2. Acids, Base and Salts

Identification Through Indicators

Reaction of an Acid or a Base with Metals

Reaction of an Acid with a Metal carbonate and Hydrogen Carbonate

Reaction Between an Acid and a Base

Reaction Between Acids and Metal Oxides

Reaction of a Non-Metallic Oxide with Base

Similarities between Acids and Bases

pH of Acids and Bases

Importance of pH in Everyday Life

pH of Salts

Chemicals from Common Salts

Hydrating and Drying of Salts

Lesson: Acids, Bases, and Salts

Question:1

A solution turns red litmus blue; its pH is likely to be:

(a) 1

(b) 4

(d) 10

Solution:

(d)

Question:2

A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains:

(a) NaCl

(b) HCl

(d) KCl

Solution:

(b)

Question: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

(d) 16 mL

Solution:

(d)

Question:4

Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(d) Antiseptic

Solution:

(c)

Question: 5

Write word equations and then balanced equations for the reaction taking place when $-$

(a) Dilute sulphuric acid reacts with zinc granules.

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

(d) Dilute hydrochloric acid reacts with iron filings.

Solution:

a) $S u l p h u r i c a c i d + Z i n c \to Z i n c s u l p h a t e + H y d r o g e n$

(b) $H y d r o c h l o r i c a c i d + M a g n e s i u m \to M a g n e s i u m c h l o r i d e + H y d r o g e n$

(d) $H y d r o c h l o r i c a c i d + I r o n \to F e r r i c c h l o r i d e + H y d r o g e n$

Question: 6

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

Solution:

· Take two nails fitted on a cork and kept in a 100-mL beaker.

· Connect the nails to two terminals of a 6-volt battery through a bulb and a switch.

· Pour dilute HCl in the beaker such that the nails dip into it.

· Switch on the current.

· Repeat the same experiment with glucose solution and alcohol solution.

Observations:

It is observed that the bulb glows in the HCl solution and does not glow in the glucose and alcohol solution.

Result

HCl dissociates into $H^{+} a n d C l^{-}$ ions. These ions conduct electricity in the solution resulting in glowing of the bulb.

Alcohol and glucose solutions do not dissociate into ions and therefore they do not conduct electricity.

That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

Question: 7

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

Solution:

Distilled water is neither acidic nor basic in nature. So it does not dissociate into ions. Therefore, it does not conduct electricity.

Rainwater becomes acidic due to its reaction with acidic gases. Like any other acid, rain water conducts electricity due to the formation of ions.

Question: 8

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

Solution:

An acid ionises on dissolving in water to produce hydrogen ions. It is the presence of these ions that makes it behave like an acid.

An acid will not show acidic behaviour in the absence of water.

Question: 9

Five solutions A, B, C, D and E when tested with universal indicator showed pH as:

4, 1, 11, 7 and 9, respectively. Which solution is:

(a) Neutral?

(b) Strongly alkaline?

(d) Weakly acidic?

(e) Weakly alkaline?

Arrange the pH in increasing order of hydrogen-ion concentration.

Solution:

(a) $N e u t r a l \to S o l u t i o n D w i t h p H 7$

(b) $S t r o n g l y a l k a l i n e \to S o l u t i o n C w i t h p H 11$

(d) $W e a k l y a c i d i c \to S o l u t i o n A w i t h p H 4$

(e) $W e a k l y a l k a l i n e \to S o l u t i o n E w i t h p H 9$

The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1.

Question: 10

Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid ( $H C l$ ) is added to test tube A, while acetic acid ( $C H_{3} C O O H$ ) 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?

Solution:

$H C l$ is a stronger acid than $C H_{3} C O O H$ . It produces hydrogen gas at a faster speed due to which the fizzing occurs. Therefore, the fizzing will occur vigorously in test tube A, in which hydrochloric acid ( $H C l$ ) is added.

Question: 11

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

Solution:

The pH of milk is 6. As it turns into curd, the pH will reduce because curd is more acidic in nature than milk.

Question: 12

A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Solution:

(a) In an alkaline condition, milk does not set as curd easily. The milkman shifts the pH of the fresh milk from 6 to slightly alkaline to keep it fresh for a longer time.

(b) When baking soda is added, milk becomes more basic than usual. Hence, acids produced to set the milk as curd are neutralized by the base added to milk. Therefore, it takes a longer time for the milk to set as curd.

Question: 13

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

Solution:

Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster of Paris absorbs water (moisture) to form a hard solid called gypsum.

$\begin{array}{l} {CaSO}_{4} . \frac{1}{2} H_{2} O+1 \frac{1}{2} H_{2} O \to {CaSO}_{4} {.2H}_{2} O \\ (POP) Gypsum (Hard Solid) \end{array}$

Question: 14

What is a neutralisation reaction? Give two examples.

