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CHAPTER

2

Acids, Bases
and Salts

Y

ou have learnt in your previous classes that the sour and bitter
tastes of food are due to acids and bases, respectively, present in them
...

Recall how we tested sour and bitter substances without tasting
them
...
Litmus is a natural indicator, turmeric is another
such indicator
...
You can also use synthetic indicators such as methyl orange and
phenolphthalein to test for acids and bases
...


Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to
the division Thallophyta, and is commonly used as an indicator
...
There are many other natural
materials like red cabbage leaves, turmeric, coloured petals of some flowers such as
Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a
solution
...


Q
1
...
One of them contains
distilled water and the other two contain an acidic solution and a basic
solution, respectively
...
1 UNDERSTANDING THE CHEMIC AL PROPERTIES OF
UNDERSTANDING
CHEMICAL
ACIDS AND BASES
2
...
1 Acids and Bases in the Laboratory
Activity 2
...

Put a drop of each of the above solutions on a watch-glass and
test with a drop of the following indicators as shown in Table 2
...

What change in colour did you observe with red litmus, blue litmus,
phenolphthalein and methyl orange solutions for each of the
solutions taken?
Tabulate your observations in Table 2
...


Table 2
...
There are some substances whose odour changes in
acidic or basic media
...
Let us try
out some of these indicators
...
2
Take some finely chopped onions in a plastic bag along with some
strips of clean cloth
...
The cloth strips can now be used to test for acids and
bases
...

Keep them on a clean surface and put a few drops of dilute HCl
solution on one strip and a few drops of dilute NaOH solution on
the other
...

Note your observations
...

Take some dilute HCl solution in one test tube and dilute NaOH
solution in another
...
Check the odour once again and
record changes in odour, if any
...


Which of these – vanilla, onion and clove, can be used as olfactory
indicators on the basis of your observations?
Let us do some more activities to understand the chemical properties
of acids and bases
...
1
...
3
CAUTION: This activity needs the teacher’s assistance
...
2
...

Take about 5 mL of dilute sulphuric acid in a test tube and add a
few pieces of zinc granules to it
...

Why are bubbles formed in the soap solution?
Take a burning candle near a gas filled bubble
...

Are the observations in all the cases the same or different?

Figure 2
...
This is seen as hydrogen gas
...
Thus,
the reaction of a metal with an acid can be summarised as –
Acid + Metal → Salt + Hydrogen gas
Can you now write the equations for the reactions you have observed?

Activity 2
...

Add 2 mL of sodium hydroxide solution and warm the contents
of the test tube
...
3 and record your
observations
...


2NaOH + Zn → Na 2 ZnO2 + H2
( Sodium zincate )
You find again that hydrogen is formed in the reaction
...


2
...
3 How do Metal Carbonates and Metal
Hydrogencarbonates React with Acids?
Activity 2
...
2
Passing carbon dioxide gas
through calcium hydroxide
solution

Take two test tubes, label them as A
and B
...
5 g of sodium carbonate
(Na2CO 3) in test tube A and about
0
...

Add about 2 mL of dilute HCl to both
the test tubes
...
2
...


The reactions occurring in the above Activity are written as –
Test tube A: Na 2 CO3 (s) + 2 HCl(aq) → 2NaCl(aq) + H2 O(l) + CO2 (g)
Test tube B: NaHCO3 (s) + HCl(aq) → NaCl(aq) + H2 O(l) + CO2 (g)
On passing the carbon dioxide gas evolved through lime water,
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2 O (l)
(Lime water)

20

(White precipitate)

Science

On passing excess carbon dioxide the following reaction takes place:
CaCO3 (s) + H2 O(l)+ CO2 (g) → Ca(HCO3 )2 (aq )
(Soluble in water)

Limestone, chalk and marble are different forms of calcium carbonate
...

Thus, the reaction can be summarised as –
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

2
...
4 How do Acids and Bases React with each other?
Activity 2
...

What is the colour of the solution?
Add dilute HCl solution to the above solution drop by drop
...

Does the pink colour of phenolphthalein reappear?
Why do you think this has happened?

In the above Activity, we have observed that the effect of a base is
nullified by an acid and vice-versa
...
In general, a neutralisation reaction
can be written as –
Base + Acid → Salt + Water

2
...
5 Reaction of Metallic Oxides with Acids
Activity 2
...

