Question

A scout on a hike becomes lost. He does not have a compass, but he does have a small needle, a magnet, and some water. He magnetizes the needle and then gently places it on the surface of the water, where it points to the north. What property of the water makes it possible for the needle to lie on the surface of the water?
A. viscosity
B. definite volume
C. surface tension
D. boiling point

Answer 1

Answer:

C. surface tension

Explanation:

Surface tension refers to the property of water or any liquid to enhance its surface area by a certain amount. It is a very important property of water due to which it resists  any external force due to cohesive nature of water molecules.

How surface tension help needle to float on water:

It is a property of water which enhance the attractive force between needle and liquid.

The molecules on the surface of water do not have any force acting from upwards because that side is exposed to atmosphere. This causes a downward force but no upward force on upper surface,so the molecule of surface tend to stay together by forming a thin sheet or imaginary layer.

If weight of any material cannot break surface tension it will float on water surface. This is what happens with needle.Therefore, when needle is placed on water surface with alot of care in horizontal direction, it will not break surface tension and hence will float on water. Please see attached image for better understanding.

Hope it clears up the picture!