Chapter 2 Acids, Bases and Salts

Acids, Bases and Salts NCERT SOLUTIONS CLASS 10 SCIENCE

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Chapter 2

Acids, Bases and Salts

Page Number: 6

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: (i) Put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus to

blue will be a basic solution. The blue litmus paper formed here can now be used to test the acidic

solution.

(ii) Put the blue litmus paper obtained above in the remaining two test-tubes, turn-by-turn. The

solution which turns the blue litmus paper to red will be the acidic solution.

(iii) The solution which has no effect on any litmus paper will be neutral and hence it will be distilled

water.

Page Number: 22

Question 1

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

Answer:

Curd and sour substances should not be kept in brass and copper vessels because these and other sour

food-stuffs contain acids which can react with the metal of the vessel to form poisonous metal

compounds which can cause food poisoning and affect our health adversely.

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:

(i) Hydrogen (H2) gas is liberated when an acid reacts with a metal.

(ii) Illustration : Set up the apparatus as shown in the given figure. Take some zinc granules in the test

tube. Add about 5 mL dilute hydrochloric acid slowly. Soon the reaction between zinc and

hydrochloric acid starts and hydrogen gas is evolved.


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(iii) Test for H2 gas :

H2 gas is not soluble in water. When passed through soap solution, it gets trapped into bubbles.

Bring a burning candle near the soap bubble filled with gas. The soap bubble bursts and hydrogen gas

burns with a pop sound.

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:

As the end product is calcium chloride and the gas formed is carbon dioxide, the metal compound A

must be calcium carbonate. Therefore, the reaction between calcium carbonate and hydrochloric acid

is

Page Number: 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:

H+ ions in aqueous solution are responsible for acidic character. HCl, HNO3, etc. give H+ ions in

water while alcohol and glucose do not give H+ ion in water. Therefore, alcohol and glucose do not

show acidic character.

Question 2

Why does an aqueous solution of an acid conduct electricity?

Answer:


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The aqueous solution of an acid conducts electricity due to the presence of charged particles called

ions in it.

Question 3

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

Answer: Dry HCl gas does not give H+ ions and therefore does not change the colour of dry litmus paper.

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: 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 to dilute it, then a large amount of heat is evolved

at once. This heat changes some of the water to steam explosively which can splash the acid on one’s

face or clothes and cause acid burns.

Question 5

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

Answer:

When a given amount of an acid is added to water, there is a fixed number of hydronium ions per

volume of the solution. On dilution, the number of hydronium ions per volume decreases and

concentration 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 will increase when excess base is dissolved in a solution of

sodium hydroxide, but it happens to a limited extent only after which the concentration becomes

almost constant.

Page Number: 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.

Since solution A has more hydrogen ion concentration, solution A is acidic and solution B is basic.


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Question 2

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

Answer:

More the concentration of H+ ions, higher the acidic nature of the solution.

Question 3

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

Answer:

Basic solutions have H+ (aq) ions. But these are far less in number than OH– ions that is responsible

for their basic nature.

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 too acidic (having low pH) then it is treated with materials like quick lime (calcium

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

Page Number: 33

Question 1

What is the common name of the compound CaOCl2 ?

Answer:

Bleaching powder.

Question 2

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

Answer:

Slaked lime Ca (OH)2.

Question 3

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

Answer:

Sodium carbonate.

Question 4

What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the

reaction involved?

Answer:

Solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide gas is


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evolved.

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

Answer:

Textbook Chapter End Questions

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) HCl

Question 3

10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of

HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 solution (the same solution

as before) required to neutralise it will be

(a) 4 mL

(b) 8 mL

(c) 12 mL


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(d) 16 mL

Answer:

(d) 16 mL

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

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 filing

Answer:

(a) Zinc + dilute sulphuric acid → Zinc sulphate + Hydrogen

Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)

(b) Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)

(c) Aluminium powder + dil. Sulphuric acid > Aluminium sulphate + Hydrogen

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

(d) Iron filings + Dilute hydrochloric acid > Ferric chloride + Hydrogen

2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)

Question 6

Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids.