Note the colour of the solution
...
The blue-green colour of the solution is
due to the formation of copper(II) chloride in the reaction
...
Since
metallic oxides react with acids to give salts and water, similar to the
reaction of a base with an acid, metallic oxides are said to be basic oxides
...
1
...
5
...
Since this is similar to
the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic in nature
...

2
...


U

E

S

T

I

O

N

S

Why should curd and sour substances not be kept in brass and copper
vessels?
Which gas is usually liberated when an acid reacts with a metal?
Illustrate with an example
...
The gas evolved extinguishes a burning candle
...


?

WHAT
HAVE
2
...
1 we have seen that all acids have similar chemical
properties
...
3 that all acids generate hydrogen gas on reacting with metals, so
hydrogen seems to be common to all acids
...


Activity 2
...
3
Acid solution in water
conducts electricity

22

Take solutions of glucose, alcohol,
hydrochloric acid, sulphuric acid, etc
...

Connect the nails to the two terminals of a
6 volt battery through a bulb and a switch, as
shown in Fig
...
3
...

Repeat with dilute sulphuric acid
...
What do you
observe now?
Does the bulb glow in all cases?

Science

The bulb will start glowing in the case of acids, as shown in Fig
...
3
...
Glowing of the bulb indicates that there is a flow of electric
current through the solution
...

Since the cation present in acids is H+, this suggests that acids
produce hydrogen ions, H+(aq), in solution, which are responsible for
their acidic properties
...
What can you conclude from the results of this
Activity?

2
...
1 What Happens to an Acid or a Base in a Water Solution?
Do acids produce ions only in aqueous solution? Let us test this
...
9
Take about 1g solid NaCl in a clean and
dry test tube and set up the apparatus as
shown in Fig
...
4
...

What do you observe? Is there a gas coming
out of the delivery tube?
Test the gas evolved successively with dry
and wet blue litmus paper
...
4 Preparation of HCl gas

Note to teachers: If the climate is very humid, you will have to pass the gas produced
through a guard tube (drying tube) containing calcium chloride to dry the gas
...
The separation of H+ ion from HCl molecules
cannot occur in the absence of water
...
Thus hydrogen ions must always be shown as
H+(aq) or hydronium ion (H3O+)
...
Let us see
what happens when a base is dissolved in water
...
Bases which are soluble
in water are called alkalis
...
An alkali is a base that dissolves in water
...
Never taste or touch them as they may
cause harm
...
1 are alkalis?

Now as we have identified that all acids generate H+(aq) and all
–
bases generate OH (aq), we can view the neutralisation reaction as
follows –
Acid + Base → Salt + Water
H X + M OH → MX + HOH
–

H+(aq) + OH (aq) → H2O(l)
Let us see what is involved when water is mixed with an acid or a base
...
10

Figure 2
...

Add a few drops of concentrated H2SO4 to it and swirl the
beaker slowly
...

Is there a change in temperature?
Is this an exothermic or endothermic process?
Repeat the above Activity with sodium hydroxide pellets
and record your observations
...
Care must be taken while mixing concentrated nitric
acid or sulphuric acid with water
...
If water is added to a concentrated acid,
the heat generated may cause the mixture to splash out and cause burns
...
Look
out for the warning sign (shown in Fig
...
5) on the can of concentrated
sulphuric acid and on the bottle of sodium hydroxide pellets
...
Such a process is
called dilution and the acid or the base is said to be diluted
...


Why do HCl, HNO3, etc
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Why does an aqueous solution of an acid conduct electricity?

3
...


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

5
...


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

?

2
...
We have also learnt in the previous section about
dilution and decrease in concentration of H+ or OH– ions in solutions
...
The universal indicator shows different
colours at different concentrations of hydrogen ions in a solution
...
The p in pH stands for ‘potenz’ in German,
meaning power
...
pH should be thought of simply as a number which
indicates the acidic or basic nature of a solution
...

The pH of a neutral solution is 7
...
As the pH value increases from 7 to 14, it
represents an increase in OH– ion concentration in the solution, that is,
increase in the strength of alkali
...
One such paper is shown
in Fig
...
6
...
6 Variation of pH with the change in concentration of H+(aq) and OH–(aq) ions

Acids, Bases and Salts

25

Table 2
...

No
...
11
Test the pH values
of solutions given in
Table 2
...

