Page 149 - Class 10th Science Chapter 2 acid , base and salt long Questions & Answers NCERET BOOK


 Chapter 2 Acids, Bases and Salts

Page : 18

Question 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 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 identified.
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, any one 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 : 22

Question 1: 
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.
Metal + Acid → Salt + Hydrogen gas
𝑍𝑛 (𝑀𝑒𝑡𝑎𝑙) + 2𝐻2𝑆𝑂4 (𝐴𝑐𝑖𝑑) → 𝑍𝑛(𝑆𝑂4)2 𝑆𝑎𝑙𝑡 + 2𝐻2

Question 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?
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 H2SO4. Shake it and pass 
the gas produced into a soap solution. The bubbles of the soap solution are 
formed. These soap bubbles contain hydrogen gas.
𝑍𝑛 + 2𝐻2𝑆𝑂4 → 𝑍𝑛(𝑆𝑂4)2 + 2𝐻2
We can test the evolved hydrogen gas by its burning with a pop sound when a 
candle is brought near the soap bubbles.

Question 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.
Answer :
𝐶𝑎𝐶𝑂 3 + 2𝐻𝐶𝑙 → 𝐶𝑎𝐶𝑙2 + 𝐶𝑂2 + 𝐻2𝑂

Page : 25

Question 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?
Answer :
The dissociation of HCl or HNO3 to form hydrogen ions always occurs in the 
presence of water. Hydrogen ions (H+) combine with H2O to form hydronium 
ions (H3O+).
The reaction is as follows:
HCl + Water → H+  +  Cl–
H+  + H2O → H3O+
Although aqueous solutions of glucose and alcohol contain hydrogen, these 
cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic 
character.

Question 2: 
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.

Question 3: 
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 HCl 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 HCl is in the aqueous form nor the litmus 
paper is wet, therefore, the colour of the litmus paper does not change.

Question 4: 
While diluting an acid, why is it recommended that the acid should be added to 
water and not water to the acid?
Answer :
The process of dissolving an acid or a base in water is a highly exothermic one. 
Care must be taken while mixing concentrated nitric acid or sulphuric acid with 
water. The acid must always be added slowly to water with constant stirring. If 
water is added to a concentrated acid, the heat generated may cause the mixture 
to splash out and cause burns. The glass container may also break due to excessive 
local heating

Question 5: 
How is the concentration of hydronium ions (H3O+) affected when a solution of 
an acid is diluted?
Answer :
When an acid is diluted, the concentration of hydronium ions (H3O+) per unit 
volume decreases. This means that the strength of the acid decreases.

Question 6: 
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 : 28

Question 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?
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.

Question 2: 
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 H+ ion concentration, the solution becomes more acidic, while 
a decrease of H+ ion causes an increase in the basicity of the solution.

Question 3: 
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.

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

Page : 33

Question 1: 
What is the common name of the compound CaOCl2?
Answer :
The common name of the compound CaOCl2 is bleaching powder.

Question 2: 
Name the substance which on treatment with chlorine yields bleaching powder.
Answer :
Calcium hydroxide [Ca(OH)2], on treatment with chlorine Cl2, yields bleaching 
powder.

Question 3: 
Name the sodium compound which is used for softening hard water.
Answer :
Washing soda (Na2CO3.10H2O) is used for softening hard water.

Question 4: 
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 is heated, sodium carbonate and water 
are formed with the evolution of carbon dioxide gas.
2NaHCO3 →  Na2CO3 + H2O + CO2

Question 5: 
Write an equation to show the reaction between Plaster of Paris and water.
Answer :
Plaster of Paris is a white powder and on mixing with water, it changes to gypsum 
giving a hard solid mass.
𝐶𝑎𝑆𝑂4.1/2𝐻2𝑂 (𝑃𝑙𝑎𝑠𝑡𝑒𝑟 𝑜𝑓 𝑃𝑒𝑟𝑖𝑠) + 1.5𝐻2𝑂 → 𝐶𝑎𝑆𝑂4 . 2𝐻2𝑂(Gypsum)

Exercise Solution

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

Question 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 
Answer : (b) The solution contains HCl. 

Question 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 of HCl 
solution (the same solution as before) required to neutralise it will be 
(a) 4 mL (b) 8mL (c) 12 mL (d) 16 mL 
Answer : (d) 16 mL of HCl solution will be required. 

Question 4:
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.

Question 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. 
Answer :
 (a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen 
H2SO4 + Zn  → ZnSO4 + H2
(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen 
2HCl + Mg  → MgCl2 + H2 (c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen 
3H2SO4 + 2Al  → Al2(SO4)3 + 3H2(d) Hydrochloric acid + Iron → Ferric chloride + Hydrogen 
6HCl + 2Fe  → 2FeCl2 + 3H2

Question 6:
Compounds such as alcohols and glucose also contain hydrogen but are not categorized 
as acids. Describe an activity to prove it. 
Answer :
Two nails are fitted 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 HCl 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 g lows in the HCl solution and does not g low in the 
glucose solution. 
Result:
HCl dissociates into H+ and Cl− ions. These ions conduct electricity in the solution 
resulting in the g lowing 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. 

Question 7:
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. 

Question 8:
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.

Question 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. 
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 

Question 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? 
Answer :
The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. 
This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen 
gas at a faster speed due to which fizzing occurs. 

Question 11:
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.

Question 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? 
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. 

Question 13:
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 absorbs water (moisture) to form a hard solid known as gypsum. 

Question 14:
What is a neutralization reaction? Give 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: 
During indigestion (caused due to the production of excess of hydrochloric acid in the 
stomach), we administer an antacid (generally milk of magnesia, which is basic in nature). The antacid neutralizes the excess of acids and thus gives relief from 
indigestion. 

Question 15:
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 CO2 that 
makes bread or cake fluffy. 
(b) It is used in soda-acid fire extinguishers.

Dear Asif Sir