Question 13: Properties of Matter

Question

Given below are two statements :
Statement I : Pressure of a fluid is exerted only on a solid surface in contact as the fluid-pressure does not exist everywhere in a still fluid.
Statement II : Excess potential energy of the molecules on the surface of a liquid, when compared to interior, results in surface tension.
In the light of the above statements, choose the correct answer from the options given below

Accepted Answer

Statement I
Consider an imaginary cube of water out of still water in a container,
This tiny cube has mass, so gravity is pulling it down. If nothing were pushing back up against it, it would fall to the bottom. However, the fluid is still (in static equilibrium). This means the net force on our imaginary cube must be zero.
The water below our imaginary cube must be pushing up on it to counteract gravity. Similarly, the water on the sides is pushing inward to keep it from spreading.
This pushing force per unit area is exactly what pressure is. Therefore, the surrounding fluid exerts pressure on our imaginary surface inside the bulk of the liquid. Pressure is a scalar property that exists at every single point within a fluid, not just where it touches a solid container.
Therefore, the statement I is false.
Statement II
A molecule deep in the interior is completely surrounded by other identical molecules on all sides. The attractive forces pull it equally in every direction. The net force on an interior molecule is zero.
A molecule on the surface has molecules next to it and below it, but practically none above it. Because it is missing half of its neighbours, there is a net inward force pulling the surface molecule down into the bulk of the liquid.
Now, if it is to bring an interior molecule up to the surface to create more surface area. It has to be pulled against that net inward force. And whenever an object moves against an opposing force by external force, then work must be done.
By the Work-Energy Theorem, the work is stored as Potential Energy. Therefore, molecules on the surface possess higher potential energy than those in the bulk interior.
Any system always prefers the lowest possible energy state. Because having surface molecules costs extra energy, a liquid will naturally try to minimize the number of molecules on its surface. It does this by minimizing its overall surface area. This macroscopic tendency of a liquid surface to shrink and act like a stretched elastic membrane is exactly what we call surface tension.
Therefore, the statement II is true.
So, statement I is false, and Statement II is true.
Hence, the correct option is (A).

Answer

Statement I is false but Statement II is true

Answer

Statement I is true but Statement II is false

Answer

Both Statement I and Statement II are false

Answer

Both Statement I and Statement II are true

Properties of Matter

Choose the correct answer: