H O

2
KOH(s) ⎯⎯⎯⎯→ K + (aq) + OH – (aq)
H 2O
Mg(OH)2(s) ⎯⎯⎯⎯ 2+ –
→ Mg (aq)+2OH (aq)
Bases generate hydroxide (OH–) ions in water. Bases which are soluble
in water are called alkalis.
Do You
Know?

All bases do not dissolve in water. An alkali is a base that dissolves in water. They
are soapy to touch, bitter and corrosive. Never taste or touch them as they may
cause harm. Which of the bases in the Table 2.1 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.

Activity 2.10
 Take 10 mL water in a beaker.
 Add a few drops of concentrated H2SO4 to it and swirl the
beaker slowly.
 Touch the base of the beaker.
 Is there a change in temperature?
 Is this an exothermic or endothermic process?
 Repeat the above Activity with sodium hydroxide pellets
Figure 2.5 and record your observations.
Warning sign displayed
on containers containing
concentrated acids and
The process of dissolving an acid or a base in water is a highly
bases
exothermic one. Care must be taken while mixing concentrated nitric
acid or sulphuric acid with water. The acid must always be added slowly
to water with constant stirring. If water is added to a concentrated acid,
the heat generated may cause the mixture to splash out and cause burns.
The glass container may also break due to excessive local heating. Look
out for the warning sign (shown in Fig. 2.5) on the can of concentrated
sulphuric acid and on the bottle of sodium hydroxide pellets.
Mixing an acid or base with water results in decrease in the
concentration of ions (H3O+/OH–) per unit volume. Such a process is
called dilution and the acid or the base is said to be diluted.

24 Science

Reprint 2025-26
 Q U E S T I O N S
1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions

?
while solutions of compounds like alcohol and glucose do not show acidic
character?
2. Why does an aqueous solution of an acid conduct electricity?
3. Why does dry HCl gas not change the colour of the dry litmus paper?
4. 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 hydronium ions (H3O +) affected when a
solution of an acid is diluted?
6. How is the concentration of hydroxide ions (OH –) affected when excess
base is dissolved in a solution of sodium hydroxide?

2.3 HOW STRONG ARE ACID OR BASE SOLUTIONS?

We know how acid-base indicators can be used to distinguish between
an acid and a base. We have also learnt in the previous section about
dilution and decrease in concentration of H+ or OH– ions in solutions.
Can we quantitatively find the amount of these ions present in a solution?
Can we judge how strong a given acid or base is?
We can do this by making use of a universal indicator, which is a
mixture of several indicators. The universal indicator shows different
colours at different concentrations of hydrogen ions in a solution.
A scale for measuring hydrogen ion concentration in a solution, called
pH scale has been developed. The p in pH stands for ‘potenz’ in German,
meaning power. On the pH scale we can measure pH generally from
0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a
number which indicates the acidic or basic nature of a solution. Higher
the hydronium ion concentration, lower is the pH value.
The pH of a neutral solution is 7. Values less than 7 on the pH scale
represent an acidic solution. 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 (Fig. 2.6). Generally paper impregnated
with the universal indicator is used for measuring pH.

Figure 2.6 Variation of pH with the change in concentration of H+(aq) and OH–(aq) ions

Acids, Bases and Salts 25

Reprint 2025-26
 Table 2.2
S. Solution Colour of Approx- Nature of
Activity 2.11 No. pH paper -imate substance
pH value
 Test the pH values
of solutions given in 1 Saliva (before meal)
Table 2.2.
 Record your observations. 2 Saliva (after meal)
 What is the nature of each 3 Lemon juice
substance on the basis of
your observations? 4 Colourless aerated
drink
5 Carrot juice
6 Coffee
7 Tomato juice
8 Tap water
9 1M NaOH
10 1M HCl

Figure 2.7 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. If we take hydrochloric acid and
acetic acid of the same concentration, say one molar, then these produce
different amounts of hydrogen ions. 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. Can you now say what weak and strong bases are?

2.3.1 Impor tance of pH in Ever yday Life

Everyday
Are plants and animals pH sensitive?
Our body works within the pH range of 7.0 to 7.8. Living organisms can
survive only in a narrow range of pH change. When pH of rain water is
less than 5.6, it is called acid rain. When acid rain flows into the rivers, it
lowers the pH of the river water. The survival of aquatic life in such rivers
becomes difficult.

26 Science

Reprint 2025-26
Do You
Know?
Acids in other planets
The atmosphere of venus is made up of thick white and yellowish clouds of
sulphuric acid. 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. To find out
the pH required for the healthy growth of a plant, you can collect the soil
from various places and check the pH in the manner described below in
Activity 2.12. Also, you can note down which plants are growing in the
region from which you have collected the soil.

