Intext Questions

Q.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?
Sol.

First of all, we will put one strip of red litmus paper in each test tube. In one of the test tubes, the litmus paper will turn blue. It is sure that this test tube contains basic solution. The remaining two test tubes contain distilled water and acidic solution in both of which red litmus remains red.

Now, we will pour out some basic solution from the test tube so that only a little amount remains in it. Then, we will pour the contents of one of the remaining test tubes into the test tube containing basic solution. Two things may happen :

(a) The litmus may turn red again : This indicates that the second test tube contains acidic substance. And the other one contains distilled water.

(b) The litmus may remain blue : This indicates that the poured liquid is distilled water. And the other one is the acidic solution.

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Page Number – 22

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

Curd and sour substances contain acids. Acids react with metals to give salt and hydrogen gas. So, if such substances are kept in copper container, the acid will react and the container will corroded.

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Q.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?
Sol.

Usually, hydrogen gas is liberated when an acid reacts with a metal. Let us take the reaction between zinc and sulphuric acid.

Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning

The apparatus is set as shown in Fig.  About 5 ml of dil. H₂SO₄ is taken in the test tube and a few granules of zinc are added to it. The delivery tube is caped in the test tube so that the evolved gas passes through the soap solution. Soap bubbles filled with the evolved gas come out of the solution. The flame of a lighted candle is brought in contact with the bubble. The gas in the bubble burns with a pop sound. This proves that the gas evolved in the reaction between zinc and sulphuric acid is hydrogen.

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Q.3 A Metal compound 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. CaCO3+2HCl→CaCl2+H2O+CO2

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Page Number – 25

Q.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?
Sol. 

HCl, HNO₃, etc. have releasable H⁺ ion. When dissolved in water, the H⁺ ion gets separated and shows acidic character. In alcohol or glucose, there is no releasable H+ ion.

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

When dissolved in water, acids dissociate to form ions e.g.,
HCl+H2O→Cl−+H3O+
These ions are responsible for electrical conductivity.

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

Sol. Dry HCl does not dissociate to give H⁺ ions. So, it does not show acidic character.

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

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.

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

Sol. Concentration of H₃O⁺ ion decreases.

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

Sol. Concentration of H₃O⁺ ion decreases.

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

Sol. Concentration of OH^– group increases.

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Page Number – 28

Q.1  You have two solutions, A and B. The pH solution A is 6 and pH of solution B is 8. While solution has more hydrogen ion concentration. Which of this is acidic and which one is basic ? 

Sol.

The solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

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

Sol. As the concentration of H⁺ ions increases the solution becomes more acidic.

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

Sol. This H⁺ ion comes from water.

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Q.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)?

Sol. If the farmer finds his soil turned acidic, he should use the bases to neutralise it.

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Page Number – 33

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

Sol. Bleaching powder.

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

Sol. Dry slaked lime [Ca(OH)₂]

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

Sol. Washing soda i.e., sodium carbonate (Na₂CO₃).

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

The following reaction take place when it is heated during cooking :

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

Sol. CaSO4.12H2O+112H2O→CaSO4.2H2O

Exercise

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

Sol.       (d) 10

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Q.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) HCl

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Q.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

Sol.        (d) 16 mL

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

Sol.        (c) Antacid

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Q.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.

Sol.

(a) Dil. Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

H2SO4(aq)+Zn→ZnSO4(aq)+H2(g)

(b) Dil. Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

2HCl(aq)+Mg→MgCl2(aq)+H2(g)

(c) Dil. Suphuric acid + Aluminium → Aluminium sulphate + Hydrogen gas

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

(d) Dil. Hydrochloric acid + Iron → Ferric chloride + Hydrogen gas

6HCl(aq)+3Fe(s)→3FeCl2(aq)+3H2(g)

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

Sol.

The apparatus is set as shown in figure. Solutions of glucose is added and the switch is turned on. The bulb does not glow. This means glucose does not dissociate into ions on dissociation. So, glucose is not an acid. Similarly, alcohol is also not an acid

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

Distilled water does not dissociate into ions. So, does not conduct electricity. Rain water has dissolved in it which becomes carbonic acid.

CO2+H2O→H2CO3

This carbonic acid dissociate into ions.

H2CO3+2H2O→(H3O+)2+CO−23

These ions are responsible for electrical conductivity rain water.

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

In absence of water, acids do not dissociated they do not show acidic behaviour.

