Question

Assertion: A small drop of mercury is spherical but bigger drops are oval in shape.
Reason: Surface tension of liquid decreases with an increase in temperature.
(A) Both Assertion and Reason are correct and Reason is the correct explanation of 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

Hint: Mercury is the only metal that will be in a liquid state at ordinary temperatures. Mercury is also called quicksilver. Mercury is a heavy, silvery-white liquid metal. The surface tension of the mercury is even higher of $4.6 \times {10^{ - 1}}\, J{m^{ - 2}}$, this is due to the even stronger metallic bonds between the atoms of mercury.

Complete step by step solution:
About Assertion:
Two kinds of forces acting upon the droplet are:
1. Surface tension forces, which tend to keep the drop form into a ball, in which it has its lowest surface energy.
2. Gravitational forces, which tend to flatten the drop, to make its gravitational potential energy as small as possible.
If it is a small drop of mercury, Due to surface tension the gravitational potential energy is negligible in comparison to the potential energy. Consequently, to maintain the drop in equilibrium, the mercury drop’s surface tends to shrink so that the mercury’s surface area will be the least to form a sphere and so that the drops will be spherical.
If it is a bigger drop of mercury, Due to surface tension the potential energy due to gravity is predominant over the potential energy. Consequently, to maintain the drop in equilibrium let the mercury drop tends to assume minimum potential energy as possible so that the drop becomes oval in shape and lower center of gravity.

About Reason:
By increasing heat energy, the molecules which present in the element gain the excess kinetic energy and move forcefully and by increasing intermolecular space between them, the intermolecular force decreases. Thus, the surface tension of liquid decreases with increasing intermolecular space between the molecules and decreasing the intermolecular force of attraction between the molecules due to increased supplement of heat energy.

$\therefore$ Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion. Hence, option (B) is correct.

Note:
Metallic mercury is used in many household products, such as thermometers, fluorescent light bulbs, and barometers. In these devices, the mercury is trapped. If mercury has reached surface waters or soils then the microorganisms can convert the mercury to methylmercury, a substance that can be absorbed quickly by most of the organisms and it will cause nerve damage.