Question

Assertion: More is the cohesive force; more is the surface tension.
Reason: More cohesive force leads to more shrinking of liquid surface.
Which of the following is correct for the above given assertion and reason?
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Assertion is incorrect but reason is correct.

Answer 1

Hint: The cohesive force is directly proportional to the physical phenomenon of the liquid. Thus, when the cohesive force is more it leads to more surface tension. But the molecules at the surface don't have that much cohesive force attracted to the molecules placed inside. Hence, those molecules on the surface cohere strongly those directly associated with them on the surface. This leads to more shrinking of the liquid surface.

Step by step solution:
(i) Cohesive force:
The cohesive force is defined as the force of intermolecular attraction between the portion of the substance such as solid or liquid. It is more in solids and comparatively low in liquid.

(ii) Surface tension:
The surface tension is defined as the tension on the surface of the liquid which is due to the force of attraction of the molecules in the surface by the liquid molecules present below the surface. It leads to minimizing the surface area.

Hence, from (i) and (ii) it is clear that the cohesive force is the factor which leads to the surface tension. Thus, when there is an increase in the cohesive force leads to the increase in the surface tension.

When, the cohesive force between the molecules increases the force of attraction between the molecules tends to increase. Thus, the molecules on the surface will have more force of attraction on other molecules which are directly associated on the surface of the liquid. So, the increase in cohesive force leads to more shrinking of the liquid surface.

Hence, the option (A) is correct.

Note: The cohesive force is the cause for the surface tension of the liquid, thus an increase in the cohesive force leads to increase in the surface tension. The molecules on the surface exerts the cohesive force partially on the liquid molecules below the surface and the remaining force will exert on the substance which gets in contact with the surface. Thus, the cohesive force plays a role in shrinking of the liquid surface.