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Q.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.
Sol.

(a) (D), (b) C, (c) B, (d) A, (e) E.

Increasing order of H+ concentration :

C(11) < E (9) < D (7) < A (4) < B (1).

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Q.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 ?
Sol.

In test tube A, fizzing occur more vigorously because HCl is a strong acid and dissociate more.

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Q.11  Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Sol.    Curd is sour which means its pH will decrease from 6.

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Q.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 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Sol.

(a) Baking soda is alkaline which causes the alkalinity of milk.

(b) Curd needs acidic condition to set.

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

Plaster of Paris easily absorbs water and forms hard gypsum. Thus, if Plaster of Paris is not kept in a moisture-proof container, then all Plaster of Paris will get converted into gypsum.

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

The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction.

NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)

Ca(OH)2(aq)+H2OSO4(aq)→CaSO4+H2O

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Q.15   Give two important uses of washing soda and baking soda.
Sol.

Uses of Washing Soda –
(i) Sodium carbonate (washing soda) is used in glass soap and paper industries.

(ii) It is used in the manufacture of sodium compound such as borax.

Uses of Baking Soda –
(i) Sodium hydrogencarbonate is also an mgredient antacids. Being alkaline, it neutralises excess acid in stomach and provides relief.

(ii) It is also used in soda–acid fire extinguishers.

Exemplar

Multiple Choice Questions : –

Q.1 What happens when a solution of an acid is mixed with a solution of a base in a test tube?

(i) The temperature of the solution increases.

(ii) The temperature of the solution decreases.

(iii) The temperature of the solution remains the same.

(iv)  Salt formation takes place

(a) (I) only (b) (I) and (III)

(c) (II) and (iii) (d) (I) and (IV)

Sol.  (d)

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Q.2 An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

(a) Baking powder

(b) Lime

(c) Ammonium hydroxide solution

(d) Hydrochloric acid

Sol.   (d)

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Q.3 During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

(a) Absorb the evolved gas

(b) Moisten the gas

(c) Absorb moisture from the gas

(d) Absorb Cl^– ions from the evolved gas

Sol.   (c)

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Q.4 Which of the following salts does not contain water of crystallisation?

(a) Blue vitriol

(b) Baking soda

(c) Washing soda

(d) Gypsum

Sol.  (b)

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Q.5 Sodium carbonate is a basic salt because it is a salt of

(a) Strong acid and strong base

(b) Weak acid and weak base

(c) Strong acid and weak base

(d) Weak acid and strong base

Sol.   (d)

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Q.6 Calcium phosphate is present in tooth enamel. Its nature is

(a) basic

(b) acidic

(c) neutral

(d) Amphoteric

Sol.  (c)

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Q.7 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?

(a) Lemon juice

(b) Vinegar

(c) Common salt

(d) An antacid

Sol.  (d)

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Q.8 Which of the following gives the correct increasing order of acidic strength?

(a) Water < Acetic acid < Hydrochloric acid

(b) Water < Hydrochloric acid < Acetic acid

(c) Acetic acid < Water < Hydrochloric acid

(d) Hydrochloric acid < Water < Acetic acid

Sol.  (a)

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Q.9 If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

(a) Wash the hand with saline solution

(b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate

(c) After washing with plenty of water, apply solution of sodium hydroxide on the hand

(d) Neutralise the acid with a strong alkali

Sol. (b)

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Q.10 Sodium hydrogen carbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved?

(i) It turns lime water milky

(ii) It extinguishes a burning splinter

(iii) It dissolves in a solution of sodium hydroxide

(iv) It has a pungent odour

(a) (i) and (ii)

(b) (i), (ii) and (iii)

(c) (ii), (iii) and (iv)

(d) (i) and (iv)

Sol. (a)

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Q.11 Common salt besides being used in kitchen can also be used as the raw material for making

(i) Soda

(ii) Bleaching powder

(iii) Baking soda

(iv) Slaked lime

(a) (i) and (ii)

(b) (i), (ii) and (iv)

(c) (i) and (iii)

(d) (i), (iii) and (iv)

Sol.  (c)

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Q.12 One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is

(a) Hydrochloric acid

(b) Tartaric acid

(c) Acetic acid

(d) Sulphuric acid

Sol. (b)

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Q.13 To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is

(a) Acidic

(b) Neutral

(c) Basic

(d) Corrosive

Sol. (c)

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Q.14 Which of the following statements is correct about an aqueous solution of an acid and of a base?