Describe an activity to prove it.

Answer:

Though compounds like alcohol and glucose contain hydrogen but they do not ionise in the solution

to produce H+ ions on passing current through them.

(i) Take solutions of alcohols and glucose.

(ii) Fix two nails on a cork, and place the cork in 100 mL beaker.

(iii) Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in

the given Figure.


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(iv) Now pour alcohol in the beaker and switch on the current.

(v) The bulb does not glow.

(vi) Repeat the experiment with glucose. The bulb does not glow in this case also.

(vii) This means no ions or H+ ions are present in the solution.

This shows that alcohols and glucose are not acids.

Question 7

Why does distilled water not conduct electricity, whereas rainwater does ?

Answer:

Distilled water does not conduct electricity because it does not contain any ionic compound (like

acids, bases or salts) dissolved in it.

Rainwater, while falling to the earth through the atmosphere, dissolves an acidic gas carbon dioxide

from the air and forms carbonic acid (H2CO3). Carbonic acid provides hydrogen ions, H+ (aq) and

carbonate ions, CO(aq)32to rainwater. Hence, due to the presence of carbonic acid which provides ions

to rainwater, the rainwater conducts electricity.

Question 8

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

Answer:

The acidic behaviour of acids is due to the presence of hydrogen ions, [H+ (aq) ions], in them. The

acid produces hydrogen ions only in the presence of water. So in the absence of water, an acid will not

form hydrogen ions and hence will not show its 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:


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(a) D

(b) C

(c) B

(d) A

(e) E

Increasing order of hydrogen ion concentration

11 < 9 < 7 < 4 < 1

i. e., C < E < D < A < B

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:

Fizzing will occur more vigorously in test tube A. Hydrochloric acid (HCl) is a strong acid whereas

acetic acid (CH3COOH) is a weak acid. Being strong acid, the hydrochloric acid solution contains a

much greater amount of hydrogen ions in it due to which the fizzing will occur more vigorously in

test tube A (containing hydrochloric acid). The fizzing is due to the evolution of hydrogen gas which

is formed by the action of acid on the magnesium metal of magnesium ribbon.

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:

pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process.

Lactic acid present in it reduces its pH value.

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) Milk is made slightly alkaline so that it may not get sour easily due to the formation of lactic acid

in it.

(b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to

first neutralise the alkali present in it.

Question 13

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

Answer:

Plaster of Paris should be stored in a moisture proof container because the presence of moisture can


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cause slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of

Paris useless after sometime.

Question 14

What is a neutralisation reaction? Give two examples.

Answer:

The reaction between an acid and a base to form salt and water is called a neutralisation reaction.

Examples:

Question 15

Give two important uses of washing soda and baking soda.

Answer:

Uses of washing soda:

(i) Washing soda is used in glass, soap and paper industries.

(ii) It is used for removing permanent hardness of water.

Uses of baking soda :

(i) Baking soda is used as an antacid in medicines to remove acidity of the stomach.

(ii) Baking soda is used for making baking powder (used in making cakes, bread, etc.).

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?

Solution:

The contents of each test tube would be identified by change in colour of red litmus paper. For

example, when we wet the red litmus paper with the basic solution, it changes into blue colour. Put

the changed blue litmus paper in the solution which turns the blue to red will be the acidic solution.

The solution, which has no effect on any litmus paper, will be neutral and hence it will be distilled

water.

Question 2

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

Solution:

Curd and other sour foodstuffs contain acids, which can react with the metal of the vessel to form

poisonous metal compounds which can cause food poisoning and damage our health.


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Question 3

Which gas is usually liberated when an acid reacts with a metal?

Solution:

When an acid reacts with metal, a salt and hydrogen gas is formed. i.e.

Question 4

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.

