Acids, Bases and Salts

Acids, Bases and Salts
Acids, Bases and Salts

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

Solution: Curd and sour food substances contain acids; these acidic substances combine with

metal. This reaction turns food to poison which damage people’s health.

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?

Solution: When an acid reacts with any metal, salt and hydrogen gas are formed.

Metal + Acid → Salt + Hydrogen gas

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

Solution: As metal compound released is Calcium Chloride the gas evolved here is CO2.

Hence metal A should be Calcium Carbonate. Hence the reaction between Calcium Carbonate

and HCl is

CaCO3 (s) + 2HCl (Aq) → CaCl2( Aq) + CO2 (g) + H2O (l)

If the red litmus paper changes to blue colour the solution is a basic solution.

If the red litmus paper experience no change in acidic solution.

If the red litmus paper changes to purple colour the solution is distilled water.

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?

Solution: Release of H+ ion in water will make a compound acidic or non-acidic. Acids are the

substance which upon dissociating with water results in production of Hydrogen ions. Some

compounds show acidic character as they dissociate in the aqueous solution which results in

the production of hydrogen ions (acids like HCl, HNO3).

Compounds similar to glucose or alcohol do contain hydrogen element but they do not show

signs of acidic nature. The fact that the hydrogen in them will not separate as like the hydrogen

in the acids. They will not separate to become hydrogen ions, on dissolving in the water.

Hence dissociation of hydrogen gas will decide the acidic or non-acidic nature of a compound.

2. Why does an aqueous solution of an acid conduct electricity?

Solution: Charged particles are responsible for the conductance of electricity in an acid. These

charged particles called as ions are the reason behind conductance of electricity in acid.

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

Solution: HCL does not give out Hydrogen ions, therefore HCL does not show any acidic

behaviour and colour of the litmus paper remain the same on reacting with HCl gas.

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

Solution: While diluting an acid, it is recommended that the acid should be added to water

and not water to the acid because if water is added to concentrated acid, it release huge amount

of heat which may result in explosion and can cause acid burns o face, clothes and body parts.

Hence it is safe to add acid to water but not water to acid.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of
an acid is diluted?

Solution: When acid is added to water there will be a fixed amount of hydronium present in

the fixed volume of solution. If we dilute the solution hydronium ion per volume of solution

decrease, this in-turn decreases Hydronium concentration in the solution.

6. How is the concentration of hydroxide ions (OH–) affected when excess base is
dissolved in a solution of sodium hydroxide?

Solution: When base is dissolved in sodium hydroxide solution its hydroxide ions increase

but it will reach saturation at some point. After saturation point hydroxide ion concentration is

not affected even after adding base further.

In-text questions set 4

Page number – 33

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?

Solution: In order to find the hydrogen ion concentration, we can use the rule that states,

“The pH of any solution is inversely proportional to the hydrogen ion concentration”.

Therefore, it means that the solution that has a lower pH number will have a higher hydrogen

ion concentration. Hence, solution A will have a higher hydrogen ion concentration. In

addition, solution B will be basic and A will be acidic.

2. What effect does the concentration of H+(aq) ions have on the nature of the

Solution: Hydrogen ion concentration decides the nature of the solution. If Hydrogen ion

concentration increase then solution turn acidic and similarly if Hydrogen ion concentration

decreases then solution turn basic.

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

Solution: Basic solutions has H+ ions, but hydroxide ions present in basic solution are more

in basic solution. Hence Hydroxide ions turn solution to basic.

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)?

Solution: If the soil is acidic in nature (PH below 7) then such field should be treated with

quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

In text questions set 5

Page number – 34-35

1. What is the common name of the compound CaOCl2?

Solution: Common name of CaOCl2 is bleaching powder.

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

Solution: The substance which on treatment with chlorine yields bleaching powder is Calcium


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

Solution: Sodium carbonate is the compound which is used for softening hard water.

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

Solution: Heating sodium hydrocarbonate yields sodium carbonate and carbon dioxide gas is

liberated in the process.

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

Solution: The chemical equation for the reaction of Plaster of Paris and water is

 CaSO4.1/2H2O + 3/2H2O → CaSO4.2H2O

Exercise questions Page number – 33

1. A solution turns red litmus blue, its pH is likely to be

(a)1 (b) 4 (c) 5 (d) 10

Solution: Answer is 10 because litmus paper turns blue when reacts with basic solution (PH

more than 7). Hence 10 is the answer.

