Science — AI-generated practice question

(a) Fresh milk has a pH of 6 (slightly acidic). The milkman adds baking soda (NaHCO₃) to shift the pH to slightly alkaline. This prevents the milk from turning sour quickly, as bacteria that cause souring (by producing lactic acid) are inhibited in an alkaline medium, thus increasing the shelf life of milk.

(b) Curd formation requires the action of lactic acid bacteria, which lower the pH of milk by producing lactic acid. Since the milk is now alkaline due to added baking soda, the bacteria need more time to first neutralise the base and then make the milk acidic enough to set into curd. Hence, this milk takes a longer time to set.

Source: Acids, Bases and Salts (Chapter 2), Exercise Q.12

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• (a) Key idea: alkaline pH inhibits the bacteria that cause souring, extending freshness. Examiners want: reason for shifting pH (prevent souring / spoilage).
• (b) Key idea: the lactic acid bacteria must first neutralise the added base before the pH can drop enough for curd to set — that extra step takes more time.
• Do not confuse the two parts: (a) is about why he makes it alkaline; (b) is about the consequence for curd formation.
• This is directly from Exercise Q.12 of Chapter 2; knowing Q.11 (pH drops as milk turns to curd) helps link both parts.