CAPF AC 2021 Q15: Science & Technology – Chemistry

CAPF AC 2021, Paper I (General Ability and Intelligence) · Science & Technology

1. (a) strongly alkaline
2. (b) weakly alkaline
3. (c) strongly acidic
4. (d) weakly acidic

Answer: (b) weakly alkaline

Explanation

Baking soda added to milk

How to think: first fix the pH of fresh milk, then see what baking soda does to it.

Fresh milk has a pH of about 6, which is slightly acidic.

Baking soda (sodium hydrogencarbonate, NaHCO₃) is a mild base.

A mild base in a small amount can only push the pH a little above 7.

Why the milkman does it: milk turns sour when bacteria make lactic acid.

In alkaline milk the acid is neutralised as it forms, so the milk does not sour quickly.

Such milk also takes longer to set as curd.

Answer: (b) weakly alkaline.

Why each option is right or wrong

• (a) Incorrect. Baking soda is a mild base and only a small amount is added.
• (b) CORRECT. The pH moves from about 6 to slightly alkaline.
• (c) Incorrect. A base cannot make milk more acidic.
• (d) Incorrect. Weakly acidic is fresh milk before the baking soda is added.

Key point

Fresh milk pH ≈ 6. A little baking soda (mild base) makes it slightly alkaline, so lactic acid is neutralised and the milk does not sour quickly; it also takes longer to set as curd.

Remember

Baking soda in milk: slightly alkaline, sours late.

Common mistakes

• Marking weakly acidic, which is the state of fresh milk before the addition.
• Calling baking soda a strong base; caustic soda (NaOH) is the strong one.
• Confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃).

Exam tip

Educated guess: a base raises pH, so both acidic options go. A small amount of a mild base cannot make anything strongly alkaline.

Sources

• NCERT Class X Science — Acids, Bases and Salts (exercise question on the milkman and baking soda)
• NCERT Class VII Science — Acids, Bases and Salts (baking soda as a base)

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