do acids not show acidic behaviour in the absence of water? Ans. Water helps in the dissociation of an acid to give hydronium ions (H3O

+). In the absence of water, these ions are not produced. Hence, acids show acidic behaviour only in the presence of water.

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. Given pH for the solutions are A = 4, B = 1, C = 11, D = 7, E = 9. (a) Solution with pH = 7 is neutral. Hence, D is the neutral solution. (b) Higher the pH than 7, more alkaline is the solution. Solution C has

highest pH and hence is strongly alkaline solution. (c) Lesser the pH than 7, more acidic is the solution. Solution B has

lowest pH and hence is strongly acidic. (d) The solution with pH less than 7 but closer to 7 is weakly acidic.

Hence, solution A is weakly acidic.

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(e) The solution with pH greater than 7 but close to 7 is weakly alkaline. Hence, solution E is weakly alkaline.

Hydrogen ion concentrations of the given solutions will be A = 10–4 M, B = 10–1 M, C = 10–11 M, D = 10–7 M, E = 10–9 M Hence, increasing order of H+ ion concentration of different solutios

will be C (10–11 M) < E (10–9 M) < D (10–7 M) < A (10–4 M) < B (10–1 M) 10. Equal lengths ofmagnesium ribbons are taken in test tubesA andB.

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 aresame.Inwhichtesttubewillthefizzingoccurmorevigorouslyandwhy?

Ans. MorevigorousfizzingwilloccurintesttubeAcontaininghydrochloricacid(HCl). This is because hydrochloric acid is stronger than acetic acid. The reaction of magnesium with hydrochloric acid to give out hydrogen gas is faster than with acetic acid

Mg + 2HCl Fast → MgCl2 + H2 Magnesium Hydrochloric Magnesium Hydrogen acid chloride

Mg + 2CH3COOH Slow → (CH3COO)2Mg + H2 Magnesium Acetic acid Magnesium Hydrogen acetate

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. When milk changes into curd, pH will decrease. This is because curd is more acidic than milk (because curd contains lactic acid). More acidic is a substance, lesser is its pH.

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) Fresh milk is acidic and it turns sour easily to become more acidic.

In presence of baking soda, milk becomes alkaline and does not turn sour easily because the alkali does not allow the milk to become more acidic easily.

(b) When the milk sets to curd, the pH decreases, i.e., it becomes more acidic. The presence of alkali does not allow it to become more acidic easily. Hence, it will take a long time to set as curd.

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13. Plaster of Paris should be stored in a moisture-proof container. Explain why? Ans. Plaster of Paris is CaSO4.½H2O. It gains moisture to become gypsum with

the formula CaSO4.2H2O which sets to a hard mass. Hence, it should be stored in moisture-proof container.

14. What is a neutralisation reaction? Give two examples. Ans. When an acid reacts with a base to form salt and water, it is called

neutralisation reaction. For example, (a) NaOH + HCl → NaCl + H2O Sodium Hydrochloric Sodium Water hydroxide acid chloride (Base) (Acid)

(b) KOH + HNO3 → KNO3 + H2O Potassium Nitric acid Potassium Water hydroxide (Acid) nitrate (Base)

15. Give two important uses of washing soda and baking soda. Ans. Two important uses of washing soda and baking soda are as follows : (1) Washing soda : (a) It is used in glass, soap, and paper industries. (b) It is used to remove permanent hardness of water. (2) Baking soda : (a) It is used as baking powder. Baking powder is a mixture of baking

soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 thatmakesbreadorcakefluffy.

(b) Itisusedinsoda-acidfireextinguishers.