Solution:

A reaction in which an acid reacts with a base to give a salt and water is called neutralisation reaction. Given below are two examples of neutralisation reaction:

1. $N a O H + H C l \to N a C l + H_{2} O$

2. $M g {(O H)}_{2} + 2 H C l \to M g C l_{2} + 2 H_{2} O$

Question: 15

Give two important uses of washing soda and baking soda.

Solution:

Washing soda is used:

(a) In glass, soap, and paper industries.

(b) To remove permanent hardness of water.
Baking soda is used:

(a) In the food industries, mainly in the bakeries, to make the bread or cake fluffy.

(b) In soda-acid fire extinguishers.

Lesson: 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)

(d) (i) and (iv)

Solution:

(d)

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

(d) Hydrochloric acid

Solution:

(d)

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

(d) Absorb $C l^{-}$ ions from the evolved gas

Solution:

(c)

Question: 4

Which of the following salts does not contain water of crystallisation?

(a) Blue vitriol

(b) Baking soda

(d) Gypsum

Solution:

(b)

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

(d) Weak acid and strong base

Solution:

(d)

Question:6

Calcium phosphate is present in tooth enamel. Its nature is

(a) Basic

(b) Acidic

(d) Amphoteric

Solution:

(a)

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

(d) An antacid

Solution:

(d)

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

(d) Hydrochloric acid < Water < Acetic acid

Solution:

(a)

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 hydrogen carbonate.

(d) Neutralise the acid with a strong alkali.

Solution:

(b)

Question: 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)

(d) (i) and (iv)

Solution:

(b)

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)

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

Solution:

(c)

Question: 12

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

(a) Hydrochloric acid

(b) Tartaric acid

(d) Sulphuric acid

Solution:

(b)

Question: 13

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

(a) Acidic

(b) Neutral

(d) Corrosive

Solution:

(c)

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)

(d) (ii) and (iv)

Solution:

(d)

Question: 15

The pH of the gastric juices released during digestion is

(a) Less than 7

(b) More than 7

(d) Equal to 0

Solution:

(a)

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)

(d) (ii) and (iv)

Solution:

(b)

Question: 17

Which one of the following can be used as an acid $-$ base indicator by a visually impaired student?

(a) Litmus

(b) Turmeric

(d) Petunia leaves

Solution:

(c)

Question: 18

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

(a) Marble

(b) Limestone

(d) Lime

Solution:

(d)

Question: 19

Which of the following is acidic in nature?

(a) Lime juice

(b) Human blood

(d) Antacid

Solution:

(a)

Question: 20

In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Figure) 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 $N a O H$ 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)

Solution:

(c)

Question: 21

Which of the following is used for dissolution of gold?

(a) Hydrochloric acid

(b) Sulphuric acid

(d) Aqua regia

Solution:

(d)

Question: 22

Which of the following is not a mineral acid?

(a) Hydrochloric acid

(b) Citric acid

(d) Nitric acid

Solution:

(b)

Question: 23

Which among the following is not a base?

(a) $N a O H$

(b) $K O H$

(d) $C_{2} H_{5} O H$

Solution:

(d)

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

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

Solution:

(b)

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 $C l_{2}$
(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)

(d) A $—$ (ii), B $—$ (iv), C $—$ (i), D $—$ (iii)

Solution:

(c)

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)

(a) Red

(b) Yellow

(d) Blue

Solution:

(c)

Question: 27

Which of the following is(are) true when $H C l (g)$ 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

(d) (iii) and (iv)

Solution:

(c)

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.

(d) Bitter and change blue litmus to red.

Solution:

(c)

Question: 29

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

a) $H_{3} O^{+} + C l^{-}$

b) $H_{3} O^{+} + O H^{-}$

c) $C l^{-} + O H^{-}$

d) $U n i o n i s e d H C l$

Solution:

(a)

Question: 30

Identify the correct representation of reaction occurring during chloralkali process.

a) $2 N a C l (l) + 2 H_{2} O (l) \to 2 N a O H (l) + C l_{2} (g) + H_{2} (g)$

b) $2 N a C l (a q) + 2 H_{2} O (a q) \to 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$

c) $2 N a C l (a q) + 2 H_{2} O (l) \to 2 N a O H (a q) + C l_{2} (a q) + H_{2} (a q)$

d) $2 N a C l (a q) + 2 H_{2} O (l) \to 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)$

Solution:

(d)

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

Solution:

(a) $—$ (iv)

(b) $—$ (iii)

(d) $—$ (i)

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) $C a {(O H)}_{2}$
b) Gypsum	ii) $C a S O_{4} . \frac{1}{2} H_{2} O$
c) Bleaching Powder	iii)
d) Slaked Lime	iv) $C a O C l_{2}$

Solution: $C a S O_{4} .2 H_{2} O$

(a) $—$ (ii)

(b) $—$ (iii)

(d) $—$ (i)

Question: 33

What will be the action of the following substances on litmus paper? Dry gas, Moistened $N H_{3}$ gas, Lemon juice, carbonated soft drink, Curd, Soap solution. $H C l$

Solution:

Substance	Action on Litmus paper
Dry $H C l$ gas	No change
Moistened $N H_{3}$ gas	Turns red to blue
Lemon juice	Turns blue to red
Carbonated soft drink	Turns blue to red
Curd	Turns blue to red
Soap solution	Turns red to 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.

