NCERT Solutions for Class 10 Science Chapter 2 - Acids, Bases And Salts
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NCERT Solutions for Class 10 Science Chapter 2 Acids, Bases And Salts are provided here with simple step-by-step explanations. These solutions for Acids, Bases And Salts are extremely popular among class 10 students for Science Acids, Bases And Salts Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of class 10 Science Chapter 2 are provided here for you for free. You will also love the ad-free experience on Meritnation’s NCERT Solutions. All NCERT Solutions for class 10 Science are prepared by experts and are 100% accurate.

Page No 9:

Question 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)

Answer:

When a solution of an acid is mixed with a solution of a base, salt and water are produced. This reaction is known as a neutralisation reaction. Heat is evolved in this reaction. Therefore, it is an exothermic reaction and results in the increase of temperature of the solution.

Hence, the correct answer is option D.

Page No 9:

Question 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

Answer:

Basic solutions change red litmus solution to blue. Thus, the given aqueous solution is basic in nature. Acid solutions change blue litmus solution to blue. Therefore, the addition of hydrochloric acid will reverse the change.

Hence, the correct answer is option D.

Page No 9:

Question 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

Answer:

Calcium chloride taken in the guard tube acts as a dehydrating agent. It absorbs moisture from the hydrogen chloride gas that flows through it to obtain dry hydrogen chloride gas.

Hence, the correct answer is option C.

Page No 9:

Question 4:

Which of the following salts does not contain water of crystallisation?
(a) Blue vitriol
(b) Baking soda
(c) Washing soda
(d) Gypsum

Answer:

Blue vitriol is ${CuSO}_{4} . 5 H_{2} O$
Baking soda is ${NaHCO}_{3_{}}$
Washing soda is ${Na}_{2} {CO}_{3} . 10 H_{2} O$
Gypsum is ${CaSO}_{4} . 2 H_{2} O$

Hence, the correct answer is option B.

Page No 10:

Question 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

Answer:

Strong bases and weak acids react to produce basic salt and water. When sodium hydroxide, a strong base reacts with carbonic acid, a weak acid, sodium carbonate salt and water are produced.
$2 NaOH + H_{2} {CO}_{3} \to {Na}_{2} {CO}_{3} + 2 H_{2} O$

Hence, the correct answer is option D.

Page No 10:

Question 6:

Calcium phosphate is present in tooth enamel. Its nature is
(a) basic
(b) acidic
(c) neutral
(d) amphoteric

Answer:

Calcium phosphate is a basic salt as it is formed by the reaction of a strong base, calcium hydroxide and a weak acid, phosphoric acid.

$\underset{(Phosphoric acid)}{2 H_{3} {PO}_{4}} + \underset{(Calcium hydroxide)}{3 Ca {(OH)}_{2}} \to \underset{(Calcium phosphate)}{{Ca}_{3} ({PO}_{4})_{2}} + 6 H_{2} O$

Hence, the correct answer is option A.

Page No 10:

Question 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

Answer:

Since this solution turns the pH paper yellowish-orange, it is acidic in nature. An antacid would change the colour of this pH paper to greenish-blue because it is basic in nature.

Hence, the correct answer is option D.

Page No 10:

Question 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

Answer:

Distilled water is neutral. Acetic acid is an organic acid. So, it is a weak acid. Hydrochloric acid is a mineral acid. So, It is a strong acid.

Hence, the correct answer is option A.

Page No 10:

Question 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 hydrogencarbonate
(c) After washing with plenty of water apply solution of sodium hydroxide on the hand
(d) Neutralise the acid with a strong alkali

Answer:

If a few drops of a concentrated acid accidentally spills over the hand, it should be washed immediately with plenty of water to remove the acid from the hand. After this, a paste of sodium hydrogencarbonate should be applied to neutralise the remaining acid on hand. Sodium hydroxide or any strong alkali is highly corrosive, it will burn the hand.

Hence, the correct answer is option B.

