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Acids Bases and Salts : NCERT Exercise Questions


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Q.1  A solution turns red litmus blue, its pH is likely to be
(a) 1          (b) 4            (c) 5            (d) 10

Sol.       (d) 10


Q.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

Sol.       (b) HCl


Q.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, 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

Sol.        (d) 16 mL


Q.4  Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic   (b) Analgesic   (c) Antacid   (d) Antiseptic

Sol.        (c) Antacid


Q.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.

Sol.

(a) Dil. Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

{H_2}S{O_4}\left( {aq} \right) + Zn \to ZnS{O_4}\left( {aq} \right) + {H_2}\left( g \right)

(b) Dil. Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

2HCl\left( {aq} \right) + Mg \to MgC{l_2}\left( {aq} \right) + {H_2}\left( g \right)

(c) Dil. Suphuric acid + Aluminium → Aluminium sulphate + Hydrogen gas

3{H_2}S{O_4}\left( {aq} \right) + 2Al\left( s \right) \to A{l_2}{\left( {S{O_4}} \right)_3}\left( {aq} \right) + 3{H_2}\left( g \right)

(d) Dil. Hydrochloric acid + Iron → Ferric chloride + Hydrogen gas

6HCl\left( {aq} \right) + 3Fe\left( s \right) \to 3FeC{l_2}\left( {aq} \right) + 3{H_2}\left( g \right)


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

Sol.

The apparatus is set as shown in figure. Solutions of glucose is added and the switch is turned on. The bulb does not glow. This means glucose does not dissociate into ions on dissociation. So, glucose is not an acid. Similarly, alcohol is also not an acid
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Q.7  Why does distilled water not conduct electricity, whereas rain water does?
Sol.

Distilled water does not dissociate into ions. So, does not conduct electricity. Rain water has dissolved in it which becomes carbonic acid. 

C{O_2} + {H_2}O \to {H_2}C{O_3}

This carbonic acid dissociate into ions. 

{H_2}C{O_3} + 2{H_2}O \to {\left( {{H_3}{O^ + }} \right)_2} + CO_3^{ - 2}

These ions are responsible for electrical conductivity rain water.


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

In absence of water, acids do not dissociated they do not show acidic behaviour.


Q.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.
Sol.

(a) (D), (b) C, (c) B, (d) A, (e) E. 

Increasing order of H+ concentration :

C(11) < E (9) < D (7) < A (4) < B (1).


Q.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. In which test tube will the fizzing occur more vigorously and why ?
Sol.

In test tube A, fizzing occur more vigorously because HCl is a strong acid and dissociate more.


Q.11  Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Sol.    Curd is sour which means its pH will decrease from 6.


Q.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 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Sol.

(a) Baking soda is alkaline which causes the alkalinity of milk.

(b) Curd needs acidic condition to set.


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

Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.


Q.14 What is a neutralisation reaction? Give two examples.
Sol.

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

NaOH\left( {aq} \right) + HCl\left( {aq} \right) \to NaCl\left( {aq} \right) + {H_2}O\left( l \right)

Ca{\left( {OH} \right)_2}\left( {aq} \right) + {H_2}OS{O_4}\left( {aq} \right) \to CaS{O_4} + {H_2}O


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

Uses of Washing Soda –
(i) Sodium carbonate (washing soda) is used in glass soap and paper industries.

(ii) It is used in the manufacture of sodium compound such as borax.

Uses of Baking Soda –
(i) Sodium hydrogencarbonate is also an mgredient antacids. Being alkaline, it neutralises excess acid in stomach and provides relief.

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



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