Solution:

The acid present in ant sting is methanoic acid (formic acid) and its chemical formula is $H C O O H$ .

The ant’s sting can be neutralised by rubbing a mild basic like baking soda $(N a H C O_{3})$ on the affected area.

Question: 35

What happens when nitric acid is added to egg shell?

Solution:

When nitric acid is added to egg shell, it reacts with calcium carbonate present in the egg shell to give carbon dioxide, calcium nitrate, and water. The reaction can be given as:

$C a C O_{3} + 2 H N O_{3} \to C a {(N O_{3})}_{2} + H_{2} O + C O_{2}$

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?

Solution:

The student can distinguish between the two solutions by using chemical indicator like phenolphthalein or natural indicators like turmeric, china rose etc.

	In acidic solution	In basic solution
Phenolphthalein	Colourless	Fuchsia
Turmeric	Yellow	Bright red
China rose	Dark pink	Green

Question: 37

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

Solution:

The chemical formula of baking powder is sodium hydrogen carbonate $(N a H C O_{3})$ and that of washing soda is sodium carbonate $(N a_{2} C O_{3} .10 H_{2} O)$ .

On heating, sodium hydrogen carbonate produces $C O_{2}$ gas. $C O_{2}$ turns lime water milky.

$2 N a H C O_{3} \overset{H e a t}{\to} N a_{2} C O_{3} + H_{2} O + C O_{2}$

On heating sodium carbonate, the following reaction takes place.

$N a_{2} C O_{3} .10 H_{2} O \overset{H e a t}{\to} N a_{2} C O_{3} + 10 H_{2} O$

In this case, no such gas is obtained from sodium carbonate.

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.

Solution:

Salt A is baking powder $(N a H C O_{3})$ or sodium hydrogen carbonate. It is commonly used in bakery products.

Salt B, which is used for removal of hardness in water, is sodium carbonate.

When sodium hydrogen carbonate is heated, sodium carbonate $(N a_{2} C O_{3})$ is formed. In the process, $C O_{2}$ gas is released. When $C O_{2}$ gas is passed through lime water, it turns the lime water milky.

The balanced equation for this heating is

$2 N a H C O_{3} \overset{H e a t}{\to} N a_{2} C O_{3} + H_{2} O + C O_{2}$

Therefore,

A is $N a H C O_{3}$

B is ${Na}_{2} {CO}_{3}$

C is ${CO}_{2}$ gas.

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.

Solution:

Sodium chloride is used in the manufacture of sodium hydroxide. In this process, chlorine and hydrogen gases are formed as by-products along with sodium hydroxide.

When chlorine gas reacts with lime water, it forms calcium oxychloride, which is used as a bleaching agent in chemical industries.

$\begin{array}{l} 2 N a C l (a q) + 2 H_{2} O (l) \to 2 N a O H (a q) + C l_{2} (g) + H_{2} (g) \\ C a {(O H)}_{2} (s) + C l_{2} (g) \to C a O C l_{2} (s) + H_{2} O \end{array}$

Therefore,

X is chlorine gas

Y is calcium oxychloride (bleaching powder)

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	$N H_{4} C l$	$N H_{4} O H$	$—$
(ii)Copper sulphate	$—$	$—$	$H_{2} S O_{4}$
(iii)Sodium chloride	$N a C l$	$N a O H$	$—$
(iv)Magnesium nitrate	$M g {(N O_{3})}_{2}$	$—$	$H N O_{3}$
(v)Potassium sulphate	$K_{2} S O_{4}$	$—$	$—$
(vi)Calcium nitrate	$C a {(N O_{3})}_{2}$	$C a {(O H)}_{2}$	$—$

Solution:

Name of the salt	Formula	Salt obtained from
Name of the salt	Formula	Base	Acid
(i) Ammonium chloride	$N H_{4} C l$	$N H_{4} O H$	$H C l$
(ii) Copper sulphate	$C u S O_{4}$	$C u {(O H)}_{2}$	$H_{2} S O_{4}$
(iii) Sodium chloride	$N a C l$	$N a O H$	$H C l$
(iv) Magnesium nitrate	$M g {(N O_{3})}_{2}$	$M g {(O H)}_{2}$	$H N O_{3}$
(v) Potassium sulphate	$K_{2} S O_{4}$	$K O H$	$H_{2} S O_{4}$
(vi) Calcium nitrate	$C a {(N O_{3})}_{2}$	$C a {(O H)}_{2}$	$H N O_{3}$

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.

