Science (086) — AI-generated practice question

pH change as milk turns to curd: The pH decreases (becomes more acidic). During the formation of curd, bacteria act on milk and produce lactic acid. This increases the H⁺ ion concentration, lowering the pH below 6.

Effect of baking soda added by the milkman:

(a) Baking soda is alkaline, so it raises the pH of fresh milk from 6 to slightly alkaline. This makes the milk last longer before turning sour.

(b) Since lactic acid is produced during curd formation, the added baking soda neutralises this acid. The milk therefore takes a longer time to set as curd, because the acid produced must first neutralise the baking soda before the pH can drop enough for curdling to occur.

Source: Chapter 2 — Acids, Bases and Salts; Section 2.3.1 and Exercise Q.11–12

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• Examiners expect students to name lactic acid as the cause of pH decrease — this is a key fact from the chapter.
• For the baking soda part, the core idea is neutralisation: baking soda (basic) neutralises the lactic acid produced, delaying the pH drop needed for curdling. Mentioning this mechanism earns full marks.
• Table 2.3 confirms lactic acid is present in sour milk/curd, supporting the answer.
• Don't just say "pH increases/decreases" — always explain why using ion concentration or chemical reasoning.