Page No 10:

Question 10:

Sodium hydrogencarbonate 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)

Answer:

When sodium hydrogencarbonate is added to acetic acid, the following reaction takes place:
${NaHCO}_{3} + {CH}_{3} COOH \to {CH}_{3} COONa + {CO}_{2} ↑ + H_{2} O$
Evolved gas is carbon dioxide which is a fire extinguisher and therefore, extinguishes a burning splinter.
On passing the carbon dioxide gas evolved through lime water, it turns lime water milky due to formation of white precipitate of calcium carbonate.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The carbon dioxide gas, being a non-metallic oxide dissolves in a solution of sodium hydroxide to form sodium carbonate and water.
CO₂ + 2NaOH → Na₂CO₃ + H₂O

Hence, the correct answer is option B.

Page No 11:

Question 11:

Common salt besides being used in kitchen can also be used as the raw material for making
(i) washing 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)

Answer:

Common salt besides being used in kitchen can also be used as the raw material for making (i) washing soda, (ii) bleaching powder and (iii) baking soda.

Disclaimer: In this question, none of the above options is correct.

Hence, no option is correct.

Page No 11:

Question 12:

One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is
(a) hydrochloric acid
(b) tartaric acid
(c) acetic acid
(d) sulphuric acid

Answer:

Baking powder is a mixture of baking soda (sodium hydrogencarbonate) and a mild edible acid such as tartaric acid.

Hence, the correct answer is option B.

Page No 11:

Question 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

Answer:

Tooth decay starts when the pH of the mouth is lower than 5.5. Bacteria present in the mouth produce acids by degrading food particles. Toothpastes, which are generally basic, neutralise this excess acid and helps in preventing tooth decay.

Hence, the correct answer is option C.

Page No 11:

Question 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)

Answer:

On the pH scale, we can measure pH generally from 0 (very acidic) to 14 (very alkaline). The pH of a neutral solution is 7. Values less than 7 on a pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in hydroxide ion concentration in the solution, that is, an increase in the strength of alkali.

Hence, the correct answer is option D.

Page No 11:

Question 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

Answer:

Our stomach produces hydrochloric acid which helps in the digestion of food. Hydrochloric acid is one of the components of gastric juice. The pH of gastric juice is about 1.2.

Hence, the correct answer is option A.

Page No 11:

Question 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)

Answer:

When a small amount of acid is added to water, it dissociates into its ions. This process is known as ionisation. Addition of water results in the decrease of concentration of ions per unit volume. This process is known as dilution.

Hence, the correct answer is option B.

Page No 12:

Question 17:

Which one of the following can be used as an acid-base indicator by a visually impared student?
(a) Litmus
(b) Turmeric
(c) Vanilla essence
(d) Petunia leaves

Answer:

Vanilla essence works as an olfactory indicator. It changes its odour in acidic or basic medium.

Hence, the correct answer is option C.

Page No 12:

Question 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

Answer:

All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water. Marble and limestone are different forms of calcium carbonate $({CaCO}_{3})$ . Baking soda $({NaHCO}_{3})$ is a hydrogen carbonate. So, they will release carbon dioxide on treatment with dilute acid. Lime ( $CaO$ ) will not do so.

Hence, the correct answer is option D.

Page No 12:

Question 19:

Which of the following is acidic in nature?
(a) Lime juice
(b) Human blood
(c) Lime water
(d) Antacid

Answer:

Human blood is slightly basic (pH 7.4). Lime water $Ca {(OH)}_{2}$ and antacid are basic in nature. Lime juice is acidic, it contains citric acid.

Hence, the correct answer is option A.

Page No 12:

Question 20:

In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus 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
(d) (iv) only

Answer:

Strong acids and bases are good conductors of electricity because they dissociate completely to furnish ions. NaOH is a strong base, it dissociates into ${Na}^{+}$ and ${OH}^{-}$ .

Hence, the correct answer is option C.

Page No 12:

Question 21:

Which of the following is used for dissolution of gold?
(a) Hydrochloric acid
(b) Sulphuric acid
(c) Nitric acid
(d) Aqua regia

Answer:

Aqua Regia is a highly corrosive mixture of conc. nitric acid and conc. hydrochloric acid in the ratio of 1:3. It can dissolve noble metals including gold.

