Acids, Bases
and Salts
CHAPTER2
You have learnt in your previous classes that the sour and bitter
tastes of food are due to acids and bases, respectively, present in them.
If someone in the family is suffering from a problem of acidity after
overeating, which of the following would you suggest as a remedy– lemon
juice, vinegar or baking soda solution?
Which property did you think of while choosing the remedy?
Surely you must have used your knowledge about the ability of
acids and bases to nullify each other’s effect.
Recall how we tested sour and bitter substances without tasting
them.
You already know that acids are sour in taste and change the colour
of blue litmus to red, whereas, bases are bitter and change the colour of
the red litmus to blue. Litmus is a natural indicator, turmeric is another
such indicator. Have you noticed that a stain of curry on a white cloth
becomes reddish-brown when soap, which is basic in nature, is scrubbed
on it? It turns yellow again when the cloth is washed with plenty of
water. You can also use synthetic indicators such as methyl orange and
phenolphthalein to test for acids and bases.
In this Chapter, we will study the reactions of acids and bases, how
acids and bases cancel out each other’s effects and many more interesting
things that we use and see in our day-to-day life.
Do You Know?
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. When the litmus
solution is neither acidic nor basic, its colour is purple. 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. These are called acid-base indicators or sometimes simply indicators.
18 Science
2.1 UNDERSTANDING THE CHEMICAL PROPERTIES OF
ACIDS AND BASES
2.1.1 Acids and Bases in the Laboratory
Activity 2.1
These indicators tell us whether a substance is acidic or basic by
change in colour. There are some substances whose odour changes in
acidic or basic media. These are called olfactory indicators. Let us try
out some of these indicators.
Q U E S T I O N
? 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?
Collect the following samples from the science laboratory–
hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3),
acetic acid (CH3COOH), sodium hydroxide (NaOH), calcium
hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium
hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).
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 2.1.
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 2.1.
Table 2.1
Sample Red Blue Phenolph- Methyl
solution litmus litmus -thalein orange
solution solution solution solution
Activity 2.2
Take some finely chopped onions in a plastic bag along with some
strips of clean cloth. Tie up the bag tightly and leave overnight in
the fridge. The cloth strips can now be used to test for acids and
bases.
Take two of these cloth strips and check their odour.
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.
Acids, Bases and Salts 19
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.
2.1.2 How do Acids and Bases React with Metals?
Rinse both cloth strips with water and again check their odour.
Note your observations.
Now take some dilute vanilla essence and clove oil and check their
odour.
Take some dilute HCl solution in one test tube and dilute NaOH
solution in another. Add a few drops of dilute vanilla essence to
both test tubes and shake well. Check the odour once again and
record changes in odour, if any.
Similarly, test the change in the odour of clove oil with dilute HCl
and dilute NaOH solutions and record your observations.
Activity 2.3
CAUTION: This activity needs the teacher’s assistance.
Set the apparatus as shown in Fig. 2.1.
Take about 5 mL of dilute sulphuric acid in a test tube and add a
few pieces of zinc granules to it.
What do you observe on the surface of zinc granules?
Pass the gas being evolved through the soap solution.
Why are bubbles formed in the soap solution?
Take a burning candle near a gas filled bubble.
What do you observe?
Repeat this Activity with some more acids like HCl, HNO3 and
CH3COOH.
Are the observations in all the cases the same or different?
Figure 2.1 Reaction of zinc granules with dilute sulphuric acid and testing hydrogen
gas by burning
20 Science
Note that the metal in the above reactions displaces hydrogen from
the acids. This is seen as hydrogen gas. The metal combines with the
remaining part of the acid and forms a compound called a salt. 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 2.4
The reactions occurring in the above Activity are written as –
Test tube A: Na CO HCl(aq) Cl(aq) H O(l) + CO 2 3 2 2 (s) + 2 → 2Na + (g)
Test tube B: NaHCO HCl(aq) Cl(aq) H O(l) + CO 3 2 2 (s) + → Na + (g)
On passing the carbon dioxide gas evolved through lime water,
Ca(OH) CO H O(l) 2 2 2 (aq) (g) CaCO s 3 + → ( ) +
(Lime water) (White precipitate)
Place a few pieces of granulated zinc metal in a test tube.
