The common sight of a white plume rising from a boiling kettle or a hot shower has led to a widespread misunderstanding about the state of matter known as “steam.” While this visible cloud is what most people instinctively call steam, the scientific reality is more complex and depends on the precise definition of a gas. Answering whether steam is a gas requires clarifying the difference between the physical state of water molecules and the visual phenomenon we observe.
The Scientific Definition of a Gas
A gas is a state of matter characterized by the complete separation and high energy of its constituent molecules. These molecules move chaotically and rapidly, possessing neither a fixed volume nor a fixed shape. Because the distance between individual gas molecules is vast compared to their size, the intermolecular forces holding them together are exceedingly weak, allowing them to expand indefinitely to fill any container.
The most distinguishing characteristic of a gas is its transparency. Gases are, by definition, invisible to the naked eye because their molecules are too small and too far apart to effectively interact with and scatter visible light. When light passes through a pure gas, it encounters no large surfaces to reflect off of, which is why air, a mixture of gases like nitrogen and oxygen, appears perfectly clear. This rule of invisibility applies to all pure gases.
Water Vapor: The Invisible Gas
Applying the scientific definition of a gas to water, the true gaseous form of $\text{H}_2\text{O}$ is called water vapor. This invisible gas is formed when liquid water absorbs enough thermal energy to break the hydrogen bonds holding the molecules together, allowing them to escape the liquid surface. While this phase change occurs when water reaches its boiling point of 100 degrees Celsius at standard atmospheric pressure, evaporation allows water vapor to exist at lower temperatures as well.
True steam is scientifically synonymous with water vapor that is at or above the boiling temperature, and it is completely transparent. If one were to observe the immediate opening of a kettle spout, there is a small, clear gap directly above the nozzle where the water exists as pure, high-temperature gas. This invisible region is the actual gaseous state of water, before it has mixed significantly with the surrounding air.
Why We See the White Cloud
The visible white cloud that people typically call “steam” is not a gas, but rather a collection of microscopic liquid water droplets suspended in the air. It forms when the invisible, high-temperature water vapor—the true gas—is rapidly cooled by mixing with the much cooler ambient air.
As the hot gas loses thermal energy, the $\text{H}_2\text{O}$ molecules slow down and cluster together, undergoing condensation to revert back to their liquid state. These tiny liquid droplets are large enough to scatter all wavelengths of visible light equally, which is why the cloud appears white. The white plume is merely a localized cloud, identical to the formation of fog or clouds in the sky.