Water exists in three fundamental states of matter: liquid, solid (ice), and gas. Matter changes between these states based on its internal energy, a process continuously happening on Earth. While the liquid and solid forms are readily seen in oceans, rivers, and glaciers, water also has a gaseous form. Understanding this third state helps clarify many natural phenomena, from weather patterns to the physics of boiling.
Water Vapor: The Gaseous State
Water can be a gas, and this invisible form is scientifically known as water vapor. Water vapor consists of individual water molecules (H₂O) that are no longer bound by the strong forces holding them in a liquid state. In this gaseous phase, the molecules move rapidly and independently, filling any volume they occupy, much like any other gas.
Water vapor is transparent and odorless, making it impossible to see in its pure form. It is present in the atmosphere at all times and is a major component of the Earth’s hydrologic cycle. Evaporation is the natural transition of liquid water into this gaseous state, occurring even below the boiling point. The amount of water vapor in the air directly relates to humidity.
The Role of Temperature and Energy
The transition of water from a liquid to a gas is driven entirely by the addition of thermal energy, or heat, which increases the kinetic energy of the water molecules. As water is heated, the molecules move faster, overcoming the hydrogen bonds and other intermolecular forces that hold them in the liquid phase. Once the kinetic energy is high enough to break these bonds, the molecules escape into the atmosphere as a gas.
The boiling point marks the specific temperature where this transition happens rapidly throughout the entire liquid. At standard atmospheric pressure, water boils at 100° Celsius (212° Fahrenheit), the point where the water’s vapor pressure equals the surrounding atmospheric pressure. However, even after reaching this temperature, a significant amount of energy is required to complete the phase change.
This additional energy is known as the latent heat of vaporization. This heat is absorbed by the liquid without causing a further increase in temperature. Instead, this energy is used exclusively to break the remaining intermolecular bonds, converting the liquid water into gaseous water vapor. For water, this value is quite high, requiring approximately 40.65 kilojoules per mole to fully transition to a gas at its boiling point.
Distinguishing Steam from Water Vapor
A common source of confusion is the distinction between water vapor and the visible white plume often called “steam,” such as that rising from a boiling kettle. True water vapor is an invisible gas, meaning the visible cloud is not the pure gaseous state. The visible plume is technically an aerosol or a mist, which is a suspension of tiny, condensed droplets of liquid water.
This visible “steam” forms when the hot, invisible water vapor mixes with the cooler surrounding air. The rapid cooling causes the gaseous water molecules to lose kinetic energy and condense back into microscopic liquid droplets. The white cloud is therefore a miniature fogbank, demonstrating the presence of liquid water suspended in the air, rather than the pure, invisible gas.