Based on the given figure, the number of correct statement/s is/are __________.
A. Surface tension is the outcome of equal attractive and repulsive forces acting on the liquid molecule in bulk.
B. Surface tension is due to uneven forces acting on the molecules present on the surface.
C. The molecule in the bulk can never come to the liquid surface.
D. The molecules on the surface are responsible for vapour pressure if the system is a closed system.

Solution

Chemistry ma’am · 3 steps

Compare the two marked molecules: the bulk one is pulled equally all round, the surface one inward.

1. Read the figure. The molecule in the bulk has neighbours on every side, so the attractive pulls cancel and the net force is zero. The molecule on the surface has no liquid above it, so it feels a net inward pull.
2. Where tension comes from. That unbalanced inward pull makes the liquid shrink its surface area. This is the origin of surface tension. It comes from the surface molecules, not from the balanced molecules in the bulk.

   Statement	Check	Verdict
   A	Balanced forces in the bulk give a net force of zero, so they cannot produce surface tension	Incorrect
   B	Surface molecules feel uneven forces, with a net pull inward	Correct
   C	Molecules move all the time, so bulk and surface molecules keep exchanging places	Incorrect
   D	Only surface molecules can escape into the vapour. In a closed container they set up the vapour pressure	Correct

3. So. Statements B and D are correct, so the number of correct statements is 2.

The common slip: 3. Accepting A because the forces in the bulk really are balanced gives 3. Balanced forces add up to zero net force, so they cannot be the cause of surface tension.

The idea to keep. Surface molecules are pulled unevenly, which gives surface tension, and they are the ones that escape, which gives vapour pressure. Bulk molecules feel balanced forces and keep swapping places with them.

Answer from the NTA final answer key. Solution by PrepWiser.