NCERT Exemplar Class 10 Science Solutions Chapter 2 Acids Bases And Salts

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: (d)
When an acid reacts with a base, neutralisation reaction takes place to form salt an water and it is an exothermic reaction. So, the temperature of the solution increases.

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: (d)

Since the aqueous solution turns red litmus solution blue, that means it is a basic compound. An acidic solution like $(HCl)$ would reverse the change. That is why $HCl$ will be used turn blue litmus solution into red.

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:(c)
The role of calcium chloride $\left ( CaCl_{2} \right )$ is to absorb moisture from the gas, as calcium chloride is a good dehydrating agent. Calcium chloride is strongly hygroscopic (absorbs water from its surroundings), so it removes moisture from the air, making it dryer.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: (b)
Baking soda is $NaHCO_{3}$ (sodium hydrogen carbonate).
blue vitriol is $CuSO_{4}.5H_{2}O$
washing soda is $Na_{2}CO_{3}.10H_{2}O$
Gypsum is $CuSO_{4}.2H_{2}O$
The water molecules that form part of the structure of a crystal (of a salt) are called water of crystallization. The salts which contain water of crystallization are called hydrated salts. Baking powder does not contain water for crystallization.

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: (d)
Sodium carbonate $(Na_{2}CO_{3})$ is the salt of weak acid carbonic acid $(H_{2}CO_{3})$ and a strong base sodium hydroxide $(NaOH)$
$2NaOH+H_{2}CO_{3}\rightarrow Na_{2}CO_{3}+2H_{2}O$
The salts of weak acids and strong bases give basic solutions (or alkaline solutions) having pH more than 7.

Question:6

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

Answer: (a)
$3Ca(OH)_{2}+2H_{3}PO_{4}\rightarrow Ca_{3}\left ( PO_{4} \right )_{2}+6H_{2}O$
Calcium phosphate $Ca_{3}\left ( PO_{4} \right )_{2}$ is basic salt, as it is a salt of weak acid (phosphoric acid) and slightly stronger base (calcium hydroxide). Also, when the pH of our mouth falls below 5.5 due to eating sweets etc., the dissolution of enamel (calcium phosphate) starts which shows that calcium phosphate is basic.

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: (d)
As pH paper turns greenish blue for weakly basic compound and antacids contain weak bases like $Mg(OH)_{2}$. So, an antacid would change the colour of this pH paper to greenish-blue.

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: (a)
Hydrochloric acid is a mineral acid and ionises completely in water, that's why it is a strong acid.
Acetic acid is an organic acid and ionises only partially in water, hence, it is a weak acid. Water is neutral .
Thus, the order of acidity is
water < acetic acid < hydrochloric acid.

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: (b)

Wash the hand immediately with plenty of water to wash away most of the acid and then apply a baking soda paste $(NaHCO_{3})$ to neutralise the little acid left. Here a strong base cannot be used to neutralise the acid because of its corrosive nature.

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: (b)
When sodium hydrogen carbonate is added to acetic acid then carbon dioxide $(CO_{2})$ gas is evolved
$NaHCO_{3}+CH_{3}COOH\; \rightarrow CH_{3}COONa+CO_{2}+H_{2}O$
$CO_{2}$ turns lime water milky; it is a non-supporter of combustion and is absorbed by strong alkalies like $NaOH$.

The gas that evolved is carbon dioxide gas which has the property of turning lime water milky. It can extinguish the burning candle or splinter and it gets dissolved in sodium hydroxide solution to form sodium carbonate but this gas is odourless.

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: (c)
Common salt (sodium chloride) is used as a raw material for making a large number of chemicals in industry such as sodium hydroxide, washing soda, baking soda, hydrochloric acid, hydrogen, chlorine and sodium metal.

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: (b)
Baking powder is a mixture of baking soda $NaHCO_{3}$, (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid.

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: (c)
The tooth paste commonly used is basic so that the extra acid formed during tooth decay is neutralised and prevent tooth decay.
When pH of our mouth falls below 5.5 due to eating of sweets etc, (ie, our mouth is moderately acidic) then the acid becomes strong enough to attack the enamel which is made up of calcium phosphate of our teeth and corrodes it which leads to tooth decay.