Hence, the correct answer is option D.

Page No 13:

Question 22:

Which of the following is not a mineral acid?
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid

Answer:

A mineral acid is an acid derived from one or more inorganic compounds. Citric acid is found in citrus fruits. It is an organic acid.

Hence, the correct answer is option B.

Page No 13:

Question 23:

Which among the following is not a base?
(a) NaOH
(b) KOH
(c) NH₄OH
(d) C₂H₅OH

Answer:

Ethyl alcohol is not a base as it does not release hydroxide ion in aqueous solution.

Hence, the correct answer is option D.

Page No 13:

Question 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

Answer:

Metal oxides are basic in nature. They give alkaline solution when dissolved in water. But all bases do not dissolve in water. An alkali is a base that dissolves in water.

Hence, the correct answer is option B.

Page No 13:

Question 25:

Match the chemical substances given in Column (A) with their appropriate application given in Column (B)

Column (A)		Column (B)
(A)	Bleaching powder	(i)	Preparation of glass
(B)	Baking soda	(ii)	Production of H₂ and Cl₂
(C)	Washing soda	(iii)	Decolourisation
(D)	Sodium chloride	(iv)	Antacid

(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)

Answer:

Bleaching powder is used for bleaching (decolourising) purposes. Baking soda works as an antacid because it neutralises the excess acid in our stomach to get rid of the pain. It is a mild non-corrosive basic salt. Washing soda is used in the glass industry. Aqueous sodium chloride on electrolysis yields hydrogen and chlorine gases.

Hence, the correct answer is option C.

Page No 13:

Question 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 Figure 2.2

(a) Red
(b) Yellow
(c) Yellowish green
(d) Blue

Answer:

When equal volumes of hydrochloric acid and sodium hydroxide solutions of the same concentration are mixed, salt and water are produced. It is a neutralisation reaction.
$HCl + NaOH \to NaCl + H_{2} O$

The aqueous solution of sodium chloride is neutral in nature, i.e., it has pH 7. The resulting solution would yield yellowish-green colour on the given pH paper.

Hence, the correct answer is option C.

Page No 14:

Question 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)

Answer:

HCl produces hydrogen ions when it is passed through water.
$HCl + H_{2} O \to H_{3} O^{+} + {Cl}^{-}$
Hydrogen ions can not exist alone, but they exist as hydronium ions after combining with water molecules.
$H^{+} + H_{2} O \to H_{3} O^{+}$

Hence, the correct answer is option C.

Page No 14:

Question 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

Answer:

The word acid is derived from the Latin word acere which means sour. Acids change the colour of blue litmus to red.

Hence, the correct answer is option C.

Page No 14:

Question 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

Answer:

A a dilute aqueous solution of hydrochloric acid dissociates into hydrogen and chloride ions.
$HCl + H_{2} O \to H_{3} O^{+} + {Cl}^{-}$
Hydrogen ions combine with the water molecules to produce hydronium ions.
$H^{+} + H_{2} O \to H_{3} O^{+}$

Hence, the correct answer is option A.

Page No 14:

Question 30:

Identify the correct representation of reaction occurring during chloralkali 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)

Answer:

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide near cathode. The process is called the chlor-alkali process because of the products formed-chlor for chlorine and alkali for sodium hydroxide. Chlorine gas is given off at the anode, and hydrogen gas at the cathode.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Hence, the correct answer is option D.

