A piece of iron (specific gravity = 7.8) sinks in water, but floats on mercury (specific gravity = 13.6) - explain why.

Explanation

The density of iron is `7.8g*cm^(-3)` and the density of water is `1g*cm^(-3)`. The density of iron is greater than that of water and hence the weight of a piece of iron is greater than the upthrust of water due to the action of the resultant downward force. But the density of mercury is `13.6g*cm^(-3)` which is greater than the density of iron. Hence, the weight of the piece of iron is less than the upthrust exerted on it by mercury. As a result, the piece of iron floats on mercury due to the action of the resultant upward force.