Activity 2.12
 Put about 2 g soil in a test tube and add 5 mL water to it.
 Shake the contents of the test tube.
 Filter the contents and collect the filtrate in a test tube.
 Check the pH of this filtrate with the help of universal
indicator paper.
 What can you conclude about the ideal soil pH for the growth of
plants in your region?

pH in our digestive system

It is very interesting to note that our stomach produces hydrochloric
acid. It helps in the digestion of food without harming the stomach.
During indigestion the stomach produces too much acid and this causes
pain and irritation. To get rid of this pain, people use bases called
antacids. One such remedy must have been suggested by you at the
beginning of this Chapter. These antacids neutralise the excess acid.
Magnesium hydroxide (Milk of magnesia), a mild base, is often used for
this purpose.

pH change as the cause of tooth decay

Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth
enamel, made up of calcium hydroxyapatite (a crystalline form of calcium
phosphate) is the hardest substance in the body. It does not dissolve in
water, but is corroded when the pH in the mouth is below 5.5. Bacteria
present in the mouth produce acids by degradation of sugar and food
particles remaining in the mouth after eating. The best way to prevent
this is to clean the mouth after eating food. Using toothpastes, which are
generally basic, for cleaning the teeth can neutralise the excess acid and
prevent tooth decay.

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. Use of a mild base like baking soda
on the stung area gives relief. Stinging hair of nettle leaves inject
methanoic acid causing burning pain.

Acids, Bases and Salts 27

Reprint 2025-26
Do You Know?

Nature provides neutralisation options

Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair,
which cause painful stings when touched accidentally. This is due to the methanoic
acid secreted by them. A traditional remedy is rubbing the
area with the leaf of the dock plant, which often grows beside
the nettle in the wild. Can you guess the nature of the dock
plant? So next time you know what to look out for if you
accidentally touch a nettle plant while trekking. Are you aware
of any other effective traditional remedies for such stings?

Table 2.3 Some naturally occurring acids

Natural source Acid Natural source Acid

Vinegar Acetic acid Sour milk (Curd) Lactic acid

Orange Citric acid Lemon Citric acid
Tamarind Tartaric acid Ant sting Methanoic acid
Tomato Oxalic acid Nettle sting Methanoic acid

Q U E S T I O N S

?
1. 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?
2. What effect does the concentration of H+(aq) ions have on the nature of the
solution?
3. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
4. Under what soil condition do you think a farmer would treat the soil of his
fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or
chalk (calcium carbonate)?

2.4 MORE ABOUT SALTS

SALTS
In the previous sections we have seen the formation of salts during
various reactions. Let us understand more about their preparation,
properties and uses.

2.4.1 Family of Salts

Activity 2.13
 Write the chemical formulae of the salts given below.
Potassium sulphate, sodium sulphate, calcium sulphate,
magnesium sulphate, copper sulphate, sodium chloride, sodium
nitrate, sodium carbonate and ammonium chloride.

28 Science

Reprint 2025-26
  Identify the acids and bases from which the above salts may be
obtained.
 Salts having the same positive or negative radicals are said to
belong to a family. For example, NaCl and Na2SO4 belong to the
family of sodium salts. Similarly, NaCl and KCl belong to the family
of chloride salts. How many families can you identify among the
salts given in this Activity?

2.4.2 pH of Salts

Activity 2.14
 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 their solubility in water (use distilled water only).
 Check the action of these solutions on litmus and find the pH
using a pH paper.
 Which of the salts are acidic, basic or neutral?
 Identify the acid or base used to form the salt.
 Report your observations in Table 2.4.

Salts of a strong acid and a strong base Table 2.4

are neutral with pH value of 7. On the other
hand, salts of a strong acid and weak base Salt pH Acid used Base used
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.

2.4.3 Chemicals from Common Salt

By now you have learnt that the salt formed
by the combination of hydrochloric acid and
sodium hydroxide solution is called sodium
chloride. This is the salt that you use in food.
You must have observed in the above Activity
that it is a neutral salt.
Seawater contains many salts dissolved
in it. Sodium chloride is separated from these
salts. Deposits of solid salt are also found in
several parts of the world. These large crystals
are often brown due to impurities. This is
called rock salt. Beds of rock salt were formed
when seas of bygone ages dried up. Rock salt
is mined like coal.
You must have heard about Mahatma Gandhi’s Dandi March. Did
you know that sodium chloride was such an important symbol in our
struggle for freedom?

Acids, Bases and Salts 29

Reprint 2025-26
 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. Let us see how one substance
is used for making all these different substances.