Page No 15:

Question 31:

Match the acids given in Column (A) with their correct source given in Column (B)

Column (A)		Column (B)
(a)	Lactic acid	(i)	Tomato
(b)	Acetic acid	(ii)	Lemon
(c)	Citric acid	(iii)	Vinegar
(d)	Oxalic acid	(iv)	Curd

Answer:

$(a) Lactic acid \to (iv) Curd (b) Acetic acid \to (iii) Vinegar (c) Citric acid \to (ii) Lemon (d) Oxalic acid \to (i) Tomato$

Page No 15:

Question 32:

Match the important chemicals given in Column (A) with the chemical formulae given in Column (B)

Column (A)		Column (B)
(a)	Plaster of Paris	(i)	Ca(OH)₂
(b)	Gypsum	(ii)	CaSO₄.1/2 H₂O
(c)	Bleaching Powder	(iii)	CaSO₄.2H₂O
(d)	Slaked Lime	(iv)	CaOC1₂

Answer:

$(a) Plaster of Paris \to (ii) {CaSO}_{4} . \frac{1}{2} H_{2} O (b) Gypsum \to (iii) {CaSO}_{4} . 2 H_{2} O (c) Bleaching Powder \to (iv) {CaOCl}_{2} (d) Slaked lime \to (i) Ca {(OH)}_{2}$

Page No 15:

Question 33:

What will be the action of the following substances on litmus paper?
Dry HCl gas, Moistened NH₃gas, Lemon juice, Carbonated soft drink, Curd, Soap solution.

Answer:

Dry HCl gas will have no effect on litmus paper.
Moistened ${NH}_{3}$ gas will turn red litmus to blue.
Lemon juice will turn blue litmus to red.
Carbonated soft drink will turn blue litmus to red.
Curd will turn blue litmus to red.
Soap solution will turn red litmus to blue.

Page No 15:

Question 34:

Name the acid present in ant sting and give its chemical formula.
Also give the common method to get relief from the discomfort caused by the ant sting.

Answer:

The acid present in ant's sting is methanoic acid. Its chemical formula is HCOOH. Use of the mild base like baking soda on the stung area gives relief.

Page No 15:

Question 35:

What happens when nitric acid is added to eggshell?

Answer:

Eggshell is composed of calcium carbonate which dissolves in nitric acid and produces carbon dioxide gas.
${CaCO}_{3} + 2 {HNO}_{3} \to Ca {({NO}_{3})}_{2} + H_{2} O + {CO}_{2}$

Page No 15:

Question 36:

A student prepared solutions of (i) an acid and (ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless, how will she distinguish between the two?

Answer:

She can distinguish acid and base by using other indicators like phenolphthalein (pink colour in acidic medium), methyl orange (red colour in acidic medium), turmeric (red colour in basic medium) etc.

Page No 15:

Question 37:

How would you distinguish between baking powder and washing soda by heating?

Answer:

When baking powder is heated, the following reaction takes place:

$2 {NaHCO}_{3} \overset{Heat}{\to} {Na}_{2} {CO}_{3} + {CO}_{2} + H_{2} O$

This reaction does not happen when washing soda is heated. The water of crystallisation is given out and salt becomes anhydrous on heating washing soda.

Page No 15:

Question 38:

Salt A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of hardness of water and a gas C is evolved. The gas C when passed through lime water, turns it milky. Identify A, B and C.

Answer:

Baking soda is commonly used in the kitchen for making crispy pakodas and bakery products, etc. Sometimes, it is added for faster cooking. Hence, salt A is baking soda $({NaHCO}_{3})$ . Following reaction takes place when it is heated -

$2 {NaHCO}_{3} \overset{heat}{\to} {Na}_{2} {CO}_{3} + H_{2} O + {CO}_{2}$

Salt B is washing soda which is used for removal of permanent hardness of water. Gas C is carbon dioxide. On passing this through lime water, it turns milky because of the formation of calcium carbonate.

$\underset{(lime water)}{Ca {(OH)}_{2} (aq)} + \underset{}{{CO}_{2} (g)} \to \underset{(white ppt)}{{CaCO}_{3} (s)} + \underset{}{H_{2} O (l)}$

Page No 16:

Question 39:

In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as by product. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved.

Answer:

Sodium hydroxide is manufactured with the help of chloralkali process which is as follows -
$2 NaCl (aq) + 2 H_{2} O (l) \to 2 NaOH (aq) + {Cl}_{2} (g) + H_{2} (g)$

Bleaching powder is produced by the action of chlorine on lime water.
$Ca {(OH)}_{2} + {Cl}_{2} \to \underset{(bleaching powder)}{{CaOCl}_{2}} + H_{2} O$

Hence, gas X is chlorine. Compound Y is bleaching powder.

