Science — AI-generated practice question

The pH of milk decreases as it turns into curd.

Fresh milk has a pH of 6. When milk turns into curd, bacteria present in the milk convert lactose (milk sugar) into lactic acid. As lactic acid is produced, the concentration of H⁺ ions in the solution increases. Since a higher H⁺ ion concentration means a lower pH value, the pH of milk decreases below 6, making it more acidic.

Source: Chapter 2, Section 2.3.1 (Table 2.3 — Sour milk/curd contains lactic acid)

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• The key facts examiners look for: (1) pH decreases, (2) lactic acid is formed by bacteria, (3) lactic acid increases H⁺ concentration, (4) higher H⁺ = lower pH.
• Table 2.3 in the textbook explicitly lists "Sour milk (Curd) → Lactic acid," so mentioning lactic acid is essential.
• Don't just state the answer — you must explain the reason (acid formation → H⁺ ions → lower pH) to earn all 3 marks.