Solution:

Strong acids:

In aqueous solutions, strong acids ionise completely and provide hydronium ions.

Examples: Hydrochloric acid, sulphuric acid, nitric acid.

Weak acids:

Weak acids do not ionize completely in aqueous solutions.

Examples: Citric acid, acetic acid, formic 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.

Solution:

When zinc metal is treated with a dilute solution of a strong acid, it forms salt and hydrogen gas. The hydrogen gas is utilized in the hydrogenation of oil. Therefore, the evolved gas is hydrogen.

$Z n + 2 H C l \to Z n C l_{2} + H_{2}$

Test to detect the hydrogen gas: When a burning candle is brought near the mouth of the test tube containing hydrogen gas, it burns with a pop sound which confirms the presence of hydrogen gas.

Question: 43

In the following schematic diagram for the preparation of hydrogen gas as shown in the given below Figure, 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

(d) Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.

Solution:

a) If zinc dust is used, hydrogen gas will evolve faster than in case of zinc granules.

b) Almost the same amount of hydrogen gas is evolved when dilute hydrochloric acid is taken instead of dilute sulphuric acid.

$Z n + 2 H C l \to Z n C l_{2} + H_{2} O$

c) Copper does not react with dilute acids in normal condition because copper is less reactive. Thus, if copper turnings are taken in place of zinc, no reaction will take place.

d) If sodium hydroxide is taken in place of sulphuric acid, sodium zincate will be formed and hydrogen gas will be evolved.

$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$

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?

Solution:

(a) The chemical name of baking soda is sodium bicarbonate or sodium hydrogen carbonate. On heating, it converts into sodium carbonate which is bitter in taste.

$2 N a H C O_{3} \overset{H e a t}{\to} N a_{2} C O_{3} + H_{2} O + C O_{2}$

Thus, if baking soda is used, the cake will taste slightly bitter.

(b) Baking soda can be converted into baking powder by adding tartaric acid to it.

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.

Solution:

On reacting with hydrochloric acid, calcium carbonate gives carbon dioxide gas.

$C a C O_{3} + 2 H C l \to C a C l_{2} + C O_{2} + H_{2} O$

When carbon dioxide, so formed, is passed through lime water, the lime water

turns milky because of the formation of calcium carbonate.

$C a {(O H)}_{2} + C O_{2} \to C a C O_{3} + H_{2} O$

The gas evolved at the anode during electrolysis of brine is chlorine.

When passed through dry calcium hydroxide, chlorine gas produces bleaching powder. This is used for disinfecting drinking water.

$C a {(O H)}_{2} + C l_{2} \to C a O C l_{2} + H_{2} O$

Therefore,

X is calcium carbonate

Y is lime water

G is chlorine gas

Y is dry calcium hydroxide

Z is 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.

Solution:

Sodium hydroxide $(N a O H)$ is a commonly used base. It absorbs moisture from the atmosphere and becomes sticky.

The acidic oxides react with base to give salt and water.

The reaction between $N a O H$ and $C O_{2}$ can be given as:

$2 N a O H + C O_{2} \to N a_{2} C O_{3} + H_{2} O$

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.

Solution:

The sulphate salt discussed here is Plaster of Paris $(C a S O_{4} . \frac{1}{2} H_{2} O) .$

The two formula units of $C a S O_{4}$ share one molecule of water. As a result, it is soft.

When it is left for some time in the open, it absorbs moisture from the atmosphere and forms a hard, solid mass of gypsum.

Reaction involved:

$C a S O_{4} . \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to C a S O_{4} .2 H_{2} O$

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.

Solution:

$\begin{array}{l} 2 N a O H + Z n \to N a_{2} Z n O_{2} + H_{2} \\ N a O H + H C l \to N a C l + H_{2} O \\ N a O H + C H_{3} C O O H \to C H_{3} C O O N a + H_{2} O \end{array}$

Therefore,

X is $N a O H$ (Sodium hydroxide)

A is $N a_{2} Z n O_{2}$ (Sodium zincate)

B is $N a C l$ (Sodium chloride)

C is $C H_{3} C O O N a$ (Sodium acetate).

Lesson: Acids, Bases, and Salts

Question:1

You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Solution:

The content of the tube will be an acid if the colour of red litmus does not change when dipped into it.

It is a base if the colour of red litmus changes to blue.

If there is a slight change in the colour of red litmus (purple), then it is distilled water.

Question:2

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

Solution:

Curd and other sour substances contain acids. Therefore, when they are kept in brass or copper vessels, the metal reacts with the acid. This results in release hydrogen gas and other harmful products. This spoils the food.

Question: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?

Solution:

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

Take few pieces of zinc granules in a test tube.

