Question

Which among the following are physical or chemical changes?
a. Evaporation of petrol
b. Burning of liquefied petroleum gas
c. Heating of an iron rod to red hot
d. Curdling of milk
e. Sublimation of solid ammonium chloride
A. a, b, and c are physical changes d and e are chemical changes
B. d, b, e are physical changes a, c are chemical changes
C. a, b, e are physical changes c and d are chemical changes
D. a, c and e are physical changes and b, d are chemical changes

Answer 1

Hint: Physical change only changes the appearance of the substance and does not alter the chemical composition of the substance. But chemical changes cause alteration in its chemical composition.

Complete step by step answer:
Chemical change alters the chemical composition of the substance for example chemical change changes the chemical formula of substance also. A few examples of chemical change are digestion of food, burning of coal, rusting and so on.
A Physical change affects only physical properties such as shape, size and so on. Some examples of physical change are freezing of water, melting of wax, boiling of water and so on.
From the above options;
Curdling of milk involves the alteration in chemical composition. In curdling of milk the protein molecules, mainly casein, repel each other so they naturally distribute evenly through the liquid.
Hence, it is a chemical change.
Burning of liquefied petroleum gas is a chemical change because on heating it produces carbon dioxide and water.
Hence, it is a chemical change.
Evaporation of petrol does not involve any chemical reaction. It only gets converted from one physical state to another physical state.
Hence, it is a physical change.
Heating of an iron rod to red hot is physical change because in heating it only gets changes into red hot and no any chemical change.
Hence, it is a physical change.
Sublimation of ammonium chloride does not involve any chemical reaction. It only gets converted from one physical state that is solid to another physical state that is gas.
Hence, it is also a physical change.

So, the correct answer is Option D.

Note: A chemical change is a permanent change. For example curdling of milk, after milk gets converted into curd we can not reverse that process. But physical change is reversible for example an ice cube can be converted into liquid and also the liquid water can get converted into solid form that is ice.