Question:14

Which of the following statements is correct about an aqueous solution of an acid and of a base?
(i) The higher the pH, the stronger the acid
(ii) The higher the pH, the weaker the acid
(iii) Lower the pH, the 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: (d)
It depends on the solution i.e., the higher the pH, the weaker the acid, and the lower the pH weaker the base.

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: (a)
The proteins inside the mouth establish an acidic environment. The stomach additionally secretes hydrochloric acid (pH under 7), establishing an acidic climate.
The pH of the gastric juices in the stomach is in the range of 2-5, which is somewhat acidic in nature. The acidic climate of the stomach helps the protein-breaking catalysts to separate proteins without any problem. Diet consumption with a greater amount of antacid foods is prescribed to keep an acidic environment inside the stomach.
Our stomach produces hydrochloric acid (of pH about 1.4).

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: (b)
When water is added to an acid, their molecules dissociate to form ions.
$HCl+H_{2}O\rightarrow H^{+}+Cl^{-}+H_{2}O$
$H_{2}O+H^{+}\rightarrow H_{3}O^{+}(Hydronium\; ion)$
Mixing of an acid with water is called dilution, it results in the decrease in the concentration of ions, $(H^{+},O^{-2} )$ per unit volume.

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:(c)
Vanilla essence is an olfactory indicator. So, its smell is different in acid and basic media which can be detected easily by a visually impaired student.If a basic solution is added to vanilla extract then we cannot detect the characteristic smell of vanilla extract. An acidic solution does not destroy the smell of vanilla extract.

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: (d)
Lime, $CaO$ (calcium oxide) does not evolve $CO_{2}$ when reacted with dilute acid. Other given compounds are carbonates and hydrogen carbonates, so evolve $CO_{2}$ with dilute acid

Question:19

Which of the following is acidic in nature?
(a) Lime juice
(b) Human blood
(c) Lime water
(d) Antacid
Answer: (a)
Lime juice is acidic in nature (pH range 2-3) because it contains citric acid.
Human blood a slightly has a alkaline range of 7.35 to 7.45
Lime water is basic in nature with a pH of 12.4
pH of Antacid typically exceeds 7.0

Question:20

In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Figure 2.1) 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

Answer: (c)
An electric circuit is a path in which electrons flow from a voltage or current source. The electric current streams in a closed path.
Bulb will glow because $NaOH$, being a strong base, furnishes $OH^{-}$ and $Na^{+}$ ions (which are responsible for electrical conductivity).
Sodium hydroxide conducts electrons as sodium cations and hydroxyl anions. The particles move towards the two iron nails in the arrangement going about as cathode(- ) for getting cations and other as anode(+) for getting anions.

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: (d)
Aqua-regia is used for the dissolution of gold.
Aqua-regia is a mixture of conc. $HNO_{3}$ and conc. $HCl$ in the ratio 1 : 3

Question:22

Which of the following is not a mineral acid?
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid
Answer: (b)
Citric acid is an example of organic acid or edible acid
Mineral acids or inorganic acids are generally prepared from the minerals present in the earth's crust, eg.,$HCl$ (hydrochloric acid), $H_{2}SO_{4}$ (sulphuric acid) and $HNO_{3}$ (nitric acid) are mineral acids.
Edible acids or organic acids are produced by plants or animals. eg, acetic acid, citric acid, lactic acid etc, are edible acids.

Question:23

Which among the following is not a base?
(a) $NaOH$
(b) $KOH$
(c) $NH_{4}OH$
(d) $C_{2}H_{5}OH$
Answer: (d)
$C_{2}H_{5}OH$ is not a base. It is an example of an organic compound known as alcohol (ethyl alcohol) which has somewhat acidic nature. And also $C_{2}H_{5}OH$ does not give $OH^{-}$ ions in the solution, so it is not a base.

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: (b)
Most metal oxides are insoluble in water but some of these (e.g., $Na_{2}O, CaO$) dissolve in water to form alkalies not salt and acid.
$\text {Metal oxide + acid }\rightarrow \text {Salt+water}$
$MgCO_{3} + H_{2}SO_{4} \rightarrow MgSO_{4} + H_{2}O + CO_{2}$
$\text {Metal oxide + water}\rightarrow \text {Base}$
$MgO+H_{2}O\rightarrow Mg\left ( OH \right )_{2}$

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 $H_{2}$ and $Cl_{2}$
(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: (c)
Bleaching powder bleaches the clothes and other coloured substances.
Baking soda is a constituent of antacid.
Washing soda is used in the preparation of glass.
Sodium chloride when subjected to electrolyses gives $H_{2}$ and $Cl_{2}$ gases.