Add 5 ml of dilute $H_{2} S O_{4}$ .

Shake it and pass the gas produced through a soap solution.

Bubbles are formed in the soap solution are formed. These soap bubbles contain hydrogen gas.

$H_{2} S O_{4} + Z n \to Z n S O_{4} + H_{2} ↑$

Now test if the evolved gas is hydrogen or not.

When a candle is brought near the soap bubbles the gas burns with a pop sound.

This proves that the evolved gas is hydrogen.

Question:4

Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Solution:

The metal compound formed due to the reaction is calcium chloride. This means that the metal compound has calcium metal. The gas that extinguishes a burning candle is carbon dioxide. This means the metal compound is calcium carbonate.

$C a C O_{3} (s) + 2 H C l (a q) \to C a C l_{2} (a q) + C O_{2} (g) + H_{2} O (l)$

Question:5

Why do $H C l$ , $H N O_{3}$ , etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Solution:

When $H C l$ or $H N O_{3}$ are mixed with water, they dissolve in water to form hydrogen ion $(H^{+})$ or hydronium ion $(H_{3} O^{+})$ . Due to the presence of these ions, these compounds show their acidic character.

When alcohol and glucose are mixed with water, they do form hydrogen or hydronium ions. Hence, they do not show acidic character.

Question: 6

Why does an aqueous solution of acid conduct electricity?

Solution:

Acids ionises in aqueous solutions.These ions are responsible for conducting electricity.

Question: 7

Why does dry $H C l$ gas not change the colour of the dry litmus paper?

Solution:

Dry $H C l$ does not show it acidic property unless it is dissolved in water. When dissolved in water, they form $H^{+}$ or $H_{3} O^{+}$ ions. Due to these ions, these compounds show their acidic character. This ionization does not happen in case of dry $H C l$ gas and hence dry $H C l$ gas does not change the colour of the dry litmus paper.

Question: 8

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

Solution:

The reaction of acid and water is exothermic in nature. If water is added to an acid, the heat generated can cause burns. To slow down the generation of heat, it is always recommended that acid should be added to water.

Question: 9

How is the concentration of hydronium ions ( $H_{3} O^{+}$ ) affected when a solution of an acid is diluted?

Solution:

When water is added to a solution of acid, the solution becomes diluted.

The concentration of hydronium ions ( $H_{3} O^{+}$ ) decreases when a solution of an acid is diluted. This means that the strength of the acid decreases.

Question: 10

How is the concentration of hydroxide ions ( $O H^{-}$ ) affected when excess base is dissolved in a solution of sodium hydroxide?

Solution:

Sodium hydroxide is basic in nature. When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions ( $O H^{-}$ ) would increase.

Question: 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?

Solution:

An acid has a pH value less than 7 while a base has a pH value more than 7.

The solution with $p H = 6$ is acidic and has more hydrogen ion concentration than the basic solution.

Question: 12

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

Solution:

Concentration of $H^{+}$ (aq) can have varied effects on the nature of the solution. If the concentration of $H^{+}$ ions increases, the solution becomes more acidic. If the concentration of $H^{+}$ ions decreases, the solution becomes more basic in nature.

Question: 13

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

Solution:

Yes, basic solutions also have $H^{+}$ (aq) ions. They are basic despite having $H^{+}$ (aq) solution. This is due to the presence of less concentration of $H^{+}$ (aq) compared to the concentration of $O H^{-}$ ions in the solutions.

Question: 14

Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Solution:

A plant grows the best when the soil is neither acidic not basic. The farmer would treat the soil with quick lime or slaked lime or chalk when the soil is acidic in nature. Quick lime or slaked lime or chalk are basic in nature and neutralizes the acidic nature of the soil when they are added to it.

Question:15

What is the common name of the compound $C a O C l_{2}$ ?

Solution:

The common name of the compound $C a O C l_{2}$ is bleaching powder.

Question:16

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

Solution:

Calcium hydroxide $[C a {(O H)}_{2}]$

Question: 17

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

Solution:

Washing soda $(N a_{2} C O_{3} .10 H_{2} O)$

Question: 18

What will happen if a solution of sodium hydro carbonate is heated?

Give the equation of the reaction involved.

Solution:

When a solution of sodium hydro carbonate is heated, sodium carbonate and water are formed. Carbon dioxide too is released in this process.

$2 N a H C O_{3} \overset{H e a t}{\to} N a_{2} C O_{3} + H_{2} O + C O_{2}$

Question:19

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

Solution:

$\begin{array}{l} {CaSO}_{4} . \frac{1}{2} H_{2} O+1 \frac{1}{2} H_{2} O \to {CaSO}_{4} {.2H}_{2} O \\ POP Water Gypsum \end{array}$

Questions

Q1	Name 2 natural indicators and 2 synthetic indicators used to test for acids and bases.