Record your observations
...
7 pH of some common substances shown on a pH paper (colours are only a rough guide)

The strength of acids and bases depends on the number of H+ ions
and OH– ions produced, respectively
...
Acids that give rise to more H+ ions
are said to be strong acids, and acids that give less H+ ions are said to be
weak acids
...
3
...
0 to 7
...
Living organisms can
survive only in a narrow range of pH change
...
6, it is called acid rain
...
The survival of aquatic life in such rivers
becomes difficult
...
Do you think life can exist on this planet?

What is the pH of the soil in your backyard?
Plants require a specific pH range for their healthy growth
...
12
...


Activity 2
...

Shake the contents of the test tube
...

Check the pH of this filtrate with the help of universal
indicator paper
...
It helps in the digestion of food without harming the stomach
...
To get rid of this pain, people use bases called
antacids
...
These antacids neutralise the excess acid
...

pH change as the cause of tooth decay
Tooth decay starts when the pH of the mouth is lower than 5
...
Tooth
enamel, made up of calcium phosphate is the hardest substance in the
body
...
5
...
The best way to prevent this is to clean the mouth after eating
food
...

Self defence by animals and plants through chemical warfare
Have you ever been stung by a honey-bee? Bee-sting leaves an acid
which causes pain and irritation
...
Stinging hair of nettle leaves inject
methanoic acid causing burning pain
...
Its leaves have stinging hair,
which cause painful stings when touched accidentally
...
A traditional remedy is rubbing the
area with the leaf of the dock plant, which often grows beside
the nettle in the wild
...
Are you aware
of any other effective traditional remedies for such stings?

Table 2
...


You have two solutions, A and B
...
Which solution has more hydrogen ion concentration?
Which of this is acidic and which one is basic?

2
...


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

4
...
4 MORE ABOUT SALTS
In the previous sections we have seen the formation of salts during
various reactions
...


2
...
1 Family of Salts
Activity 2
...

Potassium sulphate, sodium sulphate, calcium sulphate,
magnesium sulphate, copper sulphate, sodium chloride, sodium
nitrate, sodium carbonate and ammonium chloride
...

Salts having the same positive or negative radicals are said to
belong to a family
...
Similarly, NaCl and KCl belong to the family
of chloride salts
...
4
...
14
Collect the following salt samples – sodium chloride, potassium
nitrate, aluminium chloride, zinc sulphate, copper sulphate,
sodium acetate, sodium carbonate and sodium hydrogencarbonate
(some other salts available can also be taken)
...

Check the action of these solutions on litmus and find the pH
using a pH paper
...

Report your observations in Table 2
...


Salts of a strong acid and a strong base Table 2
...
On the other
Salt
hand, salts of a strong acid and weak base
are acidic with pH value less than 7 and those
of a strong base and weak acid are basic in
nature, with pH value more than 7
...
4
...
This is the salt that you use in food
...

Seawater contains many salts dissolved
in it
...
Deposits of solid salt are also found in
several parts of the world
...
This is
called rock salt
...
Rock salt
is mined like coal
...
Did
you know that sodium chloride was such an important symbol in our
struggle for freedom?

Acids, Bases and Salts

29

Common salt — A raw material for chemicals
The common salt thus obtained is an important raw material for various
materials of daily use, such as sodium hydroxide, baking soda, washing
soda, bleaching powder and many more
...

Sodium hydroxide
When electricity is passed through an aqueous solution of sodium
chloride (called brine), it decomposes to form sodium hydroxide
...

2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
Chlorine gas is given off at the anode, and hydrogen gas at the cathode
...
The three
products produced in this process are all useful
...
8 shows the
different uses of these products
...
8 Important products from the chlor-alkali process

Bleaching powder
You have already come to know that chlorine is produced during the
electrolysis of aqueous sodium chloride (brine)
...
Bleaching powder is produced
by the action of chlorine on dry slaked lime [Ca(OH)2]
...

Ca(OH)2 + Cl2 → CaOCl2 + H2O
30

Science

Bleaching powder is used –
(i) for bleaching cotton and linen in the textile industry, for bleaching
wood pulp in paper factories and for bleaching washed clothes
in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) for disinfecting drinking water to make it free of germs
...
Sometimes it is added for faster cooking
...
It is
produced using sodium chloride as one of the raw materials
...
14?
Can you correlate why it can be used to neutralise an acid? It is a mild
non-corrosive base
...

