A substance formed by the neutralisation of an acid with a base is called salt. Salt may be acidic, basic or neutral in nature.
For example,
Hydrochloric acid (HCI) + Sodium hydroxide (NaOH) \(\longrightarrow\) Sodium chloride (NaCI) + Water
  Acid                                  Base                                                Salt

(a) If the wastes of a factory flow into water bodies, it can cause a threat to the lives of sea creatures and to anybody who drinks the water. Since factory wastes may contain acids or bases and it can kill the fish.
(b) If the factory waste is acidic in nature, it can be neutralised by adding basic substances.

(i) Ant bite When an ant bite injects the acidic liquid (formic acid) into the skin, the effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.
(ii) Indigestion Our stomach contains hydrochloric acid. It helps us to digest food but too much of acid in the stomach causes indigestion. Sometimes, indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia which contains magnesium hydroxide. It neutralises the effect of excessive acid.

Acids which are mostly used in the laboratory as below:
(i) Hydrochloric acid (HCI)
(ii) Sulphuric acid (H₂SO₄)
(iii) Nitric acid (HNO₃)

Sodium bicarbonate neutralises the acidity in the stomach. Hence, it is used during indigestion. Lemon contains acid. It reacts with undigested food and suffers it.

China rose petals when added to warm water, form a light pink coloured solution which may be used as an indicator. This indicator turns acidic solution to magenta (deep pink) and basic solution to green.

The nature of the solution could be basic or neutral, as if it is acidic it will change its colour to red but instead it remains same. Also, bases always turn to blue and if the solution is neutral even then its colour will not change.

The reaction between an acid and a base is known as neutralisation reaction. In this reaction, both acid and base cancel each other's effect. Neutralisation reaction results in the formation of salt and water. During this reaction, energy in the form of heat is evolved.
Acid + Base \(\longrightarrow\) Salt + Water + Heat \(\uparrow\)
e.g. when sodium hydroxide (NaOH) is added to hydrochloric acid (HCI), sodium chloride (NaCl) and water (H₂O) are obtained.
NaOH     +       HCI           \(\longrightarrow\)  NaCI      +   H₂O + Heat \(\uparrow\)
Sodium        Hydrochloric           Sodium      Water
hydroxide     Acid                        Chloride

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) 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.