Answer:
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.
2021–22
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.
Explanation:
= 1.5 mmol Fe⁺²
= 1.75 mmol MnO₂
= 0.583 mmol Mn₃O₄