NCERT Solutions For Class 7 Science Chapter 5 Acids, Bases and Salts

The reaction between the acid and the base is known as neutralisation reaction. In the neutralisation reaction, salt and water are produced with the evolution of heat.

Acid   +   Base    →     Salt   +   Water  +  Heat

Example:

Hydrochloric acid (acid) reacts with sodium hydroxide (base) to form sodium chloride (salt) and water. Heat is evolved in this process.

The nature of the solution will be either basic or neutral. This is because both basic solution and neutral solution do not change the colour of blue litmus paper.

Ammonia is basic in nature as it turns red litmus blue. Bases turn red litmus blue.

The correct option is (iv) only d.

Distilled water is neutral in nature. The same can be tested by using red and blue litmus paper. Acidic solution turns blue litmus paper red while basic solution turns red litmus paper blue. Distilled water neither turns blue litmus paper red nor red litmus paper blue. Therefore, distilled water is neutral.

Litmus is obtained from Lichens.

Uses of Litmus solution

• Litmus solution is used as an indicator. It is used to find the nature (acidic/basic/neutral) of the solution.
• Acid solution turns blue litmus solution red.
• Basic solution turns red litmus solution blue.

Put a drop each of hydrochloric acid, sodium hydroxide and sugar solution on the turmeric indicator. The solution which changes the colour of turmeric indicator to red will be basic that is sodium hydroxide.

Now add a few drops of sodium hydroxide solution in remaining two solutions i.e. both in hydrochloric acid and sugar solution. After that, put some drops of these mixtures on turmeric indicator separately.

Drop that changes the colour of turmeric indicator will be neutral in nature i.e. sugar solution. Mixture of basic solution and neutral solution will be basic in nature and change the colour of turmeric indicator to red.

Drop that does not change the colour of turmeric indicator will be acidic in nature i.e. hydrochloric acid. This is because hydrochloric acid being acidic in nature neutralises the sodium hydroxide solution.

(a) Hydrochloric acid present in our stomach helps in the digestion of food. But excess of it causes acidity. To relieve from acidity, antacid tablets are taken as it contains base such as magnesium hydroxide that neutralises the acid present in the stomach.

(b) Formic acid is present in ant’s sting. When an ant bites, it injects the solution of formic acid into skin. The effect of sting can be neutralised by rubbing calamine solution on the stung area. Calamine solution being basic in nature neutralises the formic acid.

(c) The factory wastes contain acids. If such water is released into a water body, it can harm aquatic animals like fishes. Factory wastes should be treated with calcium hydroxide to neutralise acid before it is disposed off in water.

Since, soft drinks are edible; Dorji can decide acidic, basic and neutral soft drinks by tasting them. Acidic soft drink will be sour in taste whereas basic soft drink will be bitter in taste. Neutral soft drink will has neither sour taste nor bitter.

Acidic, basic and neutral soft drinks can also be decided by using blue and red litmus paper.

Dorji can pour a few drops of one soft drink on red and blue litmus paper. If red litmus turns blue, then it will be a basic soft drink and if blue litmus changes to red, then it will be an acidic soft drink. If colour of both the litmus papers remains same, then it will be a neutral drink.

The same process can be repeated with the other soft drinks also.

  (i) Nitric acid turns red litmus blue.  (F)

 (ii) Sodium hydroxide turns blue litmus red. (F)

(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T)

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T)

(v) Tooth decay is caused by the presence of a base. (F)