Question

A piece of iron sinks in water but can float in mercury. True or false
A) True
B) False.

Answer 1

Hint: When we place something in a liquid, it can either sink or float. Every fluid exerts an upward buoyant force on an object that is immersed in the liquid. This buoyant force depends on the density of the liquid and the volume of the object immersed in the liquid. If the buoyant force is greater than the weight of the object, it will float and vice versa.

Complete answer:
Whenever a body is immersed in water, there are two forces that act on it. One is the gravitational force that attracts the object downwards that is also known as the weight of the object. However, when the object is placed in the liquid, there will be buoyant force acting on the object such that it will push the object upwards.
The magnitudes of the buoyant force depend on the density of the liquid and the volume of the object immersed in the liquid. Whether an object will sink or float in a liquid depends on whether the density of the liquid is greater than or lower than the density of the object.
A piece of iron is denser than water so it will sink in the water as no amount of buoyant force will be able to balance the weight of the piece of iron, however when placed in mercury, mercury being denser than the iron piece, the iron piece will float in mercury.
This is because the mercury will have enough buoyant force to balance the weight of the piece of iron.
So, the statement is true that choice (A) is correct.

Note:
While we don’t have to remember the densities of all materials, we should remember the densities of water and mercury as they are the most widely used liquids. The densities of water, mercury, and iron are given below.
Water: $1\,g/c{m^3}$
Mercury: $13.6\,g/c{m^3}$
Iron: $7.8\,g/c{m^3}$.