Question

Which of the following is not a chemical change?
A. Rusting of iron
B. Converting water into steam
C. Making curd from milk
D. Heating coal

Answer 1

Hint: We need to know what are the criteria to be fulfilled in order to tell whether a reaction involves chemical change or not. Chemical change results when one substance combines with another to form a new substance. The new substance has different properties from that of the substances reacting. They are generally irreversible but there are instances where they can be reversed by further chemical reactions.

Complete step by step answer:
Let us evaluate the given changes,
Rusting of iron: Rusting of iron is the property of iron to react with oxygen (mostly atmospheric oxygen or water) to form a new iron oxide called rust. Hence it is a chemical change as the new substance formed is rust.
Converting water into steam: Water is converted to steam by boiling which is a physical phenomenon. The composition of water and steam is the same hence it is a physical process and not chemical.
Making from milk curd: Making from milk curd is a chemical process as a new substance known as lactic acid is formed. It has a different taste from that of milk and the curd formed cannot be reversed to form milk.
Heating coal: The main composition of coal is carbon. When coal is heated or burnt, the carbon gets converted to carbon dioxide. Since a new substance is formed through the reaction and carbon cannot be reobtained from it, it is considered to be a chemical change.

So, the correct answer is Option B.

Note: It must be noted that chemical change requires a molecular level change to stop it from being reversed because it creates a new substance. Some properties to be kept in mind to observe a chemical change is colour change, precipitate formation, gas formation, smell change, change in temperature. In other words, a reaction is a chemical reaction involving the rearrangement of atoms.