**Question:**

**The surface tension and vapour pressure of water at 20oC is 7.28 × 10-2 N/m and 2.33 × 103 Pa, respectively. What is the radius of the smallest spherical water droplet which can form without evaporating at 20oC?**

**Solution:**

Surface tension of water, T = 7.28 × 10-2 N/m

Vapour pressure, P = 2.33 × 103 Pa

Radius of drop = r

2T/r is the excess pressure which is greater than the vapour pressure.

Vapour pressure = excess pressure in drop

Using the above relation, we can calculate r = 6.25 × 10-5 m