Solution:

The gas that extinguishes a burning candle is carbon dioxide, which is formed by the action of dilute

hydrochloric acid on a metal carbonate and produces effervescence. Now, since one of the compounds

formed is calcium chloride, it shows that the metal compound is calcium carbonate. Thus, the metal

compound A is calcium carbonate (CaCO3). Calcium carbonate reacts with dilute hydrochloric acid to

form calcium chloride, carbon dioxide and water. This can be written as:

CaCO3 + 2HCl ⇨ CaCl2 + CO2 + H2O

Question 5 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:

An acid is a substance, which dissociates on dissolving in water to produce hydrogen ions [H+(aq)

ions]. The acids like HCl, H2SO4, HNO3 and CH3COOH, etc., show acidic character because they

dissociate in aqueous solutions to produce hydrogen ions, H+(aq) ions.

The compounds such as glucose and alcohol also contain hydrogen but they do not show acidic

character. The aqueous solutions of glucose and alcohol do not show acidic character because the

hydrogen in them does not separate out as hydrogen ions [H+ (aq) ions] on dissolving in water.

Question 6

Why does an aqueous solution of acid conduct electricity?

Solution:

The aqueous solution of an acid conducts electricity due to the presence of charged particles called

ions in it.

Question 7

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

Solution:

Dry HCl gas does not contain any hydrogen ions in it, so it does not show acidic behaviour. In fact,

dry HCl gas does not change the colour of dry litmus paper as it has no hydrogen ions [H+(aq) ions] in

it.


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Question 8

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

the acid?

Solution:

Diluting an acid should be done by adding concentrated acid to water gradually with stirring and not

by adding water to concentrated acid.

The heat is evolved gradually when a concentrated acid is added to water for diluting an acid and the

large amount of water is easily absorbed.

If, however, water is added to concentrated acid to dilute it, a large amount of heat is evolved at once.

The heat generated may cause the mixture to splash the acid on our face or clothes and cause acid

burns.

Question 9

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

Solution:

When the concentrated solution of an acid is diluted by mixing water, the concentration of hydronium

ions H3O+ per unit volume decreases.

Question 10

How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in water?

Solution:

When the solution of a base is diluted by mixing more water in it, the concentration of hydroxide ions

(OH– ions) per unit volume decreases.

Question 11

You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8.

i. Which solution has more hydrogen ion concentration?

ii. Which of this is acidic and which one is basic?

Solution:

The pH of a solution is inversely proportional to its hydrogen ion concentration. This means that the

solution having lower pH will have more hydrogen ion concentration. In this case, solution A(having

a lower pH of 6) will have more hydrogen ion concentration. Solution A is acidic and solution B is

basic.

Question 12

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

Solution:

Acids produce hydrogen ions in water. So, when an acid is added to water, the concentration of

hydrogen ions in water increases. The solution of acid thus formed will have more of hydrogen ions

and it will be acidic in nature.


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Question 13 Do basic solutions also have H+(aq) ions? If yes, why are these basic?

Solution:

No, the basic solution doesn’t have H+ ions as the solution has excess of hydroxide ions.

Question 14

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:

Most often the soil in the fields is too acidic. If the soil is too acidic (having low pH), it is treated with

materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium

carbonate). Thus, a farmer should add lime or slaked lime in his fields when the soil is too acidic.

Question 15

What is the common name of the compound CaOCl2?

Solution:

The common name of the compound CaOCl2 is bleaching powder.

Question 16

Name the substance that on treatment with chlorine yields bleaching powder.

Solution:

Calcium hydroxide is the substance that on treatment with chlorine yields bleaching powder.

Question 17

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

Solution:

Sodium carbonate (washing soda) is used for softening hard water.

Question 18

What will happen if a solution of sodium hydro carbonate is heated? Give the equation of the reaction

involved.

Solution:

Sodium carbonate and carbon dioxide are evolved when sodium hydro carbonate is heated.

