Question

Assertion: A smaller drop of mercury is spherical but bigger drops are oval in shape.
Reason: Surface tension of a liquid decreases with increases in temperature.
A.)Both assertion and reason are correct and reason is the correct explanation for assertion,
B.)Both assertion and reason are correct but reason is not the correct explanation for the assertion.
C.)Assertion is correct but reason is incorrect.
D.)Assertion is incorrect but reason is correct

Answer 1

Hint: The shape of a drop depends upon the ability of the surface tension to counteract the normal forces on the drop and its weight. The surface tension tries to reduce the surface area of the drop while the weight and normal forces have an increasing effect on the surface area.

Complete step by step answer:
The surface tension in a liquid tries to pull the surface closer together and hence tries to decrease the surface area of the drop. A sphere is a shape that has the smallest ratio of its surface area to its volume and hence, surface tension tries to shape a liquid drop in the form of a sphere. However, the weight and normal forces acting on the drop try to pull apart the compact shape of the drop and increase the surface area in this process.

For smaller drops, the weight is not that much and hence, the surface tension can counter the weight easily and form spherical drops. However, for larger drops, the weight of the drop will be higher and surface tension cannot counter this large weight and gives in to the weight force. Therefore, the compact spherical shape is disturbed to form an oval shape.

Hence, the assertion is correct.

It is indeed true that the surface tension of a liquid decreases with increase in temperature.
Hence, the reason is also true but as understood from the explanation above, it is not the correct explanation for the assertion.

Hence the correct option is B) Both assertion and reason are correct but reason is not the correct explanation for the assertion.

Note: Students can remember that the surface tension of a liquid decreases with increase in temperature by keeping in mind that hot water cleans clothes better. For cleaning clothes it is imperative that the surface tension is reduced (which is one of the reasons why soap is added), so that dirt can easily break the surface and be removed from the clothes.
Another interesting fact is that if no normal forces or gravity were to act on a liquid drop, it would be perfectly spherical regardless of its volume and size. This phenomenon can often be seen in space by astronauts.