Q1. What observations demonstrate the basic nature of soap when removing curry stains from
white cloth, and how does this relate to the properties of acids and bases?
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.
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?
n 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.
n 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.
Q2. What are some examples of natural and synthetic indicators mentioned in the passage, and
how are they utilized in testing for acids and bases in everyday situations?
Rationalised 2023-24
, 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?
ANDING THE CHEMIC
UNDERSTANDING
2 . 1 UNDERST AL PROPERTIES OF
CHEMICAL
ACIDS AND BASES
2.1.1 Acids and Bases in the Laboratory
Activity 2.1
n Collect the following solutions from the science laboratory–
hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3),
acetic acid (CH 3COOH), sodium hydroxide (NaOH), calcium
hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium
hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).
n Put a drop of each of the above solutions on a watch-glass one by
one and test with a drop of the indicators shown in Table 2.1.
n What change in colour did you observe with red litmus, blue litmus,
phenolphthalein and methyl orange solutions for each of the
solutions taken?
n Tabulate your observations in Table 2.1.
Table 2.1
Sample Red Blue Phenolph- Methyl
solution litmus litmus -thalein orange
solution solution solution solution
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.
Activity 2.2
n 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.
n Take two of these cloth strips and check their odour.
n 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.
18 Science
Rationalised 2023-24
, n Rinse both cloth strips with water and again check their odour.
n Note your observations.
n Now take some dilute vanilla essence and clove oil and check their
odour.
n 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.
n Similarly, test the change in the odour of clove oil with dilute HCl
and dilute NaOH solutions and record your observations.
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?
Activity 2.3
CAUTION: This activity needs the teacher’s assistance.
n Set the apparatus as shown in Fig. 2.1.
n Take about 5 mL of dilute sulphuric acid in a test tube and add a
few pieces of zinc granules to it.
n What do you observe on the surface of zinc granules?
n Pass the gas being evolved through the soap solution.
n Why are bubbles formed in the soap solution?
n Take a burning candle near a gas filled bubble.
n What do you observe?
n Repeat this Activity with some more acids like HCl, HNO3 and
CH3COOH.
n 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
Acids, Bases and Salts 19
Rationalised 2023-24
white cloth, and how does this relate to the properties of acids and bases?
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.
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?
n 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.
n 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.
Q2. What are some examples of natural and synthetic indicators mentioned in the passage, and
how are they utilized in testing for acids and bases in everyday situations?
Rationalised 2023-24
, 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?
ANDING THE CHEMIC
UNDERSTANDING
2 . 1 UNDERST AL PROPERTIES OF
CHEMICAL
ACIDS AND BASES
2.1.1 Acids and Bases in the Laboratory
Activity 2.1
n Collect the following solutions from the science laboratory–
hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3),
acetic acid (CH 3COOH), sodium hydroxide (NaOH), calcium
hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium
hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH).
n Put a drop of each of the above solutions on a watch-glass one by
one and test with a drop of the indicators shown in Table 2.1.
n What change in colour did you observe with red litmus, blue litmus,
phenolphthalein and methyl orange solutions for each of the
solutions taken?
n Tabulate your observations in Table 2.1.
Table 2.1
Sample Red Blue Phenolph- Methyl
solution litmus litmus -thalein orange
solution solution solution solution
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.
Activity 2.2
n 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.
n Take two of these cloth strips and check their odour.
n 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.
18 Science
Rationalised 2023-24
, n Rinse both cloth strips with water and again check their odour.
n Note your observations.
n Now take some dilute vanilla essence and clove oil and check their
odour.
n 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.
n Similarly, test the change in the odour of clove oil with dilute HCl
and dilute NaOH solutions and record your observations.
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?
Activity 2.3
CAUTION: This activity needs the teacher’s assistance.
n Set the apparatus as shown in Fig. 2.1.
n Take about 5 mL of dilute sulphuric acid in a test tube and add a
few pieces of zinc granules to it.
n What do you observe on the surface of zinc granules?
n Pass the gas being evolved through the soap solution.
n Why are bubbles formed in the soap solution?
n Take a burning candle near a gas filled bubble.
n What do you observe?
n Repeat this Activity with some more acids like HCl, HNO3 and
CH3COOH.
n 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
Acids, Bases and Salts 19
Rationalised 2023-24
