Question

Write short notes on:

(1) Indicators (with proper example).

(2) pH scale (with proper diagram).

(3) Water of crystallization (with proper example).

Answer 1

(1) The acidic and basic nature of the compounds is indicated by some natural as well as chemical substances called indicators. Indicators are of following types:

(i) Natural indicators: There are many natural substances such as rose petals, turmeric and beet root that can be used as indicators to produce colours.

(ii) Synthetic indicators: Some manmade chemical substances such as phenolphthalein and methyl orange are synthetic indicators.

Uses of indicators: The indicators are used to judge the acidic or basic nature of a compound. Indicators are used in agriculture to determine the pH of the soil.

2. pH scale: It helps in measurement of hydrogen ion concentration in a solution.

In pH, p stands for ‘potenz’ meaning ‘strength’ in German.

• The scale ranges from 0 (most acidic) to 14 (most basic). The value of pH indicates acidic or basic nature of a solution. When the pH is in between 0 to 7, the solution is acidic in nature; at value 7, the solution is neutral; and from 7 to 14 the solution becomes basic in nature.

• Lower the pH of an acid, greater is the concentration of H+ ions while greater the pH of a base, greater is the concentration of OH⁻ ions.

(3) Water of crystallization: It is a fixed number of water molecules present in the crystal structure.

For example, chemical formula of hydrated copper sulphate is CuSO₄.5H₂O. So, copper sulphate has five molecules of water of crystallization. On heating, the above salts lose water of crystallization and form anhydrous substances.