Q2	What is an olfactory indicator?

Q3	Why is baking soda suggested as a remedy to a person suffering from acidity?

Q4	What is an alkali?

Q5	What does a universal indicator do?

Q6	What happens when electricity is passed through brine (a concentrated solution of sodium chloride)?

Q7	When is an acid or a base said to be diluted?

Q8	Why should water not be added to acid for dilution of the acid?

Q9	Why is the aqueous solution of hydrogen chloride considered to be a very strong acid?

Q10	How does baking powder differ from baking soda? Give 2 uses of baking soda.

Q11

When provided with three beakers containing three different colourless liquids and two strips of litmus papers (one red and another blue), how would you identify the nature of these liquids? It is disclosed that one of the liquids is an acidic solution, another a basic solution and the third one is distilled water.

Q12	Tooth decay is caused by the presence of acid. Explain.

Q13	Why are hydrated salts not wet?

Q14	What is salt? Name the following: i) Two non-hydrated crystalline salts ii) Two neutral salts iii) Two basic salts iv) Two acid salts

Q15	What happens when: a) The crystals of washing soda are exposed to air? b) Red litmus is introduced into a solution of sodium sulphate? c) Bleaching powder is treated with HCl? d) Bottle full of concentrated sulphuric acid is left open in the air by accident? e) Blue vitriol is heated?

Q16	Describe an activity with a labelled diagram to show how hydrogen gas produced can be tested.

Q17	Why is aqueous solution of sodium carbonate basic in nature and aqueous solution of ammonium sulphate acidic in nature?

Q18	Which characteristic of acids establishes that even though compounds like glucose and alcohol contain hydrogen, still they cannot be regarded as acids?

Q19	Explain the preparation of washing soda.

Q20

Depending upon their strength, how are acids classified? Explain with examples.

Questions

Q1	Name 2 natural indicators and 2 synthetic indicators used to test for acids and bases.

A1	Turmeric and litmus are 2 natural indicators while methyl orange and phenolphthalein are 2 synthetic indicators used to test for acids and bases.

Q2	What is an olfactory indicator?

A2	These are the substances whose smell changes in an acidic or basic solution. Examples: Vanilla extract, onion extract.

Q3	Why is baking soda suggested as a remedy to a person suffering from acidity?

A3

Production of excess hydrochloric acid in stomach causes a burning sensation and triggers discomfort and pain. This problem is commonly known as acidity.

Baking soda or sodium bicarbonate (NaHCO₃), which is a base, when consumed during this period of unease, neutralises the excess HCl to heal the indigestion.

Q4	What is an alkali?

A4	An alkali is a base which is soluble in water.

Q5	What does a universal indicator do?

A5	A universal indicator displays different colours for unlike concentration of hydronium or hydroxide ions in a solution and tells us how strong an acid or a base is.

Q6	What happens when electricity is passed through brine (a concentrated solution of sodium chloride)?

A6	When electricity is passed through brine, which is a concentrated solution of sodium chloride, the solution decomposes to form sodium hydroxide, chlorine and hydrogen.

Q7	When is an acid or a base said to be diluted?

A7	When a solution of an acid or base is diluted, the concentration of hydronium/hydroxide ions (H₃O⁺/OH^-) per unit volume decreases. Such a process is called dilution and the acid or base is said to be diluted.

Q8	Why should water not be added to acid for dilution of the acid?

A8

Dilution of acid is an exothermic reaction, that is, it is a chemical reaction that releases heat energy. So when acid is added to water, the evolved heat at once gets absorbed in the water. But instead, if water is added to acid, the heat evolved would convert water into water vapour causing the acid to splash out causing severe burns to the person carrying out the experiment.

Q9	Why is the aqueous solution of hydrogen chloride considered to be a very strong acid?

A9	When hydrogen chloride is dissolved in water, it separates to give hydrogen ions and chloride ions in the solution and so there is an increase in the concentration of hydrogen ions (H⁺) thereby making hydrochloric acid a very strong acid.

Q10	How does baking powder differ from baking soda? Give 2 uses of baking soda.

A10

Baking soda is a compound of sodium hydrogen carbonate which is alkaline (basic) in nature. Baking powder on the other hand is a mixture of sodium hydrogen carbonate and a mild edible tartaric acid. On mixing with water the sodium hydrogen carbonate reacts with the mild tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly thereby making the volume of the cake increase and hence come out of the oven fluffy.

Uses of baking soda:

a) Baking soda is used as baking powder to make breads and cakes soft and spongy.

b) It is used in soda-acid type fire extinguishers where the air supply is made to cut off to stop the burning process.

Q11

A11

By dipping the litmus papers into the three solutions, the nature of the liquids can be identified. The beaker containing the basic solution will turn the red litmus paper blue and will not have any effect on the blue litmus paper. On the other hand, the acidic solution will turn the blue litmus paper red and will not have any effect on the red litmus paper. The third beaker containing distilled water will have no effect either on the blue litmus or on the red litmus paper because distilled water is neutral.

