Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Surface tension is the result of cohesive forces between molecules in the bulk of a liquid.
Reason (R) : Surface tension is a measure of the energy required to increase the surface area of a liquid due to the attractive forces between molecules at the surface.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is NOT the correct explanation of (A).
• B. Both (A) and (R) are true and (R) is the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To understand surface tension molecular forces:
- In the bulk: Balanced cohesive forces \(\rightarrow\) Net force is zero.
- At the surface: Unbalanced cohesive forces \(\rightarrow\) Net force is inward.
This net inward pull is the direct cause of surface tension.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Surface tension is a surface property of liquids that arises due to cohesive forces between molecules.
It represents the tendency of liquid surfaces to shrink into the minimum possible surface area.

Step 2: Detailed Explanation:
Let us analyze the correctness of the Assertion and Reason:
- Assertion (A) is false:
The molecules in the bulk of a liquid are surrounded by other liquid molecules on all sides.
This results in equal attractive (cohesive) forces in all directions, making the net cohesive force acting on a bulk molecule exactly zero.
However, molecules at the surface of the liquid are only attracted by molecules below and adjacent to them.
This results in an unbalanced, net inward cohesive force acting on the surface molecules.
This net inward pull creates surface tension.
Therefore, stating that surface tension is the result of cohesive forces "in the bulk" is scientifically incorrect; it is caused by the *unbalanced* cohesive forces acting on surface molecules.
- Reason (R) is true:
Due to the net inward pull on surface molecules, bringing a molecule from the bulk to the surface requires work to be done against these attractive forces.
The energy required to increase the surface area of a liquid by a unit amount is called surface energy, and its magnitude is equal to the surface tension.
Thus, surface tension is indeed a measure of the energy required to increase the surface area.
Therefore, (A) is false, but (R) is true.

Step 3: Final Answer:
Assertion (A) is false but Reason (R) is true.