Question

Why will a block of iron float in mercury but sink in water?

Answer 1

Hint: To answer this question, you must recall the property of buoyancy of a liquid. When any substance is placed in a liquid, it may either float or sink in the liquid. Each and every liquid has the tendency to exert an upward force on any object that is immersed in it and this force is known as buoyant force. The buoyant force exerted depends on the density of the liquid and on the volume of the object immersed.

Complete step by step solution
If the buoyant force acting on the object is greater than the weight of the object, the object will float and if the buoyant force is less than the weight of the object, it will sink. In other words, if the object has a density lower than that of the liquid, it would float and vice versa.
We know that the density of iron is greater than water, hence it sinks in water whereas, the density of iron is lower as compared to mercury. As a result a block of iron would float on the surface of mercury.

Note
A body when immersed in a liquid, is acted upon by two forces. One is the gravitational force of the earth that attracts it downwards in the liquid. This can also be known as the weight of the object. The second force that acts on the object is the buoyant force. The buoyant force acts on the object upwards and thus acts in the opposite direction to the force of gravitation of the object.