Add 2 mL of sodium hydroxide solution and warm the contents
of the test tube.
Repeat the rest of the steps as in Activity 2.3 and record your
observations.
The reaction that takes place can be written as follows.
2 2 2 2 NaOH Zn Na ZnO H
Sodiumzincate
+ → +
( )
You find again that hydrogen is formed in the reaction. However,
such reactions are not possible with all metals.
2.1.3 How do Metal Carbonates and Metal
Hydrogencarbonates React with Acids?
Activity 2.5
Take two test tubes, label them as A
and B.
Take about 0.5 g of sodium carbonate
(Na2CO3) in test tube A and about
0.5 g of sodium hydrogencarbonate
(NaHCO3) in test tube B.
Add about 2 mL of dilute HCl to both
the test tubes.
What do you observe?
Pass the gas produced in each case
through lime water (calcium
hydroxide solution) as shown in
Fig. 2.2 and record your observations.
Figure 2.2
Passing carbon dioxide gas
through calcium hydroxide
solution
Acids, Bases and Salts 21
Activity 2.6
Take about 2 mL of dilute NaOH solution in a test tube and add
two drops of phenolphthalein solution.
What is the colour of the solution?
Add dilute HCl solution to the above solution drop by drop.
Is there any colour change for the reaction mixture?
Why did the colour of phenolphthalein change after the addition
of an acid?
Now add a few drops of NaOH to the above mixture.
Does the pink colour of phenolphthalein reappear?
Why do you think this has happened?
On passing excess carbon dioxide the following reaction takes place:
CaCO s Ca(HCO aq 3 3 ( )+ H O(l)+ CO (g) → ) ( ) 2 2 2
(Soluble in water)
Limestone, chalk and marble are different forms of calcium carbonate.
All metal carbonates and hydrogencarbonates react with acids to give a
corresponding salt, carbon dioxide and water.
Thus, the reaction can be summarised as –
In the above Activity, we have observed that the effect of a base is
nullified by an acid and vice-versa. The reaction taking place is written as –
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
The reaction between an acid and a base to give a salt and water is
known as a neutralisation reaction. In general, a neutralisation reaction
can be written as –
Base + Acid → Salt + Water
2.1.5 Reaction of Metallic Oxides with Acids
Activity 2.7
Take a small amount of copper oxide in a beaker and add dilute
hydrochloric acid slowly while stirring.
Note the colour of the solution. What has happened to the copper
oxide?
You will notice that the colour of the solution becomes blue-green
and the copper oxide dissolves. The blue-green colour of the solution is
due to the formation of copper(II) chloride in the reaction. The general
reaction between a metal oxide and an acid can be written as –
Metal oxide + Acid → Salt + Water
Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
2.1.4 How do Acids and Bases React with each other?
22 Science
Now write and balance the equation for the above 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.
2.1.6 Reaction of a Non-metallic Oxide with Base
You saw the reaction between carbon dioxide and calcium hydroxide
(lime water) in Activity 2.5. Calcium hydroxide, which is a base, reacts
with carbon dioxide to produce a salt and water. Since this is similar to
the reaction between a base and an acid, we can conclude that nonmetallic
oxides are acidic in nature.
Q U E S T I O N S
? 1. Why should curd and sour substances not be kept in brass and copper
vessels?
2. 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?
3. 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.
2.2 WHAT DO ALL ACIDS AND ALL BASES HAVE IN
COMMON?
In Section 2.1 we have seen that all acids have similar chemical
properties. What leads to this similarity in properties? We saw in Activity
2.3 that all acids generate hydrogen gas on reacting with metals, so
hydrogen seems to be common to all acids. Let us perform an Activity to
investigate whether all compounds containing hydrogen are acidic.
Activity 2.8
Take solutions of glucose, alcohol,
hydrochloric acid, sulphuric acid, etc.
Fix two nails on a cork, and place the cork in
a 100 mL beaker.