Question 19

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

Solution:

Plaster of Paris has a very remarkable property of setting into a hard mass on wetting with water. So,


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when water is added to plaster of Paris, it sets into a hard mass in about half an hour. The setting of

plaster of Paris is due to the hydration crystals of gypsum, which set to form a hard, solid mass.

Question 20

Why does distilled water not conduct electricity, whereas rainwater does?

Solution:

Distilled water does not conduct electricity because it does not contain any ionic compound (like

acids, bases or salts) dissolved in it. On the other hand, rain water conducts electricity. This can be

explained as follows: Rain water, while falling to the earth through the atmosphere, dissolves an

acidic gas carbon dioxide from the air and forms carbonic acid (H2CO3). Carbonic acid provides

hydrogen ions, H+(aq), and carbonate ions, CO2-3 (aq), to rain water. So, due to the presence of

carbonic acid (which provides ions to rain water), the rain water conducts electricity.

Question 21

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

Solution:

The acidic behaviour of acid is due to the presence of hydrogen ions. The acids will not show its

acidic behaviour in the absence of water, this is because the acids produce hydrogen ions only in the

presence of water.

Question 22

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?

(i) Neutral?

(ii) Strongly alkaline?

(iii) Strongly acidic?

(iv) Weakly acidic?

(v) Weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.

Solution:


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Question 23 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

fizzing occur more vigorously and why?

Solution:

Acetic acid (CH3COOH) is a weak acid whereas hydrochloric acid (HCl) is a strong acid. Fizzing

occurs in the test tube due to the evolution of hydrogen gas by the action of acid on magnesium

ribbon. Since hydrochloric acid is a strong acid a large amount of hydrogen gas is liberated in the test

tube A. So, fizzing occurs more vigorously in test tube A.

Question 24 Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain.

Solution:

The pH will change to below 6, as lactic acid is formed when milk turns into curd.

Question 25

Plaster of Paris should be stored in a moisture-proof container. Why?

Solution: The presence of moisture can affect the slow setting of plaster of Paris by bringing about its

hydration. This will make the plaster of Paris useless after some time.

Question 26

What is a neutralization reaction? Give two examples.

Solution:

The reaction of an acid and a base, giving rise to the corresponding salt and water is called

neutralization reaction.

Examples:

NaOH + HCl → NaCl + H2O

Mg(OH)2 + H2CO3 → MgCO3 + 2H2O

Question 27

Give two important uses of washing soda and baking soda.

Solution:

Washing soda

1. It is often used as an electrolyte.


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2. Domestically it is used as a water softener during laundry.

Baking soda

1. It is used to test garden soil for acidity. If it develops bubbles, the soil is too acidic.

2. Washing a car with it can remove dead bug bodies without damaging the paint.

MULTIPLE CHOICE QUESTIONS

Question 1. Equal pieces of zinc granules are dropped in four test tubes. Following substances are poured in all

the four test tubes. The reaction will be vigorous with [CCE 2014]

(a) CH3COOH

(b) HCl

(c) sodium bicarbonate solution

(d) lemon juice

Answer:

(b) Strong adds like HCl react vigorously with active metals like Zn and form metal salt and evolve

H2 gas.

Question 2. Which of the following statements shows the property of an acid? [CCE 2014]

(a) It turns blue litmus to red

(b) It is sour in taste

(c) It has no effect on red litmus

(d) All of the above

Answer:

(d) An acid turns blue litmus red. Thus, it has no effect on red litmus and acids are sour in taste.

Question 3. A drop of a liquid sample was put on the pH paper. It was observed that the colour of the pH paper

turned blue. The liquid sample is

(a) lemon juice

(b) sodium bicarbonate solution

(c) distilled water

(d) hydrochloric acid

Answer:

(b) The liquid sample is of sodium bicarbonate (NaHC3) solution. It is a basic solution. And we know

that a basic solution turns pH paper blue.

