Use both red and blue litmus solution and predict the colours in each case. In acidic medium, blue litmus solution turns to red. In basic medium, red litmus solution turns to blue

(i) Detergent solution-Basic solution, gives blue colour with litmus paper.
(ii) Lime water-Basic solution, gives blue colour with litmus paper.
(iii) Soft drinks-Acidic solution, gives red colour with litmus paper.
(iv) Sugar solution-Neutral solution, gives purple colour with litmus paper.
(v)Vinegar-Acidic solution, gives red colour with litmus paper.

(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.

(a) The reason for this burning pain is honeybee sting which causes pain and irritation. This is due to the acidic liquid (formic acid) injected into the skin by the honeybee.
(b) The effect of acid can be neutralised by rubbing the affected area by baking soda solution which is a mild base.
(c) Nitesh's friends are caring, supportive and helpful.

China rose (gudhal) is an acid-base indicator.
China rose solution in different medium shows following changes:
China rose + acid \(\longrightarrow\) Give dark pink colour, hence A is an acidic solution.
China rose + base \(\longrightarrow\) give green colour, hence B is a basic solution.
China rose + neutral \(\longrightarrow\) No change in colour, hence C is a neutral solution.

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.

The sting of an ant contains formic acid. When an ant bites, it injects the acidic liquid into the skin. It causes irritation and burning effect on the skin. To relieve pain, skin should be rubbed by moist baking soda or calamine which are basic in nature

(i) An acid is the compound formed by the reaction of acidic oxide with water. e.g., HCl, whereas alkali is the hydroxide of metals dissolved in water. e.g., KOH.
(ii) A base is a substance containing hydroxide group. e.g., NH₄OH, whereas alkali is the base which is soluble in water. e.g.,NaOH.
(iii) Organic acid is a weak acid derived from plants or animals. e.g., acetic acid. Mineral acid is a strong acid derived from earth minerals. e.g., HCl.