We can get solid sodium chloride by an evaporation method. Evaporation is the process by which water changes from a liquid to a gas or vapour.
Rate of evaporation increases with temperature.

Dorji will decide which drink is to be served to whom (customer) by doing the following litmus paper tests:
(i) If the drink turns blue litmus paper to red, then the drink is acidic and hence will be served to the first customer who wants the acidic drink.
(ii) If the drink turns red litmus paper to blue, then the drink is basic and hence will be served to another customer who wants the basic drink.
(iii) If the drink shows no change in colour with litmus paper, then it will be neutral drink. Thus, it will be served to the third customer.
Since the soft drinks are edible. Dorji can take the decision by tasting the drinks. Acidic drinks will be sour to taste whereas basic drinks will be bitter to taste and neutral drinks will have no taste.

Distilled water is neither acidic nor basic. When we test its nature through litmus paper strips, we have found neither red nor blue changes in colour which verifies its neutral nature.

Litmus solution is obtained from lichens. It has mauve (purple) colour in distilled water. This solution is used as indicator.

Differences between acids and bases are as follows:

(a) All indicators do not change colour with acids as well as bases, e.g. turmeric changes its colour to pink with the base but it remains yellow with acid.
Therefore, statement (i) is incorrect. There are some indicators which change colour with both acid and base, e.g. methyl orange, litmus, thus statement (ii) and (iii) are also incorrect. Statement (iv) is the correct conclusion about indicators.

Carbonic acid.

(i) Baking soda
(ii) Soap or detergent

When hydrochloric acid reacts with sodium hydroxide solution, then sodium chloride salt is formed.

The acid is formic acid which is injected into the skin of a person when an ant bites.

Neutralisation reaction.

(a) Rahul knows that our stomach contains hydrochloric acid. It helps us to digest food. But, too much acid in the stomach causes indigestion. Sometimes, indigestion is painful and some mild base should be taken to relief from this pain. So, this information given by his teacher helped him.
(b) He gave her mother baking soda solution which was available in the kitchen easily. He selects baking soda because it is a mild base and neutralises the excess acid in the stomach and it will give relief from pain.
(c) Rahul is very intelligent and caring boy.

The reason for low crop yield in farmer's field was that the soil at a place is either too acidic or too basic. The soil may be acidic or basic naturally. The excessive use of fertilisers in the fields also makes the soil too acidic. When the soil is too acidic, it is treated with base such as quicklime (calcium oxide) or slaked lime (calcium hydroxide) which neutralises the excess acid present in the soil and reduces its acidic nature. If the soil is too basic, then decaying organic matter (called manure or compost) is added it. The decaying organic matter releases acid which neutralises the excess bases present in the soil and reduces its basic nature.