Why does the milkman add a very small amount of baking soda to fresh milk?

Correct — C. Fresh milk is slightly acidic, about pH 6. Baking soda — sodium hydrogencarbonate, NaHCO₃ — is a mild base, so a pinch of it raises the pH to slightly alkaline.

Milk sours because bacteria turn lactose into lactic acid. Starting from an alkaline point, that acid has to be neutralised before the pH can fall far enough to curdle the milk, so the milk keeps longer.

The cost shows up later: the same milk then takes longer to set as curd.

Why the others are wrong

• (a)
  To maintain the pH of the fresh milk at 6 for a longer time — Baking soda does not hold milk at pH 6. It is a base, so it moves the pH upward; an option in which the pH is unchanged describes no reaction at all.
• (b)
  To reduce the pH of the fresh milk from 6 to slightly alkaline — This option contradicts itself. Reducing the pH from 6 makes milk more acidic, while alkaline means a pH above 7 — a rise, not a fall.
• (d)
  To reduce the pH of the fresh milk from 6 to slightly more acidic — Making milk more acidic would hurry the curdling along, the opposite of what a milkman wants, and baking soda is basic, so the pH cannot fall.

The whole answer sits in one line of the acid-base chapter: baking soda is a base, so it raises pH.

The pH scale runs from 0 to 14 with 7 neutral — below 7 acidic, above 7 alkaline. Fresh milk sits just under neutral at about 6.

Souring is bacterial: lactic acid bacteria convert lactose into lactic acid, and the falling pH clots the milk protein. Beginning a little on the alkaline side buys time before that point is reached.

Key facts

• Fresh milk has a pH of about 6, slightly acidic.
• Baking soda is sodium hydrogencarbonate, NaHCO₃, a mild base.
• Milk sours when bacteria convert lactose into lactic acid, lowering the pH.
• On the pH scale 7 is neutral, below 7 is acidic and above 7 is alkaline.

Common traps

• Option (b) pairs 'reduce' with 'alkaline', which cannot both be true — the direction word kills it before any chemistry is needed.
• Baking soda is a mild base, so the milk becomes slightly alkaline rather than strongly so.

No directly related past PYQ was found.