Question

Give reason why liquids have one free upper surface only.

Answer 1

The free surface of a liquid just means the surface that is in contact with air. Like the horizontal surface of water in a glass. Or the top of the ocean where boats float.

If you are on the topic of surface tension, then a soap bubble has two free surfaces, the outside and the inside. these are free surfaces that are in contact with air.

A bubble inside a liquid has one free surface. That's just the place that air is touching the liquid