Fresh milk has a pH of 6. To delay its curdling, a chemical substance is added to it, which is :
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
(d) Baking soda (Sodium hydrogen carbonate)
Fresh milk has pH 6 (slightly acidic). Adding baking soda raises the pH to slightly alkaline, which delays the formation of lactic acid (curdling). As it is a mild base, it neutralises the developing acidity.
Source: Acids, Bases and Salts, Section 2.3.1 / Exercise Q.12
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Explanation
The question is directly based on Exercise Q.12 of Chapter 2. The key logic is:
- Curdling occurs when bacteria produce lactic acid, lowering pH further.
- Adding a mild base (baking soda / NaHCO₃) shifts pH to slightly alkaline, slowing acid build-up and delaying curdling.
- Sodium carbonate (Na₂CO₃) and NaOH are too strongly basic and would spoil the milk; baking powder is a mixture, not a pure base. Only baking soda is the appropriate mild base used for this purpose.