gaseous

Imagine blowing up a balloon—the air filling it is gaseous. This word describes matter that exists as a gas rather than a solid or a liquid. Think of the steam swirling above a hot cup of tea; that's water in its gaseous form! While many gases are invisible, like the oxygen we breathe, they are always active, spreading out to fill whatever space is available.

A "gaseous planet" like Jupiter or Saturn is made mostly of hydrogen and helium, unlike a rocky planet such as Earth. You might also meet the word in a safety warning: if gas is leaking in a kitchen, it can spread quickly through the air in a gaseous form.

The word "gaseous" describes a state of matter where molecules move freely and rapidly, having no fixed shape or volume. Unlike solids (like ice) or liquids (like water), gaseous substances expand to fill any container they occupy.

Our atmosphere is a primary example, consisting of a gaseous mixture of nitrogen, oxygen, and other trace gases. You can see this state of matter in action when you notice bubbles in a carbonated drink; those are pockets of gaseous carbon dioxide. When a liquid turns into a gas, the process is called vaporization (like boiling water), and when a solid skips the liquid phase to become a gas, it is called sublimation (as with dry ice).

The term is most common in scientific fields like chemistry and astronomy. For instance, astronomers study gaseous nebulae, which are massive clouds of gas and dust in space where stars are born. While "gaseous" can sometimes suggest something airy, it usually has a technical or literal meaning. Whether you are discussing the atmosphere of a planet, steam rising from a kettle, or a leak in a pipe, "gaseous" refers to something existing in its free-flowing state as a gas.