Britannica Sources used in this answer

Why can liquids form drops and free surfaces while gases expand to fill a container?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

The difference in behavior between liquids and gases arises from the strength of intermolecular forces relative to the kinetic energy of their constituent particles.

Gases

In a gas, particles possess high kinetic energy and are separated by distances large compared to their size. Because these particles move rapidly and interact only through occasional collisions, they are not bound to one another. Consequently, gases do not maintain a fixed volume or shape and will expand to fill any available container.123

Liquids

In a liquid, particles are packed closely together, and their kinetic energy is low enough that attractive intermolecular forces can overcome the tendency of the particles to move apart. These cohesive forces allow liquids to maintain a definite volume and form a free surface.345

The formation of drops and the existence of a surface are driven by surface tension, a phenomenon where molecules at the surface are pulled inward by the bulk of the liquid. This effect acts like a stretched elastic membrane, tending to minimize the surface area—which, in the absence of external forces, results in a spherical shape.67