Class – 10 Chemistry

Chapter 2: Acid, Base and Salt

Ncert Solved Questions

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?

Ans: put the red litmus paper in all the three test tubes one by one. The solution which turns red litmus to blue is a basic solution. Now, put this one litmus paper to other two solution one by one and the solution which turns it to red is acidic solution. The third solution is of distilled water.
2. Why should curd and sour substances not be kept in brass and copper vessels?

Sol:Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, metals react with the acids to form harmful toxic products, thereby spoiling the food and damaging our health.

3.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?

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

Procedure:
Step 1: Take few pieces of zinc granules and add 5 ml of dilute H₂SO₄.
Step 2: Shake it and pass the gas produced into a soap solution. Bubbles are formed in the soap solution. These soap bubbles contain hydrogen gas.
Zn + dil. H₂SO₄ →ZnSO₄ + H₂
Test for hydrogen gas: The evolved hydrogen gas can be tested by bringing a burning candle near the soap bubbles. Hydrogen gas burns with a pop sound.

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.

Sol:Metal compound A should be a compound of Ca as a calcium salt is formed in the product. We know that CO₂ extinguishes a burning candle. Also, since carbon dioxide is liberated, therefore, it must be a carbonate. Hence, it is calcium carbonate.

CaCO₃ (S) + 2HCl (aq) → CaCl₂ (S) + CO₂ (g) + H₂O (l)

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.

Sol:HCl and HNO₃ show acidic character because they dissociate in the presence of water to form hydrogen or hydronium ions.

Although aqueous solutions of glucose and alcohol contain hydrogen, these do not dissociate in water to form hydrogen or hydronium ions. Hence, they do not show acidic character.

6. :Why does an aqueous solution of an acid conduct electricity?

Sol:Acids dissociate in aqueous solutions to form charged particles called ‘ions’. These ions are responsible for conduction of electricity.

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

Sol:Dry HCl gas does not change the colour of the dry litmus paper because it does not contain hydrogen or hydronium ions.

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

Sol:It is recommended that the acid should be added to water and not water to the acid because the process of dissolving an acid in water is exothermic. If water is added to acid, since large amount of acid is present, a large amount of heat is generated at once. Hence, the mixture can splash out and cause burns. But, if acid is added to water, then heat is evolved gradually and easily absorbed by the large amount of water.

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

Sol:When an acid is diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases. This means that the strength of the acid decreases.

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

Sol: The concentration of hydroxide ions (OH^–) would increase when excess base is dissolved in a solution of sodium hydroxide.

11 .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?

Sol:A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. The pH of a solution is inversely proportional to its hydrogen ion concentration. Hence, the solution having lower pH will have more hydrogen ion concentration. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

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

Sol:If the concentration of H⁺ ions is increased, the solution becomes more acidic. If the concentration of H⁺ ions is decreased, the solution becomes less acidic or more basic.

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

Sol:Yes, a basic solution also has H⁺(aq) ions. These come from the ionization of water in which the base is dissolved. However, their concentration is less as compared to the concentration of OH^–(aq) ions that makes the solution basic.

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)?

Sol:If the soil is too acidic and improper for cultivation, then to neutralise the acidity of the soil, the farmer would treat the soil with bases like quick lime or slaked lime or chalk.

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

Sol:The common name of the compound CaOCl₂ is bleaching powder.

16 :Name the substance which on treatment with chlorine yields bleaching powder?

Sol: Calcium hydroxide or slaked lime [Ca(OH)₂], on treatment with chlorine, yields bleaching powder.

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

Sol:Washing soda (Na₂CO₃.10 H₂O) is used for softening hard water.

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

Sol:When a solution of sodium hydro carbonate is heated, sodium carbonate and water are formed with the evolution of carbon dioxide gas.

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

Sol:The chemical equation for the reaction of Plaster of Paris and water can be represented as:

20.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.

Sol

(a) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen
              H₂S0₄ (aq) + Zn(s) → ZnSO₄ (aq) + H₂ (g)
(b) Hydrochloric acid + Magnesium ──→ Magnesium chloride + Hydrogen
            2HCl (aq) + Mg (s) —-→ MgCl₂ (aq) + H₂ (g)
(c) Sulphuric acid + Aluminium —–→ Aluminium sulphate + Hydrogen
            3 H₂SO₄ (aq) + 2 Al(s) —-→ Al₂(SO₄)₃ (aq) + 3 H₂ (g)

(d) Hydrochloric acid + Iron —-→ Ferric chloride + Hydrogen
6 HCl (aq) + 2 Fe (s) —–→ 2 FeCl₃ (aq) + 3H₂ (g)

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

Sol :Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. When rain water falls to the earth, it dissolves an acidic gas ‘carbon dioxide’ from the air and forms carbonic acid. This acid provides some hydrogen ions and carbonate ions to rain water. Hence, due to presence of these ionic species, rain water conducts electricity.

