Science — CBSE Class 10 board question

(a)
(i) During indigestion (excess acid in stomach), we take an antacid such as milk of magnesia (magnesium hydroxide) or sodium bicarbonate to neutralise the excess acid and raise the pH.

(ii) When stung by a honey bee (which injects formic acid/acidic venom) or nettle leaves, we apply a mild base such as baking soda (sodium bicarbonate) solution on the affected area to neutralise the acid and relieve the pain.

(b) When fresh milk (pH 6) changes to curd, its pH will decrease (become more acidic). This is because bacteria in milk convert lactose into lactic acid during fermentation, increasing the acidity of the curd.

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• Part (a) tests the concept of neutralisation: excess acid → use a base; acidic sting → apply a base.
• Honey bee sting is acidic; wasp sting is alkaline (use acid to counteract). Know the difference.
• Part (b): curd formation = lactic acid production → lower pH. Examiners expect both the direction of change AND the reason (lactic acid formation).