NCERT Solutions for Class 10 Science Chapter 2 Acids, Bases and Salts

NCERT
SOLUTIONS
CLASS - 10th

aglase .co

Class : 10th
Subject : Science
Chapter : 2
Chapter Name : Acids, Bases and Salts

Q1 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 test tube?

Answer. If the colour of red litmus paper gets changed to blue, then it is a base and if there is
no colour change, then it is either acidic or neutral. Thus, basic solution can be easily
identi ed.
Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red
litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue,
then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, anyone
can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the
remaining two solutions separately and then the nature of the drops of the mixtures is
checked. If the colour of red litmus turns blue, then the second solution is neutral and if there
is no change in colour, then the second solution is acidic. This is because acidic and basic
solutions neutralize each other.
Hence, we can distinguish between the three types of solutions.

Page : 18 , Block Name : Questions

Q1 Why should curd and sour substances not be kept in brass and copper vessels?

Answer. Curd and other sour substances contain acids. Therefore, when they are kept in brass
and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful
products, thereby spoiling the food.

Page : 22 , Block Name : Questions

Q2 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?

Answer. Hydrogen gas is usually liberated when an acid reacts with a metal.

Take few pieces of zinc granules and add 5 ml of dilute H SO . Shake it and pass the gas
2 4

produced into a soap solution. The bubbles of the soap solution are formed. These soap
bubbles contain hydrogen gas.
2H2 SO4 + Zn ⟶ Zn(SO4 ) + 2H2 ↑
2

we can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought
near the soap bubbles.

Page : 22 , Block Name : Questions

Q3 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?

Answer. 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.
CaCO + 2HCl
3(s)
-> CaCl
(aq)
+CO +H O
2(aq) 2(g) 2 (l)

Page : 22 , Block Name : Questions

Q1 Why do HCl, HNO , etc., show acidic characters in aqueous solutions while solutions of
3

compounds like alcohol and glucose do not show acidic character?

Answer. The dissociation of HCI or HNO to form hydrogen ions always occurs in the
3

presence of water. Hydrogen ions (H+) combine with H O to form hydronium ions 3O .
2
+

The reaction is as follows:
+ −
HCl(aq) ⟶ H + Cl
+ +
H + H2 O ⟶ H3 O

Page : 25 , Block Name : Questions

Q2 Why does an aqueous solution of an acid conduct electricity?

Answer. Acids dissociate in aqueous solutions to form ions. These ions are responsible for
conduction of electricity.

Page : 25 , Block Name : Questions

Q3 Why does dry HCl gas not change the colour of the dry litmus paper?

Answer. Colour of the litmus paper is changed by the hydrogen ions. Dry HCI gas does not
contain H+ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since
in this case, neither HCI is in the aqueous form nor the litmus paper is wet, therefore, the
colour of the litmus paper does not change.

Page : 25 , Block Name : Questions

Q4 While diluting an acid, why is it recommended that the acid should be added to water and
not water to the acid?

Answer. Since the process of dissolving an acid in water is exothermic, it is always
recommended that acid should be added to water. If it is done the other way, then it is possible
that because of the large amount of heat generated, the mixture splashes out and causes
burns.

Page : 25 , Block Name : Questions

Q5 How is the concentration of hydronium ions H O affected when a solution of an acid is
+
3

diluted?

Answer. When an acid is diluted, the concentration of hydronium ions H O per unit volume
+
3

decreases. This means that the strength of the acid decreases.

Page : 25 , Block Name : Questions

Q6 How is the concentration of hydroxide ions OH affected when excess base is dissolved in
−

a solution of sodium hydroxide?

Answer. The concentration of hydroxide ions OH would increase when excess base is
−

dissolved in a solution of sodium hydroxide.

Page : 25 , Block Name : Questions

Q1 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?

Answer. A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a
basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion
concentration than the solution of pH = 8 which is basic.

Page : 28 , Block Name : Questions

Q2 What effect does the concentration of H (aq) ions have on the nature of the solution?
+

Answer. concentration of H (aq) can have a varied effect on the nature of the solution.With
+

an increase in ion concentration, the solution becomes more acidic, while a decrease of H +

ion causes an increase in the basicity of the solution.

Page : 28 , Block Name : Questions

Q3 Do basic solutions also have H (aq) ions? If yes, then why are these basic?
+

Answer. Yes, basic solution also has H+(aq) ions. However, their concentration is less as
compared to the concentration of OH ions that makes the solution basic.
−

Page : 28 , Block Name : Questions

Q4 Under what soil condition do you think a farmer would treat the soil of his elds with quick
lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer. If the soil is acidic and improper for cultivation, then to increase the basicity of soil,
the farmer would treat the soil with quicklime or slaked lime or chalk

2NaHCO3 ->Na CO
2 3 + H2 O + CO2

Page : 28 , Block Name : Questions

Q1 What is the common name of the compound CaOCl ? 2

Answer. The common name of the compound CaOCl is bleaching powder.
2

Page : 33 , Block Name : Questions

Q2 Name the substance which on treatment with chlorine yields bleaching powder.