Q12	Tooth decay is caused by the presence of acid. Explain.

A12

At the end of each meal, the bacteria existing in the mouth acts upon the sugar present in the food and turn it into acid. This acid lowers the pH of the mouth. Tooth starts decaying when the pH value of the mouth starts falling below 5.5. The enamel of the tooth is made up of calcium phosphate. The strong acid attacks the enamel and corrodes it.

Q13	Why are hydrated salts not wet?

A13

Hydrated salts are the ones which has water of crystallisation in them. Water of crystallization is the specific number of water molecules that go into loose chemical combination when the salt crystallizes out from its saturated solution. For example, in the blue copper sulphate crystal, there are 5 molecules of water attached to it (CuSO₄.5H₂O) and the blue colour is due to the presence of these water molecules in the structure. Water of crystallisation is an integral part of the crystal structure of salts. It is not free or there is no additional water outside the crystal. So these salts are in fact completely dry.

Q14	What is salt? Name the following: i) Two non-hydrated crystalline salts ii) Two neutral salts iii) Two basic salts iv) Two acid salts

A14

A salt is an ionic compound formed as a product when:

• An acid reacts with a base (neutralisation reaction).

• An acid reacts with a metal and gives hydrogen gas.

Salts can be acidic, basic or neutral.

Some of the examples of salts are:

i) Two non-hydrated crystalline salts: sodium chloride (NaCl) and potassium nitrate (KNO₃)

ii) Two neutral salts are: sodium chloride (NaCl) and sodium sulphate (Na₂SO₄)

iii) Two basic salts are: calcium carbonate (CaCO₃) and potassium phosphate (K₃PO₄)

iv) Two acid salts are: sodium hydrogen carbonate (NaHCO₃) and sodium phosphate (Na₃PO₄)

Q15	What happens when: a) The crystals of washing soda are exposed to air? b) Red litmus is introduced into a solution of sodium sulphate? c) Bleaching powder is treated with HCl? d) Bottle full of concentrated sulphuric acid is left open in the air by accident? e) Blue vitriol is heated?

A15

a) The crystals of washing soda undergo efflorescence, that is, the water of crystallisation is partially or completely lost from the hydrated salt when exposed to the atmosphere, thereby turning the salt crystals powdery due to the formation of sodium carbonate monohydrate.

Na₂CO₃.10H₂O ------^air------> Na₂CO₂.H₂O + 9H₂O

b) Solution of sodium sulphate (Na₂SO₄), being neutral, shows no such observation of colour change of red litmus paper when it is introduced into this solution.

c) When bleaching powder is treated with dilute hydrochloric acid, all the chlorine present in it is liberated as shown in the following equation:

CaOCl₂ + 2HCl ---------> CaCl₂ + Cl₂ + H₂O

d) When a bottle full of concentrated sulphuric acid is left open in the air by accident, it is observed that the acid starts flowing out of the bottle on its own.

This happens because concentrated sulphuric acid, being highly hygroscopic, absorbs moisture from the air and increases in volume. Due to the volume increase, the acid overflows out of the bottle.

e) Blue vitriol (CuSO₄.5H₂O) on heating loses water of crystallisation and changes to anhydrous copper sulphate (CuSO₄) which is white in colour due to water loss.

Q16	Describe an activity with a labelled diagram to show how hydrogen gas produced can be tested.

A16

Apparatus is set as above.

A small quantity of dil. sulphuric acid (H₂SO₄) is taken in a test tube.

A small quantity of zinc granules are added to it.

Small bubbles appear to pass through the acid solution and then through the delivery tube, bubbling through the soap solution.

When a burning candle is brought near the bursting bubbles, the candle burns with a pop sound. As burning with a pop sound is the characteristic test for hydrogen gas, it confirms the evolution of hydrogen gas. The reaction is:

Zn + H₂SO₄ → ZnSO₄ + H₂↑

Q17	Why is aqueous solution of sodium carbonate basic in nature and aqueous solution of ammonium sulphate acidic in nature?

A17

Salt of a weak acid and a strong base yields a basic (alkaline) solution. Sodium carbonate (Na₂CO₃) is the salt of a weak carbonic acid (H₂CO₃) and a strong base of sodium hydroxide (NaOH). So an aqueous solution of sodium carbonate will be basic (alkaline) in nature.

Na₂CO₃ + 2H₂O ⇄ 2NaOH + H₂CO₃

The fact that sodium carbonate is basic in nature can be justified on the basis of hydrolysis of salt, which means splitting up of a compound by the action of water. Thus aqueous solution of sodium carbonate has pH more than 7.