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: (c)
Because the resulting solution is obtained as a result of neutralisation reaction
Strong Acid + Strong Base = Neutralisation. (Ph - 7)
$HCl+NaOH\rightarrow NaCl+H_{2}O$
The colour of the neutral solution (with pH = 7) obtained is yellowish green.

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: (c)
When $HCl$ is passed through water then $HCl$ being a polar covalent compound, ionises in water as
$HCl (aq)\rightarrow H^{+}+Cl^{-}$
$H^{+}+H_{2}O\rightarrow H_{3}O^{+}$

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: (c)
Acids are those chemical substances which have a sour taste and turn blue litmus solution to red.

Question:29

Which of the following are present in a dilute aqueous solution of hydrochloric acid?
(a) $H_{3}O^{+}+Cl^{-}$
(b) $H_{3}O^{+}+OH^{-}$
(c) $Cl^{-}+OH^{-}$
(d) unionised $HCl$
Answer: (a)
When acid is mixed with water, their molecules dissociate to form ions. The $H^{+}$ ions combine with $H_{2}O$ to form $H_{3}O^{+}$ ions.
$HCl+H_{2}O\rightarrow H^{+}+Cl^{-}+H_{2}O$
$H_{2}O+H^{+}\rightarrow H_{3}O^{+}\left ( \text {Hydronium ion} \right )$

Question:30

Identify the correct representation of reaction occurring during chloralkali process
(a) $2NaCl(l)+2H_{2}O(l)\rightarrow 2NaOH(l)+Cl_{2}\left ( g \right )+H_{2}(g)$
(b) $2NaCl(aq)+2H_{2}O(aq)\rightarrow 2NaOH(aq)+Cl_{2} \left ( g \right )+H_{2}(g)$
(c) $2NaCl(aq)+2H_{2}O(l)\rightarrow 2NaOH(aq)+Cl_{2} \left ( aq \right )+H_{2}(aq)$
(d) $2NaCl(aq)+2H_{2}O(l)\rightarrow 2NaOH(aq)+Cl_{2} \left ( g \right )+H_{2}(g)$

Answer: (d)
$2NaCl(aq)+2H_{2}O(l)\rightarrow 2NaOH(aq)+Cl_{2} \left ( g \right )+H_{2}(g)$
(because state of $Cl_{2}$ and $H_{2}$ is gaseous,$H_{2}O$ is liquid and that of $NaCl$ and $NaOH$ is aqueous)

NCERT Exemplar Class 10 Science Solutions Chapter 2 (Short Answer)

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:
We know that
Lactic acid Curd
Acetic acid Vinegar
Citric acid Lemon
Oxalic acid Tomato

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) $Ca\left ( OH \right )_{2}$
(b) Gypsum (ii) $CaSO_{4}.\frac{1}{2}H_{2}O$
(c) Bleaching Powder (iii) $CaSO_{4}.2H_{2}O$
(d) Slaked Lime (iv) $CaOCl_{2}$
Answer:
We have,

Question:33

What will be the action of the following substances on litmus paper? Dry HCl gas, Moistened NH₃ gas, Lemon juice, Carbonated soft drink, Curd, Soap solution.
Answer:
(i) Dry $HCl$ gas: No change on litmus paper
(ii) Moistened $NH_{3}$ gas (basic): Red litmus will turn blue.
(iii) Lemon Juice (contains citric acid): Blue litmus will turn red.
(iv) Carbonated soft drinks (contains carbonic acid): Blue litmus will turn red.
(v) Curd (contains lactic acid): Blue litmus will turn red.
(vi) Soap solution (basic): Red litmus will turn blue.

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:

Answer:
Formic acid (or methanoic acid) is present in the ant sting. The chemical formula is $HCOOH$. By applying some wet baking soda on the affected area, it gives relief.