Page No 16:

Question 40:

Fill in the missing data in the following table

Name of the salt		Formula	Salt obtained from
Name of the salt		Formula	Base	Acid
(i)	Ammonium chloride	NH₄Cl	NH₄OH	–
(ii)	Copper sulphate	–	–	H₂SO₄
(iii)	Sodium chloride	NaCl	NaOH	–
(iv)	Magnesium nitrate	Mg(NO₃)₂	–	HNO₃
(v)	Potassium sulphate	K₂SO₄	–	–
(vi)	Calcium nitrate	Ca(NO₃)₂	Ca(OH)₂	–

Answer:

Missing data has been written in the bold format-

Name of the salt		Formula	Salt obtained from
Name of the salt		Formula	Base	Acid
(i)	Ammonium chloride	NH₄Cl	NH₄OH	HCl
(ii)	Copper sulphate	CuSO₄	Cu(OH)₂	H₂SO₄
(iii)	Sodium chloride	NaCl	NaOH	HCl
(iv)	Magnesium nitrate	Mg(NO₃)₂	Mg(OH)₂	HNO₃
(v)	Potassium sulphate	K₂SO₄	KOH	H₂SO₄
(vi)	Calcium nitrate	Ca(NO₃)₂	Ca(OH)₂	HNO₃

Page No 16:

Question 41:

What are strong and weak acids? In the following list of acids, separate strong acids from weak acids.
Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid.

Answer:

The strength of acids depends on the no. of $H^{+}$ ions produced. Acids that give rise to more hydrogen ions are said to be strong acids and acids that give less hydrogen ions are said to be weak acids.
Hydrochloric acid, nitric acid and sulphuric acid get ionised completely, so they are strong acids.
Citric acid, acetic acid, formic acid get ionised partially, so they are weak acids.

Page No 16:

Question 42:

When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved, which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.

Answer:

Let us take a dilute solution of sulphuric acid which is a strong acid. When zinc metal is treated with it, the following reaction takes place-

$Zn + H_{2} {SO}_{4} \to {ZnSO}_{4} + H_{2}$

As you can see, hydrogen gas is evolved in this reaction which is utilised in the hydrogenation of oil. If you bring a burning candle or match stick near the hydrogen gas, it burns with a pop sound.

Page No 16:

Question 43:

In the following schematic diagram for the preparation of hydrogen gas as shown in Figure 2.3, what would happen if following changes are made?

(a) In place of zinc granules, same amount of zinc dust is taken in the test tube
(b) Instead of dilute sulphuric acid, dilute hydrochloric acid is taken
(c) In place of zinc, copper turnings are taken
(d) Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.

Answer:

(a) If zinc dust is taken in place of zinc granules, the rate of reaction will increase.
(b) If dilute hydrochloric acid is taken in place of dilute sulphuric acid, zinc chloride and hydrogen gas will be produced.
$Zn + 2 HCl \to {ZnCl}_{2} + H_{2}$
(c) If copper turnings are taken in place of zinc, no reaction will take place because copper does not react with dilute acids under normal conditions.
(d) If sodium hydroxide is taken in place of dilute sulphuric acid, sodium zincate and hydrogen gas will be produced in the following manner-
$Zn + 2 NaOH \to {Na}_{2} {ZnO}_{2} + H_{2}$

Page No 17:

Question 44:

For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake,
(a) how will it affect the taste of the cake and why?
(b) how can baking soda be converted into baking powder?
(c) what is the role of tartaric acid added to baking soda?