Question 4. Two solutions X and Y were found to have pH value of 4 and 10 respectively. The inference that can

be drawn is

(a) X is a base and Y is an acid

(b) Both X and Y are acidic solutions

(c) X is an acid and Yis a base


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(d) Both X and Y are bases

Answer:

(c) Any solution having pH > 7 will be a base while the solution having pH < 7 will surely be an acid.

Hence, it can be concluded that X is an acid (pH=4, i.e. < 7) and Y is a base (pH =10, i.e. > 7).

Question 5. A student was asked to collect apparatus from lab store, for doing experiment of pH of given sample.

Identify the article which he is not supposed to pick. [CCE 2014]

(a) pH paper

(b) Dropper

(c) Litmus paper

(d) Petri dish

Answer:

(d) Petri dish is not required for doing experiment of pH.

Question 6. Which one of the following would you need to identify the gas that evolve when you heat NaOH

solution with zinc metal?

(a) Red litmus solution

(b) Blue litmus solution

(c) A burning splinter / matchstick

(d) Lime water

Answer:

(c) When a base like NaOH is treated with any active metal like Zn, it produces H2 gas. And the

presence of the hydrogen gas can be tested by bringing a burning splinter/ matchstick near the gas

produced. The gas will burn with a pop sound confirming the presence of hydrogen gas.

Question 7. A solution has pH value of 5. On adding 10 mL of NaCl to it, what will be the pH of the new

solution?

(a) More than 5

(b) Less than 5

(c) Only seven

(d) No change in pH

Answer:

(d) As NaCl is a neutral solution due to its complete ionization in H2O and has pH = 7. Thus, it will

not affect the pH of the solution. Hence, the pH of the solution will remain unaffected.

Question 8. A salt is dissolved in water. The pH of this salt solution was found to be 7 by measuring the pH with a


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universal indicator paper. The salt is most likely to be

(a) Na2CO3

(b) KCl

(c) NH4Cl

(d) CH3COONa

Answer:

(b) Salts which are completely ionisable in water are said to be neutral salts and their pH is equal to 7.

Among the given salts, only KCl is completely ionisable in the following manner: KCl (aq) → K+ +

Cl

Hence, the pH of KCl will be 7.

Question 9. A student performed an experiment using zinc granules and sodium carbonate with sodium hydroxide

and hydrochloric acid under different

conditions as shown below.

In which set up, no gas is evolved?

(a) I

(b) II

(c) III

(d) IV

Answer:

(d) Gas will not evolve in the case of IV set up as NaOH does not react with sodium carbonate.

Question 10. Four set ups as given below are arranged to identify the gas evolved when dilute hydrochloric acid

was added to zinc granules. Which is the most appropriate set up?


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(a) I

(b) II

(c) IV

(d) III

Answer:

(c) The gas evolved can be tested as shown in IV set up because to evolve H2 gas delivery tube,

should not dip in the acid.

Question 11.

Four students I, II, III and IV were asked to examine the changes for blue and red litmus paper strips

with dil. HCl (Solution A) and dil. NaOH (Solutions). The following observations were reported by

the 4 students. The sign …………. indicates no colour change.

Litmus A B Litmus A B

I Blue — red Blue red —

II Red — blue Red — blue

III Blue red red Blue blue blue

IV Red blue blue Red red red

The correct observation would be of student

(a) I

(b) II

(c) III

(d) IV

Answer:

(c) The correct observation is taken by student III because HCl (solution A) is an acid which turns

blue litmus red and dil. NaOH (solution B) is a base which turns red litmus blue.

Question 12. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH


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paper yellowish orange. Which of the following would change the colour of this pH paper to greenish

blue?

(a) Lemon juice

(b) Vinegar

(c) Common salt

(d) An antacid

Answer:

(d) As pH paper turns greenish blue for weakly basic compound and antacids contain weak base like

Mg(OH)2. So, an antacid would change the colour of this pH paper to greenish blue. Other options (a)

and (b) contain acids and option (c) is a neutral salt.

Documents

Chapter 2 Acids, Bases and Salts