A milkman adds a very small amount of baking soda to fresh milk.
Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
The milkman adds baking soda (sodium hydrogencarbonate) to shift the pH of fresh milk from 6 to slightly alkaline because baking soda is a mild non-corrosive basic salt. Being alkaline in nature, it neutralises the slight acidity of fresh milk. This helps in preserving the milk for a longer time by preventing it from turning sour quickly.
Why does this milk take a long time to set as curd?
This milk takes a long time to set as curd because curd formation requires acidic conditions. The bacteria present in milk convert lactose into lactic acid, which helps in setting the curd. However, when baking soda is added, it makes the milk alkaline and neutralises the acid produced by bacteria. As a result, the milk does not become sufficiently acidic to set as curd quickly, thereby delaying the process.
Explanation
This question tests the student's understanding of pH, acids, bases, and neutralisation reactions. Part (a) requires explaining that baking soda is alkaline and neutralises the mild acidity of fresh milk (pH 6), shifting it towards alkaline to preserve it longer. Part (b) requires understanding that curd formation is an acidic process where bacteria produce lactic acid. The added baking soda neutralises this acid, preventing the pH from dropping to the acidic level needed for curd formation, thus delaying the process. The context clearly establishes that baking soda is 'alkaline' and 'neutralises excess acid'.