Uses of sodium hydrogencarbonate (NaHCO3)
(i) For making baking powder, which is a mixture of baking soda
(sodium hydrogencarbonate) and a mild edible acid such as
tartaric acid
...

(ii) Sodium hydrogencarbonate is also an ingredient in antacids
...

(iii) It is also used in soda-acid fire extinguishers
...
10H2O (washing soda)
...
It is also a basic salt
...
10 H2 O
( Sodium
carbonate )

Acids, Bases and Salts

31

What does 10H2O signify? Does it make Na2CO3 wet? We will address
this question in the next section
...

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

(iii) Sodium carbonate can be used as a cleaning agent for domestic
purposes
...


2
...
4 Are the Crystals of Salts really Dry?
Activity 2
...

What is the colour of the copper
sulphate after heating?
Do you notice water droplets in the
boiling tube? Where have these come
from?
Add 2-3 drops of water on the sample
of copper sulphate obtained after
heating
...
9
Removing water
of crystallisation

Copper sulphate crystals which seem to be dry contain water of
crystallisation
...

If you moisten the crystals again with water, you will find that blue
colour of the crystals reappears
...
Five water molecules are present in one
formula unit of copper sulphate
...
5H2O
...
10H2O is wet
...

It has two water molecules as water of cyrstallisation
...
2H2O
...

Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes
calcium sulphate hemihydrate ( CaSO 4
...
This is called Plaster of
2 2

Science

Paris, the substance which doctors use as plaster for supporting
fractured bones in the right position
...

CaSO4
...
2H2 O
2
2

(Plaster of Paris)

(Gypsum)

Note that only half a water molecule is shown to be attached as water
of crystallisation
...
Plaster of Paris is used for making toys, materials for decoration
and for making surfaces smooth
...


What is the common name of the compound CaOCl2?

2
...


3
...


4
...


5
...


?

What you have learnt
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the
presence of acids and bases
...

Formation of OH–(aq) ions in solution is responsible for the basic nature of a
substance
...

When a base reacts with a metal, along with the evolution of hydrogen gas a salt is
formed which has a negative ion composed of the metal and oxygen
...

Acidic and basic solutions in water conduct electricity because they produce
hydrogen and hydroxide ions respectively
...

A neutral solution has a pH of exactly 7, while an acidic solution has a pH less
than 7 and a basic solution a pH more than 7
...

Mixing concentrated acids or bases with water is a highly exothermic process
...

Water of crystallisation is the fixed number of water molecules chemically attached
to each formula unit of a salt in its crystalline form
...


E X E R C I S E S
1
...
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky
...
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a
given solution of HCl
...
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
5
...

(b) dilute hydrochloric acid reacts with magnesium ribbon
...

(d) dilute hydrochloric acid reacts with iron filings
...
Compounds such as alcohols and glucose also contain hydrogen but are not
categorised as acids
...

7
...
Why do acids not show acidic behaviour in the absence of water?
9
...
Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration
...
Equal lengths of magnesium ribbons are taken in test tubes A and B
...
In which test tube will the fizzing occur more vigorously and why?
11
...
How do you think the pH will change as it turns into
curd? Explain your answer
...
A milkman adds a very small amount of baking soda to fresh milk
...
Plaster of Paris should be stored in a moisture-proof container
...
What is a neutralisation reaction? Give two examples
...
Give two important uses of washing soda and baking soda
...

Add sufficient water to obtain the extract
...

Collect the filtrate to test the substances you may have tasted earlier
...
Pour
2 mL each of lemon juice solution, soda-water, vinegar and baking soda solution
in them respectively
...
Write your observation in a Table
...


Acids, Bases and Salts

35

(II) Preparing a soda-acid fire extinguisher
The reaction of acids with metal hydrogencarbonates is used in the fire extinguishers
which produce carbon dioxide
...

Suspend an ignition tube containing dilute sulphuric acid in the wash-bottle
(Fig
...
10)
...

Tilt the wash-bottle so that the acid from the ignition tube mixes with the sodium
hydrogencarbonate solution below
...

Direct this gas on a burning candle
...
10 (a) Ignition tube containing dilute sulphuric acid suspended in a wash-bottle containing
sodium hydrogencarbonate, (b) Carbon dioxide gas coming out of the nozzle

36

Science



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