Question

Assertion: When a drop of ink falls on a newspaper, it spreads on it.
Reason: Adhesive force between ink and paper is greater than cohesive force between ink molecules
A. Both Assertion and Reason are correct and Reason I the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

This is a reasoning question that consists of two statements that is an assertion which gives the statement of fact and a reason which gives explanation for the assertion. If the assertion is correct then the reason for the assertion should be evaluated.The concept of adhesive force and cohesive force will be utilised to check the reason.

Complete answer:
ADHESIVE FORCE: The force of attraction between two different molecules. For example, adhesive force is the attraction between water molecules and air molecules. In the case of a liquid wetting agent, adhesive force or adhesion causes the liquid to cling to the surface that is to hold to the surface on which it rests.

When water is poured on clean glass, it tends to spread and form a thin and uniform film over the surface of glass. This is due to adhesive forces between water and glass which are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the water.

COHESIVE FORCE: The force of attraction between two similar molecules. For example, attraction between water molecules.For instance, rain falls in the form of droplets rather than a fine mist, because water has a strong cohesive force or cohesion, which pulls its molecules tightly together which forms droplets.

This force is responsible for uniting the molecules of a liquid. Therefore, when a drop of ink falls on a newspaper, ink will spread on paper and the ink spreads on the paper because adhesive force between ink and paper is greater than cohesive force between ink molecules.

Hence, the correct option is A.

Note: The adhesive force is the attractive force between unlike substances whereas cohesive force is between intermolecular forces. In this problem it should be noted that the attractive force is between ink and paper whereas repulsive force is between ink molecules.