Question:35

What happens when nitric acid is added to egg shell?
Answer:

Egg shells contain calcium carbonate $\left ( CaCO_{3} \right )$. When nitric acid is added to it, brisk effervescence due to the formation of $CO_{2}$ gas is observed. The reaction is
$CaCO_{3}(s) + 2HNO_{3}(aq)\rightarrow Ca(NO_{3})_{2} (aq) + CO_{2}(g) + H_{2}O (l)$

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:
In the absence of litmus, any other indicator like methyl orange, phenolphthalein, etc., can be used. Otherwise a natural indicator like turmeric can also be used. Methyl orange has the property to colour alkaline and neutral water yellow. If the water becomes acidic, it turns red immediately.

Question:37

How would you distinguish between baking powder and washing soda by heating?
Answer:
On heating $\text {NaHCO}_{3}$ (baking soda), $\text {CO}_{2}$ (carbon dioxide) gas is given out that turns lime water milky.
$\text {2NaHCO}_{3}\rightarrow \text {Na}_{2}\text {CO}_{3}+\text {H}_{2}\text {O}+\text {CO}_{2}\text {(g)}$
While on heating $\text {Na}_{2}\text {CO}_{3}.\text {10H}_{2}\text {O}$ (washing soda) water of crystallisation is given out and the salt becomes anhydrous. The presence of water of crystallisation given as product can be tested by treating it with anhydrous $\text {CuSO}_{4}$ (white) which becomes blue in colour in its contact.

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:
Salt A is sodium bicarbonate $\text {NaHCO}_{3}$ (as it is used in bakery products and gives $\text {Na}_{2}\text {CO}_{3}$ on heating).
Salt B is sodium carbonate $\text {Na}_{2}\text {CO}_{3}$ (it is used for removal of hardness of water)
Gas C is carbon dioxide $\text {CO}$, (as it turns lime water milky).
$\text {2NaHCO}_{3}\rightarrow \text {Na}_{2}\text {CO}_{3}+\text {H}_{2}\text {O}+\text {CO}_{2}\left ( g \right )$

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:
In the manufacture of sodium hydroxide, hydrogen gas and chlorine gas (X) are formed as by-product. Chlorine gas reacts with lime water to give bleaching powder, a bleaching agent.
Thus, X is chlorine gas ($Cl_{2}$ gas)
Y is calcium oxychloride or bleaching powder $(CaOCl_{2})$.
The equation for the preparation of sodium hydroxide is
$2NaCl (aq)+2H_{2}O (l) \rightarrow 2NaOH (aq) +Cl_{2} (g) + H_{2}(g)$
$Cl_{2} +Ca(OH)_{2} \rightarrow CaOCl_{2} + H_{2}O$

Question:40

Fill in the missing data in the following table

Answer:
(i) Acid : $HCl$
$\left [ NH_{4}OH+HCl\rightarrow NH_{4}Cl+H_{2}O \right ]$
(ii) Formula : $CuSO_{4}$
Base : $Cu\left ( OH \right )_{2}$
$\left [ Cu\left ( OH \right )_{2}+H_{2}SO_{4}\rightarrow CuSO_{4}+2H_{2}O \right ]$
(iii) Acid : $HCl$
$\left [ NaOH+HCl\rightarrow NaCl+H_{2}O \right ]$
(iv) Base : $Mg\left ( OH \right )_{2}$
$\left [ Mg\left ( OH \right )_{2}+2HNO_{3}-Mg\left ( NO_{3} \right )_{2}+2H_{2}O \right ]$
(v) Base: $KOH$
Acid: $H_{2}SO_{4}$
$\left [ 2KOH+H_{2}SO_{4}\rightarrow K_{2}SO_{4}+2H_{2}O \right ]$
(vi) Acid :$HNO_{3}$
$\left [ Ca\left ( OH \right )_{2}+2HNO_{3}\rightarrow Ca\left ( NO_{3} \right )_{2}+2H_{2}O \right ]$

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:
Strong acid: The acid that ionises completely in aqueous solution, thus producing a high concentration of $H_{3}O^{+}$ ions, is called a strong acid, e.g., $HCl, H_{2}SO_{4}, HNO_{3}$ etc
Weak acid Weak acid ionises only partially in aqueous solution and thus it produces ions as well as molecules, e.g., acetic acid, carbonic acid

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:
When zinc reacts with dilute solution of strong acid (like hydrochloric acid $HCl$), it forms salt and hydrogen gas is evolved which is used in hydrogenation of oil
$Zn+2HCl\rightarrow ZnCl_{2}+H_{2}(g)$
To test the presence of $H_{2}$ gas when a burning splinter is brought near the mouth of the test tube, the gas burns with a pop sound.