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

Solution: Answer is HCl.

Egg shells contains calcium carbonate, which on reaction with HCl liberates CO2 gas which

turn lime water to milky.

CaCO3 + 2HCl → CaCl2 + H2O + CO2

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

Solution: Since 10 ml of NaOH requires 8 mL of HCL, 20 ml of NaOH require 8 x 2 = 16mL of

HCl Hence the answer id option d 16mL.

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

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Solution: Indigestion is due to excess production of acid in the stomach. Medicines used to

treat indigestion is called as Antacid.

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.


(a) dilute sulphuric acid reacts with zinc granules:

=> dilute sulphuric acid + zinc → Zinc Sulphate + Hydrogen Gas

=> H2SO4(aq) + Zn → ZnSO4(aq) + H2(g)

(b) dilute hydrochloric acid reacts with magnesium ribbon.

=> dilute Hydrochloric + Magnesium → Magnesium Chloride + Hydrogen Gas

=> 2HCl(aq) + Mg → MgCl2(aq) + H2(g)

(c) dilute sulphuric acid reacts with aluminium powder.

=> dilute Sulphuric Acid + Aluminium → Aluminium Sulphate + Hydrogen Gas

NCERT Solution for class 10 Science Chapter 2 Acids, Bases and Salts

=> 3H2SO4(aq) + 2Al(s) → Al2(SO4)3(aq) + 3H2(g)

(d) dilute hydrochloric acid reacts with iron filings.

=> dilute Hydrochloric Acid + Iron → Ferrous Chloride + Hydrogen Gas

=> 6HCl(aq) + 3Fe(s) → 3FeCl2(aq) + 3H2(g)

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

Solution: Insert two nails on the wooden or rubber cork and place them on a beaker as shown

in figure. Connect iron nail to a bulb, 6 volt battery and a wire connected to switch. Pour some

alcohol or glucose so as to dip the nails in glucose or alcohol. Turn the switch on and you the

see the bulb not glowing despite of connection to switch. Now empty the beaker and add HCL

solution. This time bulb glows. This proves acid can conduct electricity but alcohol and glucose

does not conduct electricity.

7. Why does distilled water not conduct electricity, whereas rain water does?


• Distilled water does not contain any ionic compounds in it.

• Whereas rainwater has a lot, more compounds.

• Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms

carbonic acid. This means that it has hydrogen ions and carbonate ions. Therefore, with

the presence of acids, rainwater can conduct electricity.

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

Solution: The acidic behaviour from acids is because of the presence of hydrogen ions. 

NCERT Solution for class 10 Science Chapter 2 Acids, Bases and Salts

Hydrogen ions can only be produced in the presence of water and therefore water is definitely

needed if acids are to show their acidic behaviour.

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?

Solution: In increasing order of hydrogen ion concentration:

pH 11(B) -> pH 9(E) -> pH 7(A) -> pH 4(D) -> pH 1 (B)

PH11 - Strongly alkaline

pH9 - weakly alkaline

PH7 - Neutral

pH4 - Weakly acidic

pH1 - Strongly acidic

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

Solution: HCl is a strong acid whereas acetic is a weaker acid. Fizzing occurs because of the

production of the hydrogen gas obtained due to reaction of the acid on the magnesium ribbon.

Since HCl is a very strong acid there is a lot of liberation of hydrogen gas from test tube A.

therefore, more fizzing take place in test tube A.

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

Solution: Fresh milk is turned to curd due to production of lactic acid. Lactic acid reduces the

pH of the milk.

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?

Solution: (a) He shifted the pH of the fresh milk from 6 to slightly alkaline to prevent milk

from getting sour due to production of lactic acid.

(b) This milk takes long time to set into curd because the lactic acid produced here first

neutralises the pH then the pH is reduced to turn milk to curd.

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

Solution: Plaster of Paris should be stored in moisture-proof container because moisture can

affect plaster of Paris by slowing down the setting of the plaster because of hydration. This will

turn plaster useless.

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

Solution: The reaction of the acid + base gives a product of salt + water, which is considered

as neutralization reaction.


NaOH + HCl → NaCl + H2O

Mg(OH)2+ H2CO3 → MgCO3 + 2H2O