Answer:

The chemical name of baking soda is sodium hydrogencarbonate ( ${NaHCO}_{3}$ ). It is a mild non-corrosive basic salt.
(a) If your mother uses baking soda instead of baking powder in a cake, the taste of the cake will be bitter because of the formation of sodium carbonate.
$\underset{(baking soda)}{2 {NaHCO}_{3}} \overset{heat}{\to} \underset{(sodium carbonate)}{{Na}_{2} {CO}_{3}} + H_{2} O + {CO}_{2}$

(b) Baking soda can be converted into baking powder by mixing a mild edible acid such as tartaric acid into it.
(c) Tartaric acid is added to baking soda so that it can neutralise the sodium carbonate and hence, the cake will not taste bitter.

Page No 17:

Question 45:

A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z.

Answer:

When calcium carbonate reacts with hydrochloric acid, the following reaction takes place-
${CaCO}_{3} + 2 HCl \to {CaCl}_{2} + H_{2} O + {CO}_{2}$

On passing the carbon dioxide gas evolved through lime water, it turns milky because of the formation of calcium carbonate. Hence, we got it back.
${CO}_{2} + Ca {(OH)}_{2} \to \underset{(white ppt)}{{CaCO}_{3}} + H_{2} O$

When electricity is passed through brine (an aqueous solution of sodium chloride), it decomposes to form sodium hydroxide.
$2 NaCl + 2 H_{2} O \to 2 NaOH + H_{2} + {Cl}_{2}$
Hydrogen gas is given off at the cathode and chlorine gas at the anode. Chlorine gas is used in the manufacture of bleaching powder which is produced by the action of chlorine on dry slaked lime and represented as ${CaOCl}_{2}$ though the actual composition is quite complex. It is used to make drinking water free from germs.
$Ca {(OH)}_{2} + {Cl}_{2} \to {CaOCl}_{2} + H_{2} O$

Hence, the metal carbonate X is ${CaCO}_{3}$ , Solution Y is lime water $[C a {(O H)}_{2}]$ , gas G is chlorine $({Cl}_{2})$ , Y is dry slaked lime $[Ca {(OH)}_{2}]$ , Z is bleaching powder $({CaOCl}_{2})$ .

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Question 46:

A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by–product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution.

Answer:

In the chloralkali process, when electricity is passed through the brine (an aqueous solution of sodium chloride), sodium hydroxide, chlorine gas and hydrogen gas are produced.
$2 NaCl (aq) + 2 H_{2} O (l) \to 2 NaOH (aq) + {Cl}_{2} (g) + H_{2} (g)$

So, B is sodium hydroxide. When it is kept in open, it absorbs moisture and turns sticky. Sodium hydroxide is a base. When it is treated with an acidic oxide like carbon dioxide, sodium carbonate (salt) and water are produced. Hence, this is a neutralisation reaction.
$2 NaOH + {CO}_{2} \to {Na}_{2} {CO}_{3} + H_{2} O$

Page No 17:

Question 47:

A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved.

Answer:

Calcium sulphate is a salt of group 2 element (calcium), which is a white, soft substance and can be moulded into different shapes by making its dough. It is commonly known as Plaster of Paris. When this compound is left in open for some time, it changes to gypsum giving a hard solid mass.
$\underset{(Plaster of Paris)}{{CaSO}_{4} . \frac{1}{2} H_{2} O} + 1 \frac{1}{2} H_{2} O \to \underset{(Gypsum)}{{CaSO}_{4} . 2 H_{2} O}$

Page No 17:

Question 48:

Identify the compound X on the basis of the reactions given below.
Also, write the name and chemical formulae of A, B and C.

Answer:

When zinc reacts with sodium hydroxide, the following reaction takes place-
$Zn + 2 NaOH \to {Na}_{2} {ZnO}_{2} + H_{2}$

When hydrochloric acid reacts with sodium hydroxide, the following reaction takes place-
$NaOH + HCl \to NaCl + H_{2} O$

When acetic acid reacts with sodium hydroxide, the following reaction takes place-
${CH}_{3} COOH + NaOH \to {CH}_{3} COONa + H_{2} O$

Hence, compound X is sodium hydroxide $(NaOH)$ , A is sodium zincate $({Na}_{2} {ZnO}_{2})$ , B is sodium chloride $(NaCl)$ , C is sodium acetate $({CH}_{3} COONa)$ .

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