Connect the nails to the two terminals of a
6 volt battery through a bulb and a switch, as
shown in Fig. 2.3.
Now pour some dilute HCl in the beaker and
switch on the current.
Repeat with dilute sulphuric acid.
What do you observe?
Repeat the experiment separately with
glucose and alcohol solutions. What do you
observe now?
Does the bulb glow in all cases?
Figure 2.3
Acid solution in water
conducts electricity
Acids, Bases and Salts 23
The bulb will start glowing in the case of acids, as shown in Fig. 2.3.
But you will observe that glucose and alcohol solutions do not conduct
electricity. Glowing of the bulb indicates that there is a flow of electric
current through the solution. The electric current is carried through the
solution by ions.
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.
Repeat the same Activity using alkalis such as sodium hydroxide,
calcium hydroxide, etc. What can you conclude from the results of this
Activity?
2.2.1 What Happens to an Acid or a Base in a Water Solution?
Do acids produce ions only in aqueous solution? Let us test this.
Activity 2.9
Take about 1g solid NaCl in a clean and
dry test tube and set up the apparatus as
shown in Fig. 2.4.
Add some concentrated sulphuric acid to
the test tube.
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.
In which case does the litmus paper change
colour?
On the basis of the above Activity, what do
you infer about the acidic character of:
(i) dry HCl gas
(ii) HCl solution?
Figure 2.4 Preparation of HCl gas
This experiment suggests that hydrogen ions in HCl are produced
in the presence of water. The separation of H+ ion from HCl molecules
cannot occur in the absence of water.
HCl + H2O → H3O+ + Cl–
Hydrogen ions cannot exist alone, but they exist after combining
with water molecules. Thus hydrogen ions must always be shown as
H+(aq) or hydronium ion (H3O+).
H+ + H2O → H3O+
We have seen that acids give H3O+ or H+(aq) ion in water. Let us see
what happens when a base is dissolved in water.
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.
24 Science
Mg(OH)2(s) Mg (aq)+ OH (aq)
H O 2 2→ + 2
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
bases
The process of dissolving an acid or a base in water is a highly
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.
Acids, Bases and Salts 25
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 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. Generally paper impregnated with the
universal indicator is used for measuring pH. One such paper is shown
in Fig. 2.6.
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?
Figure 2.6 Variation of pH with the change in concentration of H+(aq) and OH–(aq) ions
26 Science
Activity 2.11
Test the pH values
of solutions given in
Table 2.2.
Record your observations.
What is the nature of each
substance on the basis of
your observations?
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 Importance of pH in Everyday Life
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.
Table 2.2
S. Solution Colour of Approx- Nature of
No. pH paper -imate substance
pH value
1 Saliva (before meal)
2 Saliva (after meal)
3 Lemon juice
4 Colourless aerated
drink
5 Carrot juice
6 Coffee
7 Tomato juice
8 Tap water
9 1M NaOH
10 1M HCl
Acids, Bases and Salts 27
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.
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?
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 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.
Do You
Know?
28 Science
2.4 MORE ABOUT 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 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.
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)? ?
Do You Know?
Acids, Bases and Salts 29
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
are neutral with pH value of 7. On the other
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.
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?
Table 2.4
Salt pH Acid used Base used
30 Science
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) → 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.
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 CaOCl2, though the actual composition is quite
complex.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
Figure 2.8 Important products from the chlor-alkali process
Acids, Bases and Salts 31
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.
Baking soda
The soda commonly used in the kitchen for making tasty crispy pakoras
is baking soda. 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 + H2O + CO2 + NH3 NH4Cl + 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 base. The following reaction takes place when it is heated
during cooking –
2NaHCO3 Na2CO3 +H2O+CO2
Heat→
(Sodium (Sodium
hydrogencarbonate) carbonate)
Sodium hydrogencarbonate has got various uses in the household.
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. 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 causes 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.
Na CO H O Na CO H O
Sodium
carbonate
2 3 + 10 2 → 2 3.10 2
(
)
32 Science
What does 10H2O signify? Does it make Na2CO3 wet? We will address
this question in the next section.
Sodium carbonate and sodium hydrogencarbonate are useful
chemicals for many industrial processes as well.
