(a) What is the i

(a) (i) Living organisms can survive only in a narrow range of pH change. Acidic rainwater when

(ii) flows into the rivers, it lowers the pH value of river water and makes the survival of aquatic life in such river water difficult. Plants require a specific pH range for their healthy growth. Our stomach and intestines work in a specific pH range. Stomach acts in slightly acidic medium while small intestine digests the food in slightly alkaline medium.

(iii) Tooth decay starts when the pH of the mouth is lower than 5.5.

(b) When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. This process is called Chlor-alkali process the products formed are chlorine and alkali NaOH.

2NaCl (aq) + 2H₂O (l) 2NaOH (aq) + Cl₂ + H₂ (g)

Cl₂ gas is given off at the anode and H₂ gas at the cathode while sodium hydroxide solution is formed near the cathode.