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

Sol:Acids do not show acidic behaviour in the absence of water because there are no hydrogen ions available for conduction of electricity. Acids dissociate in the presence of water to give free hydrogen ions. It is the hydrogen ions that are responsible for the acidic behaviour.

23.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.

Ans:

(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

pH is inversely proportional to hydrogen ion concentration. Hence, the pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1

24. 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?

ans: 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 contains a much greater amount of hydrogen ions; therefore it produces hydrogen gas at a faster speed due to which fizzing occurs.

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

ans:The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature due to formation of lactic acid.

26.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?

ans:(a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not get sour easily.

(b) Since this milk is slightly basic than usual milk, lactic acid produced to set the curd is neutralised by the base. Therefore, it takes a longer time for the curd to set.

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

ans: Plaster of Paris 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.

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

ans:A reaction in which an acid and base react with each other to give a salt and water is termed as neutralisation reaction. In this reaction, energy is evolved in the form of heat. For example:
(i) NaOH (base) + HCl (acid) → NaCl (salt) + H₂O (water)
(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 neutralises the excess of acid produced and thus gives relief from indigestion.

Mg(OH)₂ + 2 HCl → MgCl₂ + 2H₂O

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

ans:Two important uses 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 gas that makes bread or cake fluffy.
(b) It is used in soda-acid fire extinguishers.

30. Dry hydrogen chloride gas does not turn blue litmus, whereas hydrochloric acid does. Why?

Ans. In the dry state, hydrogen chloride (HCl) does not release H+ ions. Therefore, it cannot behave as an acid. When dissolved in water, it forms hydrochloric acid. It dissociates to give H+ ions in solution and behaves as an acid.

31. Acid when react with metals release hydrogen gas but there is one acid which when reacts with metals does not release hydrogen except for two metals. Prove this statement.

Ans. Acid + Metal → Salt + Hydrogen

e.g. 2HCl + 2Na →2NaCl + H2

H2SO4 + 2Na → Na2SO4 + H2

HNO3 + Na →No hydrogen gas.

Because nitric acid is strong oxidising agent. Nitric acid reacts only with Mg and Mn to give hydrogen gas.

Mg + 2HNO3→ Mg(NO3)2 + H2

Mn + 2HNO3→Mn(NO3)2 + H2

32. Dry hydrogen chloride gas does not turn blue litmus whereas hydrochloric acid does. Give one reason.

HCl (aq) —–> H+ (aq) + Cl- (aq)

33. Why is Plaster of Paris written as CaSO4. ½ H2O ? How is it possible to have half a water molecule attached to CuSO4?

Ans. The actual formula of Plaster of Paris is 2CaSO4.H2O which means that one molecule of H2O is associated with two molecules of CaSO4. The formula for simplicity is written as CaSO4. ½H2O.

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

Ans. Mixing of water and acid is a highly exothermic process. When water is added to acid , due to small amount of water, evolved heat will change it to vapour and acid will spill out and may cause injury. So it is advised to mix acid to water and not water to acid.

35.How does baking powder differ from baking soda?

Answer: Baking soda is a single compound which has sodium hydrogen carbonate which is alkaline (basic) in nature Baking powder is a mixture of sodium hydrogencarbonate and a mild edible(eatable) acid such as tartaric acid. When baking powder mixes with water, then the sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly making the cake to rise and hence ‘soft and spongy’.The equation which takes place can be shown as:

NaHCO3 + H+ —-> Na+ + CO2 + H2O

36. The pH of hydrochloric acid solution is 3. Does it mean that it has only hydronium ions? If not, how are OH- ions generated?

Answer: It is found that for any aqueous solution (neutral, acidic or basic), the product of H+ ion concentration and OH- ion concentration is always constant and equal to at 25°C.This is called ionic product of water(Kw) Kw= [H+] [OH – ] = .Here The pH of hydrochloric acid solution is 3.

