Q37
General SciencePhysics
Two drops of liquid merge to form a single drop. In this process energy is
- a.absorbed
- b.released
- c.may be absorbed or released depending upon the specific heat of the liquid
- d.neither absorbed nor released
Answer: (B) released
When two drops merge, the total volume stays the same but the total surface area falls. Two drops of radius $r$ have area $8\pi r^2$, while the merged drop of radius $2^{1/3}r$ has area of only about $6.35\pi r^2$. Since surface energy = surface tension × area, the surplus energy is released, mostly as heat, and the liquid warms slightly. The reverse also holds: breaking one drop into many needs work, so energy is absorbed. Specific heat has nothing to do with the direction of the energy change. Surface tension is also why free drops are spherical (least area for a given volume); it falls as temperature rises or when a detergent is added.