NCERT Solutions for Class 10 Science Chapter 2 Acids, Bases and Salts

Indicators are substances which give different colour in acid or bases solutions. Natural indicators from plants are:
(a) Litmus,
(b) Vanilla extract.

Antacids are mild alkalies. These are used for getting relief from acidity and indigestion and sometimes, even headache. When taken orally, it reacts with hydrochloric acid present in the stomach and reduces its strength by consuming some of it. For example, milk of magnesia is an antacid.

Olfactory indicators are substances which have different odour in acid and base solutions. For example, vanilla essence has characteristics pleasant smell in acid solution and no smell in alkali solution.

Tap water contains some impurities in the forms of salts. Due to presence of salts, it conducts electricity. Distilled water is free from all kinds of salts and hence does not conduct electricity.

When an acid is mixed with water, there is evolution of large amount of heat. Therefore, acid is slowly added to water. If on the other hand, water is added to acid, it might spill on your body and clothes due to explosion and evolution of sudden and large amount of heat.

pH is a term which indicates whether a solution is acidic of basic and to what extent. Mathematically it is a measure of H+ ions concentration in water.
The rain water collected from city B is more acidic.

Sugar present in chocolates and sweets gets broken to acids by bacteria present in the mouth. This lowers the pH in the mouth. Tooth enamel is made up of calcium phosphate, which gets corroded when the pH in the mouth is below 5.5. To prevent tooth enamel from decay, toothpaste is used because it is alkaline and neutralises the acid produced in the mouth and helps to prevent lowering of pH in mouth.

Bleaching powder is soluble in water. However, commercial samples of bleaching powder contains slaked lime that does not react with chlorine gas during the manufacture of bleaching powder. The insoluble part of bleaching powder is this white solid, i.e., slaked lime.

When the effect of a base is nullified by an acid and vice versa, it is called neutralization reaction. In general, a neutralization reaction is written as: Base + Acid ―> Salt + Water

Distilled water is a pure form of water not containing any ionic species. Therefore, it does not conduct electricity. Rain water, being an impure form of water, contains many ionic species (dissolved in air) such as acids and therefore it conducts electricity.

Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions (H+ ) from an acid occurs in the presence of water only. It is the hydrogen ions (H+ ) that are responsible for the acidic behaviour.

The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature. The acids present in it decrease the pH.

Acids dissociate in aqueous solutions to form ions. These ions are responsible for conduction of electricity.

Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain H+ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCl is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.