Sodium hydroxide
When electricity is passed through an aqueous solution of sodium
chloride (called brine), it decomposes to form sodium hydroxide. The
process is called the chlor-alkali process because of the products formed–
chlor for chlorine and alkali for sodium hydroxide.
2NaCl(aq) + 2H2O(l) Æ 2NaOH(aq) + Cl2(g) + H2(g)
Chlorine gas is given off at the anode, and hydrogen gas at the cathode.
Sodium hydroxide solution is formed near the cathode. The three
products produced in this process are all useful. Figure 2.8 shows the
different uses of these products.

Figure 2.8 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). This chlorine gas is used
for the manufacture of bleaching powder. Bleaching powder is produced
by the action of chlorine on dry slaked lime [Ca(OH)2]. Bleaching powder
is represented as Ca(ClO)2, though the actual composition is quite
complex.
2Ca(OH)2 + 2Cl2 Æ Ca(ClO)2 + CaCl2 + 2H2O

30 Science

Reprint 2025-26
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) to make drinking water free from germs.

Baking soda
The baking soda is commonly used in the kitchen for making tasty crispy
pakoras, etc. Sometimes it is added for faster cooking. The chemical
name of the compound is sodium hydrogencarbonate (NaHCO3). It is
produced using sodium chloride as one of the raw materials.

NaCl + H2 O + CO2 + NH3 → NH4 Cl + NaHCO3

(Ammonium (Sodium
chloride) hydrogencarbonate)

Did you check the pH of sodium hydrogencarbonate in Activity 2.14?

Can you correlate why it can be used to neutralise an acid? It is a mild
non-corrosive basic salt. The following reaction takes place when it is
heated during cooking –
Heat
2NaHCO3 ⎯⎯⎯⎯ → Na 2 CO3 + H2O + CO2
(Sodium (Sodium
hydrogencarbonate) carbonate)

Sodium hydrogencarbonate has got various uses in the household.

Uses of Baking soda
(i) For making baking powder, which is a mixture of baking soda
(sodium hydrogencarbonate) and a mild edible acid such as
tartaric acid. When baking powder is heated or mixed in water,
the following reaction takes place –
NaHCO3 + H+ → CO2 + H2O + Sodium salt of acid
(From any acid)

Carbon dioxide produced during the reaction can cause bread or

cake to rise making them soft and spongy.
(ii) Sodium hydrogencarbonate is also an ingredient in antacids.
Being alkaline, it neutralises excess acid in the stomach and
provides relief.
(iii) It is also used in soda-acid fire extinguishers.

Washing soda
Another chemical that can be obtained from sodium chloride is
Na2CO3.10H2O (washing soda). You have seen above that sodium
carbonate can be obtained by heating baking soda; recrystallisation of
sodium carbonate gives washing soda. It is also a basic salt.
Na2 CO3 + 10 H2 O → Na 2 CO3 .10 H2O
( Sodium
carbonate )

Acids, Bases and Salts 31

Reprint 2025-26
 8. Why do acids not show acidic behaviour in the absence of water?
9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as
4,1,11,7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric
acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test
tube B. Amount and concentration taken for both the acids are same. In which test
tube will the fizzing occur more vigorously and why?
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into
curd? Explain your answer.
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?
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
14. What is a neutralisation reaction? Give two examples.
15. Give two important uses of washing soda and baking soda.

Group Activity

(I) Prepare your own indicator

 Crush beetroot in a mortar.
 Add sufficient water to obtain the extract.
 Filter the extract by the procedure learnt by you in earlier classes.
 Collect the filtrate to test the substances you may have tasted earlier.
 Arrange four test tubes in a test tube stand and label them as A,B,C and D. Pour
2 mL each of lemon juice solution, soda-water, vinegar and baking soda solution
in them respectively.
 Put 2-3 drops of the beetroot extract in each test tube and note the colour change
if any. Write your observation in a Table.
 You can prepare indicators by using other natural materials like extracts of red
cabbage leaves, coloured petals of some flowers such as Petunia, Hydrangea and
Geranium.

Acids, Bases and Salts 35

Reprint 2025-26
 (II) Preparing a soda-acid fire extinguisher
The reaction of acids with metal hydrogencarbonates is used in the fire extinguishers
which produce carbon dioxide.
 Take 20 mL of sodium hydrogencarbonate (NaHCO3) solution in a wash-bottle.
 Suspend an ignition tube containing dilute sulphuric acid in the wash-bottle
(Fig. 2.10).
 Close the mouth of the wash-bottle.
 Tilt the wash-bottle so that the acid from the ignition tube mixes with the sodium
hydrogencarbonate solution below.
 You will notice discharge coming out of the nozzle.
 Direct this discharge on a burning candle. What happens?

Figure 2.10 (a) Ignition tube containing dilute sulphuric acid suspended in a wash-bottle containing
sodium hydrogencarbonate, (b) Discharge coming out of the nozzle

36 Science

Reprint 2025-26