Uses of washing soda
(i) Sodium carbonate (washing soda) is used in glass, soap and
paper industries.
(ii) It is used in the manufacture of sodium compounds such as borax.
(iii) Sodium carbonate can be used as a cleaning agent for domestic
purposes.
(iv) It is used for removing permanent hardness of water.
2.4.4 Are the Crystals of Salts really Dry?
Activity 2.15
Heat a few crystals of copper sulphate
in a dry boiling tube.
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.
What do you observe? Is the blue
colour of copper sulphate restored?
Figure 2.9
Removing water
of crystallisation
Copper sulphate crystals which seem to be dry contain water of
crystallisation. When we heat the crystals, this water is removed and the
salt turns white.
If you moisten the crystals again with water, you will find that blue
colour of the crystals reappears.
Water of crystallisation is the fixed number of water molecules present
in one formula unit of a salt. Five water molecules are present in one
formula unit of copper sulphate. Chemical formula for hydrated copper
sulphate is Cu SO4. 5H2O. Now you would be able to answer the question
whether the molecule of Na2CO3.10H2O is wet.
One other salt, which possesses water of crystallisation is gypsum.
It has two water molecules as water of cyrstallisation. It has the formula
CaSO4.2H2O. Let us look into the use of this salt.
Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes
calcium sulphate hemihydrate ( 4 2
1
CaSO . H O
2
). This is called Plaster of
Acids, Bases and Salts 33
Paris, the substance which doctors use as plaster for supporting
fractured bones in the right position. Plaster of Paris is a white powder
and on mixing with water, it changes to gypsum once again giving a
hard solid mass.
CaSO .
1
2
H O+1
1
2
4 2 H2O CaSO4 .2H2O
(Plaster of Paris) Gypsum)
→
(
Note that only half a water molecule is shown to be attached as water
of crystallisation. How can you get half a water molecule? It is written in
this form because two formula units of CaSO4 share one molecule of
water. Plaster of Paris is used for making toys, materials for decoration
and for making surfaces smooth. Try to find out why is calcium sulphate
hemihydrate called ‘Plaster of Paris’ ?
1. What is the common name of the compound CaOCl2?
2. Name the substance which on treatment with chlorine yields bleaching
powder.
3. Name the sodium compound which is used for softening hard water.
4. What will happen if a solution of sodium hydrocarbonate is heated?
Give the equation of the reaction involved.
5. Write an equation to show the reaction between Plaster of Paris and
water.
Q U E S T I O N S
?
What you have learnt
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the
presence of acids and bases.
Acidic nature of a substance is due to the formation of H+(aq) ions in solution.
Formation of OH–(aq) ions in solution is responsible for the basic nature of a
substance.
When an acid reacts with a metal, hydrogen gas is evolved and a corresponding
salt is formed.
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.
When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives
the corresponding salt, carbon dioxide gas and water.
Acidic and basic solutions in water conduct electricity because they produce
hydrogen and hydroxide ions respectively.
34 Science
The strength of an acid or an alkali can be tested by using a scale called the pH
scale (0-14) which gives the measure of hydrogen ion concentration in a solution.
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.
Living beings carry out their metabolic activities within an optimal pH range.
Mixing concentrated acids or bases with water is a highly exothermic process.
Acids and bases neutralise each other to form corresponding salts and water.
Water of crystallisation is the fixed number of water molecules chemically attached
to each formula unit of a salt in its crystalline form.
Salts have various uses in everyday life and in industries.
E X E R C I S E S
1. A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky.
The solution contains
(a) NaCl (b) HCl (c) LiCl (d) KCl
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 (c) 12 mL (d) 16 mL
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
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.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
6. Compounds such as alcohols and glucose also contain hydrogen but are not
categorised as acids. Describe an Activity to prove it.
7. Why does distilled water not conduct electricity, whereas rain water does?
Acids, Bases and Salts 35
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. 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.
(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.
Group Activity
36 Science
(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 an effervescence coming out of the nozzle.
Direct this gas 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) Carbon dioxide gas coming out of the nozzle
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