Answer. Calcium hydroxide [Ca(OH) ], on treatment with chlorine, yields bleaching powder.
2

Page : 33 , Block Name : Questions

Q3 Name the sodium compound which is used for softening hard water.

Answer. Washing soda (Na C0 .10H 0) is used for softening hard water.
2 3 2

Page : 33 , Block Name : Questions

Q4 What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of
the reaction involved.

Answer. When a solution of sodium hydrocarbonate (sodium hydrogen carbonate) is heated,
sodium carbonate and water are formed with the evolution of carbon dioxide gas.

Page : 33 , Block Name : Questions

Q5 Write an equation to show the reaction between Plaster of Paris and water.

Answer. The chemical equation for the reaction of Plaster of Paris and water can be
represented as
1 1
CaSO4 ⋅ H2 O + 1 H2 O ⟶ CaSO4 ⋅ 2H2 O
2 2

Page : 33 , Block Name : Questions

Q1 A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10

Answer. (d) Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH
value more than 7. Since the solution turns red litmus blue, its pH is likely to be 10.

Page : 34 , Block Name : Exercise

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

Answer. (b) The solution contains HCI.

Page : 34 , Block Name : Exercise

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

Answer. (d) 16 mL of HCI solution will be required.

Page : 34 , Block Name : Exercise

Q4 Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer. (c) Antacid is used for treating indigestion.

Page : 34 , Block Name : Exercise

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 lings.

Answer. (a) Sulphuric acid + Zinc -> Zinc sulphate + Hydrogen
(b) Hydrochloric acid + Magnesium -> Magnesium chloride + Hydrogen
(c) Sulphuric acid + Aluminium -> Aluminium sulphate + Hydrogen
(d) Hydrochloric acid + Iron -> Ferric chloride + Hydrogen

Page : 34 , Block Name : Exercise

Q6 Compounds such as alcohols and glucose also contain hydrogen but are not categorised as
acids. Describe an Activity to prove it.

Answer. Two nails are tted on a cork and are kept it in a 100 mL beaker. The nails are then
connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute
HCI is poured in the beaker and the current is switched on. The same experiment is then
performed with glucose solution and alcohol solution.
Observations:
It will be observed that the bulb glows in the HCI solution and does not glow in the glucose
solution.

Result:
HCI dissociates into 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.

Page : 34 , Block Name : Exercise

Q7 Why does distilled water not conduct electricity, whereas rain water does?

Answer. Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it
does not conduct electricity. Rain water, being an impure form of water, contains many ionic
species such as acids and therefore it conducts electricity.

Page : 34 , Block Name : Exercise

Q8 Why do acids not show acidic behaviour in the absence of water?

Answer. Acids do not show acidic behaviour in the absence of water because the dissociation
of hydrogen ions from an acid occurs in the presence of water only. It is the hydrogen ions that
are responsible for the acidic behaviour.

Page : 35 , Block Name : Exercise

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

Answer. (a) Neutral Solution -> D with pH 7
(b) Strongly alkaline -> Solution C with pH 11
(c) Strongly acidic -> Solution B with pH 1
(d) Weakly acidic -> Solution A with pH 4
(e) Weakly alkaline -> Solution E with pH 9
The pH can be arranged in the increasing order of the concentration of hydrogen ions as: ( 11
< 9 < 7 < 4 < 1 ).

Page : 35 , Block Name : Exercise

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 (CH COOH) is added to
3

test tube B. Amount and concentration taken for both the acids are same.
In which test tube will the zzing occur more vigorously and why?

Answer. The zzing will occur strongly in test tube A, in which hydrochloric acid (HCI) is
added. This is because HCI is a stronger acid CH COOH than and therefore produces
3

hydrogen gas at a faster speed due to which zzing occurs.

Page : 35 , Block Name : Exercise

Q11 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd?
Explain your answer.

Answer. The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in
nature. The acids present in it decrease the PH.

Page : 35 , Block Name : Exercise

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?

Answer. (a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in
alkaline condition, milk does not set as curd easily.
(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are
neutralized by the base. Therefore, it takes a longer time for the curd to set.

Page : 35 , Block Name : Exercise

Q13 Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer. Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster
of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.
1 1
CaSO4 ⋅ H2 O + 1 H2 O ⟶ CaSO4 ⋅ 2H2 O
2 2

Page : 35 , Block Name : Exercise

Q14 What is a neutralisation reaction? Give two examples.

Answer. A reaction in which an acid and base react with each other to give a salt and water is
termed as neutralization reaction. In this reaction, energy is evolved in the form of heat.
For example: NaOH + HCl ⟶ NaCl + H O 2

(ii) During indigestion (caused due to the production of excess of hydrochloric acid in
the stomach), we administer an antacid (generally milk of magnesia, Mg(OH)2 which is basic
in nature). The antacid neutralizes the excess of acids and thus gives relief from indigestion.
Mg(OH)2 + 2HCl → MgCl + 2H2 O
2

Page : 35 , Block Name : Exercise

Q15 Give two important uses of washing soda and baking soda.

Answer. Two important used 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 CO that makes bread
2

or cake uffy.
(b) It is used in soda-acid re extinguishers.

Page : 35 , Block Name : Exercise