NCERT Exemplar Class 10 Science Solutions Chapter 2 (Long Answer)

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 same amount of zinc dust is taken in the test tube then the reaction will be comparatively faster and hydrogen gas will evolve with greater speed. It is because zinc dust has larger surface area than zinc granules.
(b) With dilute hydrochloric acid, almost same amount of gas is evolved.
(c) With copper turnings, hydrogen gas will not evolve because copper is less reactive, so, it does no react with dil. $H_{2}SO_{4}$ or dil. $HCl$. Hence, no reaction will take place.
(d) Zinc also react with $NaOH$. So, if sodium hydroxide is taken, then hydrogen gas will be evolved.
$Zn+2NaOH\rightarrow Na_{2}ZnO_{2}+H_{2}(g)$

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:
(a) The advantage of using baking powder is that tartaric acid present in baking powder reacts with sodium carbonate $\left ( Na_{2} CO_{3}\right )$ produced during decomposition of $NaHCO_{3}$ and neutralises it. If only sodium hydrogen carbonate (baking soda) is used in making cake, then sodium carbonate formed from it by the action of heat (during baking) will give a bitter taste to cake.
(b) By adding tartaric acid to baking soda we can form baking powder.
(c) Tartaric acid neutralises the sodium carbonate formed during decomposition of $NaHCO_{3}$ hence, making the cake tasty and not bitter in taste.

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:
X is calcium carbonate and the gas evolved is carbon dioxide, when calcium carbonate reacts with acid
$CaCO_{3}+Dil.2HCl\rightarrow CaCl_{2}+H_{2}O+CO_{2}$
Solution Y is lime water $Ca\left ( OH \right )_{2}$ because, when $CO_{2}$ is passed through it, it gives the carbonate back as shown by the given equation.
$Ca\left ( OH \right )_{2}+CO_{2}\rightarrow CaCO_{3}+H_{2}O$
The gas evolved at anode during electrolysis of brine is chlorine (G)
Electrolysis $2NaCl+2H_{2}O\rightarrow 2NaOH+Cl_{2}(g)+H_{2}(g)$
When chlorine gas is passed through dry $Ca(OH)_{2}(Y),$ it produces bleaching powder (Z), used for disinfecting drinking water.
$Ca(OH)_{2}+Cl_{2}\rightarrow CaOCl_{2}+H_{2}O$
Hence, Z is calcium oxy-chloride $\left ( CaOCl_{2} \right )$ or bleaching powder.

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:
Sodium hydroxide $\left ( NaOH \right )$ is commonly used base and is hygroscopic (silica gel used in packing is also hygroscopic which protects things from moisture), that is, it absorbs
moisture from the atmosphere and becomes sticky.
Thus, base B is $NaOH$ (sodium hydroxide) It is also a by-product of chlor-alkali process.
The acidic oxide reacts with base to give salt and water.
If $CO_{2}$ is the acidic oxide taken, then the following reaction takes place with ‘B'.

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 belongs to group 2. Calcium sulphate is a white soft substance.
The sulphate salt which is used for making different shapes is plaster of Paris. Its chemical name is calcium sulphate hemihydrate $\left ( CaSO_{4}.\frac{1}{2}H_{2}O \right )$ . The two formula units of $CaSO_{4}$ share one molecule of water. As a result, it is soft. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum which is a hard solid mass.
$CaSO_{4}.\frac{1}{2}H_{2}O+1\frac{1}{2}H_{2}O\rightarrow CaSO_{4}.2H_{2}O$
plaster of paris Gypsum
Plaster of Paris shows such behavior because of presence half water molecule as water of crystallization. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard solid mass.
So, gypsum sets as a hard solid mass and cannot be used for moulding purposes.

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:

Compound X is $NaOH$ (sodium hydroxide).
$2NaOH+Zn\rightarrow Na_{2}ZnO_{2}+H_{2}(g)$
$NaOH+HCl\rightarrow NaCl+H_{2}O$
$NaOH+CH_{3}COOH\rightarrow CH_{3}COONa+H_{2}O$
X is sodium hydroxide, A is sodium zincate, B is sodium chloride and C is sodium acetate.