Salt of a strong acid and a weak base yields an acidic solution. Ammonium sulphate (NH₄)₂SO₄ is the salt of a strong sulphuric acid (H₂SO₄) and a weak ammonium hydroxide (NH₄OH) base. So an aqueous solution of ammonium sulphate is acidic in nature. Ammonium sulphate, in its aqueous solution gets hydrolysed (to split up a compound by the action of water) to some extent, to form ammonium hydroxide (NH₄OH) and sulphuric acid (H₂SO₄).

(NH₄)₂SO₄+ H₂O ⇄ NH₄OH + H₂SO₄

Being a strong acid, H₂SO₄ is fully ionized to give large amount of hydrogen ions (H⁺).Since ammonium sulphate solution contains more of hydrogen ions, it reveals its acidic nature of having pH less than 7, by turning blue litmus red.

Q18	Which characteristic of acids establishes that even though compounds like glucose and alcohol contain hydrogen, still they cannot be regarded as acids?

A18

Just as all acids contain hydrogen, even compounds like glucose and alcohol contain hydrogen, yet these cannot be categorized as acids. This can be demonstrated by testing the solution of these two substances with both red and blue litmus papers. Neither glucose nor alcohol would show colour change. This would prove that they both are neither acidic nor basic but are neutral.

Moreover, it is the nature of acidic compounds to dissociate into hydrogen ions (H⁺) in the aqueous state, that is, when dissolved in water. It is not the case with compounds like glucose and alcohol. This can be confirmed by carrying out the following experiment:

When electricity is passed through a beaker containing dilute hydrochloric acid, it is observed that current flows through the circuit, which can be tested by the glowing of bulb connected to this circuit.

When glucose and alcohol are separately used, turn by turn, to replace the hydrochloric acid solution, under the same experimental set up, it is observed that the bulb which glows in the hydrochloric acid solution does not do so when either of the other two solutions is used.

The reason is that hydrochloric acid solution dissociates into H⁺ and Cl^– ions which help to conduct electricity in the hydrogen chloride solution while glucose and alcohol solutions do not dissociate into ions and so, do not conduct electricity. From here one can come to the conclusion that all acids contain hydrogen but all compounds containing hydrogen are not acids. Due to this reason compounds such as alcohol and glucose, although contain hydrogen yet they are not regarded as acids.

Q19	Explain the preparation of washing soda.

A19

Washing soda, which is the common name for sodium carbonate is manufactured in the following process:

When a cold and concentrated solution of sodium chloride is reacted with ammonia (NH₃) and carbon dioxide (CO₂), both ammonium chloride (NH₄Cl) and sodium hydrogen carbonates (NaHCO₃) are formed and the latter precipitates as a solid as it is slightly soluble in water.

NaCl + CO₂ + NH₃ + H₂O → NH₄Cl + NaHCO₃

The precipitate of sodium hydrogen carbonate formed is separated by filtration and is dried. Next on heating, it decomposes to form sodium carbonate (Na₂CO₃), which is also known as soda ash.

2NaHCO₃ → Na₂CO₃ +CO₂ + H₂0

The soda ash is at first dissolved in water and then recrystallised to get washing soda crystals each containing 10 molecules of water of crystallisation, and thus is known as sodium bicarbonate decahydrate.

Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O

Q20	Depending upon their strength, how are acids classified? Explain with examples.

A20

Depending upon their strength, acids can be classified into strong and weak acids, but this does not mean the same as concentrated or dilute. The strength of an acid depends on the degree of ionisation in water and concentration of hydronium ions (H₃O⁺) produced by that acid in aqueous solution.

(i) Strong acids: Strong acids dissociate to ionise vigorously and completely in aqueous solution, which is, when they are dissolved in water, they produce a high concentration of hydronium ions (H₃O⁺). These strong acids have a pH of 1.

For example, hydrochloric acid (HCl), when dissolved in water, all of the hydrogen chloride molecules become hydrogen ions and chloride ions. Sulphuric acid (H₂SO₄), nitric acid (HNO₃) are also considered to be strong acids.

(ii) Weak acids: Weak acids dissociate to ionise only partially in aqueous solution, thereby producing ions as well as molecules. Weak acids have a pH of 3 to 5. Examples of weak acids include acetic acid, carbonic acid, ethanoic acid, and others.

Acetic acid: CH₃COOH + H₂O → H₃O⁺ + CH₃COO^-(acetate ion)

Carbonic acid: H₂CO₃ + 2H₂O → 2 H₃O⁺ + CO₃^2-(carbonate ion)

Ethanoic acid:CH₃CO₂H + H₂O ⇄ H₃O⁺ + CH₃CO₂^-(acetate ion)

In the above cited examples, some of the acetic, carbonic and ethanoic acid molecules respectively become ions in water but most of them stay as molecules.

Acids, Bases, and Salts F

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Solution: $C a S O_{4} .2 H_{2} O$