Question

Making of curd from milk is a physical change.How?

Answer 1

Making of curd from milk is a chemical change.The bacteria Lactobacillus when added to milk converts it into curd.The bacteria breakdown the lactose in milk to lactic acid.This increases the acidity of milk due to which the milk protein casein coagulates as curd.This process is known as fermentation and requires a temperature of 35 - 40 degree Celcius.

The chemical reaction involved is:

C₁₂H₂₂O₁₁ (lactose) + H₂O (water) → 4 CH₃CHOHCOOH (lactic acid)