Question

Two droplets merge with each other and form a large droplet. In this process:

• A. Energy liberated
• B. Energy absorbed
• C. Neither liberated nor absorbed
• D. Some mass is converted to energy

Answer 1

The correct option is A Energy liberated

We know that the volume of sphere is $\frac{4}{3}\pi r^3$

We know that the surface area of sphere is $4\pi r^2$

When the two droplets merge, their volume gets added. But their total surface area decreases. For convenience, assume that the 2 droplets are equal in size and their radius is $r.$ Consider the large drop's radius is $R.$

Calculate their before and after surface area. You will realise that their surface area decreases.

In the case of a droplet, it is spherical and its surface is exposed to air. The molecules on the surface experience an inward force and hence the surface tries to contract. Since it cannot contract beyond a point the surface remains under tension. This tension is in the form of surface energy. When the two droplets merge, their surface area decreases and hence total surface energy decreases. The remaining energy is liberated.