(i) Higher the pH, stronger the acid

(ii) Higher the pH, weaker the acid

(iii) Lower the pH, stronger the base

(iv) Lower the pH, weaker the base

(a) (i) and (iii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (ii) and (iv)

Sol.  (c)

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Q.15 The pH of the gastric juices released during digestion is

(a) Less than 7

(b) More than 7

(c) Equal to 7

(d) Equal to 0

Sol.  (?)

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Q.16  Which of the following phenomena occur, when a small amount of acid is added to water?

(i) Ionisation

(ii) Neutralisation

(iii) Dilution

(iv) Salt formation

(a) (i) and (ii)

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (ii) and (iv)

Sol.  (b)

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Q.17 Which one of the following can be used as an acid–base indicator by a visually impaired student?

(a) Litmus

(b) Turmeric

(c) Vanilla essence

(d) Petunia leaves

Sol. (c)

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Q.18 Which of the following substance will not give carbon dioxide on treatment with dilute acid?

(a) Marble

(b) Limestone

(c) Baking soda

(d) Lime

Sol. (d)

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Q.19 Which of the following is acidic in nature?

(a) Lime juice

(b) Human blood

(c) Lime water

(d) Antacid

Sol. (b)

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Q.20 In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Given Figure Below) was set up.

Which among the following statement(s) is (are) correct?

(i) Bulb will not glow because electrolyte is not acidic

(ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction.

(iii) Bulb will not glow because circuit is incomplete

(iv) Bulb will not glow because it depends upon the type of electrolytic solution

(a) (i) and (iii)

(b) (ii) and (iv)

(c) (ii) only

(c) (iv) only

Sol.  (c)

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Q.21 Which of the following is used for dissolution of gold?

(a) Hydrochloric acid

(b) Sulphuric acid

(c) Nitric acid

(d) Aqua regia

Sol. (d)

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Q.22 Which of the following is not a mineral acid?

(a) Hydrochloric acid

(b) Citric acid

(c) Sulphuric acid

(d) Nitric acid

Sol.  (b)

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Q.23 Which among the following is not a base?

(a) NaOH

(b) KOH

(c) NH₄OH

(d) C₂H₅ OH

Sol. (d)

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Q.24 Which of the following statements is not correct?

(a) All metal carbonates react with acid to give a salt, water and carbon dioxide

(b) All metal oxides react with water to give salt and acid

(c) Some metals react with acids to give salt and hydrogen

(d) Some non metal oxides react with water to form an acid

Sol.  (b)

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Q.25 Match the chemical substances given in Column (A) with their appropriate application given in

(a) A—(ii), B—(i), C—(iv), D—(iii)

(b) A—(iii), B—(ii), C—(iv), D—(i)

(c) A—(iii), B—(iv), C—(i), D—(ii)

(d) A—(ii), B—(iv), C—(i), D—(iii)

Sol.  (c)

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Q.26 Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in given Figure

(a) Red

(b) Yellow

(c) Yellowish green

(d) Blue

Sol. (c)

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Q.27 Which of the following is (are) true when HCl (g) is passed through water?

(i) It does not ionise in the solution as it is a covalent compound.

(ii) It ionises in the solution

(iii) It gives both hydrogen and hydroxyl ion in the solution

(iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule

(a) (i) only

(b) (iii) only

(c) (ii) and (iv)

(d) (iii) and (iv)

Sol.  (c)

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Q.28 Which of the following statements is true for acids?

(a) Bitter and change red litmus to blue

(b) Sour and change red litmus to blue

(c) Sour and change blue litmus to red

(d) Bitter and change blue litmus to red

Sol. (c)

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Q.29 Which of the following are present in a dilute aqueous solution of hydrochloric acid?

(a) H₃O⁺ + Cl^–

(b) H₃O⁺ + OH^–

(c) Cl^– + OH^–

(d) Unionised HCl

Sol.  (a)

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Q.30 Identify the correct representation of reaction occurring during “chlor-alkali” process

(a) 2NaCl (l) + 2H₂O (l) → 2NaOH (l) + Cl₂ (g) + H₂ (g)

(b) 2NaCl (aq) + 2H₂O (aq) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

(c) 2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (aq) + H₂ (aq)

(d) 2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

Sol. (d)