Question

Radha kept milk for curdling in a copper vessel while Priya used a steel vessel for the same. Which of the two would be edible and why?

Answer 1

Curd is acidic in nature as its pH is less than 7. We know that curd is acidic due to the presence of lactic acid ${{C_3}{H_6}{O_3}}$. Acidic compounds react with metals to give salt and hydrogen gas.

Complete step by step answer: Firstly, we will learn about the concept of curdling of milk in terms of chemistry. Milk is slightly acidic in nature but when pH is lowered more by adding another acidic ingredient, protein molecules present in the milk stop repelling one another which allows them to stick together forming clumps termed as curd.
We have already learnt that curd is acidic in nature due to the presence of lactic acid. When curd is kept in metallic containers or vessels, it will react with metal to give toxic substances, which is unfit for our health.
Copper is a metal and it possesses all the metallic properties like malleability, lustrous, ductility, strong and many more. When a metal reacts with an acid or acidic compound, it produces salt and hydrogen gas (${H_2}$).
${\rm{Metal}} + {\rm{Acid}} \to {\rm{Salt}} + {\rm{hydrogen}}\;{\rm{gas}}$
The hydrogen gas (${H_2}$), which is produced after the reaction is poisonous if it combines with metals present in our body i.e. potassium, sodium, iron etc.
Therefore, we can store curd in steel vessels for a few days, as steel is not a metal it is an alloy, which does not react with curd. Hence, Priya’s curd would be edible.

Note:
We need to remember one important thing about curdling milk, which is denaturation of proteins. Denaturation of protein in curdling of milk occurs due to the formation of lactic acid (from the lactose present in milk).