Science — AI-generated practice question

(a) Bacteria in milk produce lactic acid, which lowers the pH and causes milk to spoil. By adding baking soda, the milkman makes the milk slightly alkaline (higher pH), which neutralises the acid produced, thus slowing bacterial activity and preserving milk longer.

(b) For milk to set as curd, bacteria must produce enough lactic acid to lower the pH sufficiently. Since the milk is now alkaline, bacteria need more time to first neutralise the added base before the pH can drop enough to set curd.

(c) This tells us that bacteria responsible for curd formation are pH-sensitive and function optimally only within a specific (acidic) pH range. Living beings, including microorganisms, carry out their metabolic activities within an optimal pH range.

Source: Chapter 2, Exercises Q.12; "Living beings carry out their metabolic activities within an optimal pH range."

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• Key concept tested: pH affects biological/microbial activity — a core idea from Ch. 2.
• (a) Links alkaline pH → neutralises acid produced by bacteria → preservation. Don't just say "alkaline stops bacteria"; explain why (neutralisation of acid).
• (b) The logic is sequential: bacteria must first overcome the alkalinity before acidifying the milk enough to set curd — hence longer time.
• (c) Ties back to the textbook line: "Living beings carry out their metabolic activities within an optimal pH range." Examiners expect this line or its paraphrase.
• Avoid writing vague answers like "bacteria don't work in alkaline conditions" — always explain the pH mechanism.