Question

Conversion of milk into curd is:
A. a physical property of milk
B. a chemical property of milk
C. a physical change
D. a chemical change

Answer 1

Hint: Think about whether milk spontaneously converts into milk due to attributes of its own composition or does it need some external incentive. Also think about whether the change is reversible or irreversible.

Complete step by step solution:
Physical properties include factors like colour, texture, odour, melting point, boiling point, etc. They can be measured without changing the material chemically.
Chemical properties are the qualities a material has that can be observed only when it is changed chemically. Toxicity, flammability, heat of combustion, etc.
Properties of materials are inherent. When milk is converted into curd, the globular protein casein coagulates into curd. This is not an inherent property of casein, it needs some external, acidic conditions for its coagulation like lemon juice or vinegar. Hence it is not a physical or chemical property of milk.
A physical change is usually reversible and chemical change is not. After milk is converted into curd, the change is not reversible since the acidity of the medium in which the casein coagulates cannot be reversed.

Hence, the answer is ‘D. a chemical change’

Additional information:
When the bacterium Lactobacillus is added to milk, it breaks down the lactose present in the milk to lactic acid. This increases the acidity of milk and urges the casein to coagulate. Lactobacillus uses lactose to fulfill its energy requirements. Hence, this change is irreversible.

Note: Please note that although the coagulation of casein may be a physical change, the conditions that are required for it to coagulate arise due to a chemical change. Thus, the answer is ‘D. a chemical change’ and not ‘C. a physical change’.