Let OH- ion concentration = p

[H+] [OH – ] =

1.0 × x p =

p = [ /[ 1.0 × ] =1.0 ×

Thus for a solution if H+ concentration is known, OH- ion concentration is fixed (because their product is always constant).Hence we express the acidic, basic or neutral character of any solution by expressing its H+ ion concentration in terms of pH where p stands for potenz ie power.

OBJECTIVE QUESTIONS.

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

Sol: (d)

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)

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

Sol : (d)

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

Sol (c)

5. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?

Lemon juice
Vinegar
Common salt
An antacid
sol: (d)

6.Which of the following gives the correct increasing order of acidic strength?

Water < Acetic acid < Hydrochloric acid
Water < Hydrochloric acid < Acetic acid
Acetic acid < Water < Hydrochloric acid
Hydrochloric acid < Water < Acetic acid
sol: (a)

7. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

Wash the hand with saline solution
Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate
After washing with plenty of water apply solution of sodium hydroxide on the hand
Neutralise the acid with a strong alkali
Ans: (b)

8.Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved?

It turns lime water milky
It extinguishes a burning splinter
It dissolves in a solution of sodium hydroxide
It has a pungent odour
1. (i) and (ii)
1. (i), (ii) and (iii)
1. (ii), (iii) and (iv)
1. (i) and (iv)
  Ans: (a)

9.One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is

hydrochloric acid
1. tartaric acid
1. acetic acid
1. sulphuric acid
  sol: (b)

10.To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is

acidic b. neutral c. basic d. corrosive
sol: (c)

11.Which of the following statements is correct about an aqueous solution of an acid and of a base?

Higher the pH, stronger the acid
Higher the pH, weaker the acid
Lower the pH, stronger the base
Lower the pH, weaker the base
1. (i) and (iii)
1. (ii) and (iii)
1. (i) and (iv)
1. (ii) and (iv)
  sol: (c)

12.The pH of the gastric juices released during digestion is

less than 7
1. more than 7
1. equal to 7
1. equal to 0
  sol: (a)

13..Which of the following phenomena occur, when a small amount of acid is added to water?

A). Ionisation B).Neutralisation

C).Dilution D).Salt formation

Choose correct one

A.)(i) and (ii) B.)(i) and (iii)

C.)(ii) and (iii) D.)(ii) and (iv)
sol: (b)

14.Which one of the following can be used as an acid–base indicator by a visually impared student?

Litmus
Turmeric
Vanilla essence
Petunia leaves
sol: (c)

15.Which of the following substance will not give carbon dioxide on treatment with dilute acid?

Marble
Limestone
Baking soda
Lime
sol: (d)

16.Which of the following is acidic in nature?

A . Lime juice B.Human blood

C.Lime water D.Antacid
sol: (a)

17. Which of the following is not a mineral acid?

Hydrochloric acid
Citric acid
Sulphuric acid
Nitric acid
sol: (b)

18. Which among the following is not a base?

NaOH
KOH
NH₄OH
C₂H₅OH
sol: (d)

19. Which of the following statements is not correct?

All metal carbonates react with acid to give a salt, water and carbon dioxide
All metal oxides react with water to give salt and acid
Some metals react with acids to give salt and hydrogen
Some non metal oxides react with water to form an acid
sol: (b)

20.when colour of PH paper becomes red, the solution is ——and pH is between —–

Strong acid , PH =1 to 2
Weakly acid ,PH=6 to 7
Strongly basic, PH=12 to 13
Weakly basic ,PH=7 to 8 sol: a)

21. A student adds 2 mL of acetic acid to a test tube containing 2 mL of distilled water. He then shakes the test tube well and leaves it to settle for some time. After about 5 minutes he observes that in the test tube there is:

(A) a clear transparent colorless solution

(B) a clear transparent pink solution

(D) a layer of water over the layer of acetic acid

Sol. (A)

22. An acid can react with

AgCl
Na2 CO3
PbSO4
Na2 SO4

sol (b)

23. Which of the following gives CO2 on heating?

Slaked
Quick lime
Lime stone
Soda ash.

Sol (c)

24. Plaster of Paris is made from

Lime stone
Slaked Lime
Quick lime
Gypsum

Sol (d)

25. Which is a base and not alkali?

NaOH
KOH
Fe( OH)3
None of these

sol(c)

26. Chemical formula of baking soda is

(a) MgSO4

(b) Na2 CO3

(d) MgCO3

Sol(c)