Science — AI-generated practice question

When milk turns into curd, its pH decreases (becomes more acidic, below 6). This is because bacteria in milk convert lactose into lactic acid, increasing H⁺ ion concentration.

(a) The milkman adds baking soda (a base) to shift the pH of fresh milk from slightly acidic (6) to slightly alkaline. This prevents the milk from turning sour quickly, increasing its shelf life.

(b) Since baking soda makes the milk alkaline, the lactic acid produced by bacteria must first neutralise the added base before the pH can drop enough for the milk to set as curd. This neutralisation takes extra time, so the milk takes longer to set.

Source: Acids, Bases and Salts (Chapter 2)

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• Key link: pH below 7 = acidic; curd formation = lactic acid production → pH falls.
• Examiner expects three clear points: (1) pH drops when milk → curd due to lactic acid, (2) baking soda raises pH to preserve milk, (3) longer setting time = lactic acid must neutralise the base first.
• Avoid writing only "pH increases" for the main intro — it decreases (becomes more acidic).
• Exercise Q11 and Q12 in Chapter 2 directly match this question, so use that language.