Water vapor

Invisible water vapor over a cup of hot tea. The visible mist is not water vapor. It is liquid water drops.

Water vapor is water that is in the form of a vapor, or gas. It is a part of the water cycle. When liquid water is heated to boiling point, 100 °C (212 °F), it turns into vapor. Water vapor can also be produced directly from ice; this is called sublimation. Steam is water vapor, but clouds are liquid water. The amount of water vapor in air is called humidity and it affects weather conditions. In the cold, breathing out causes the water vapor in the breath to freeze.

Water vapor weighs less than air, thus it slightly reduces the lift produced by an aircraft wing. It is also transparent, which means the small droplets that look like mist are actually liquid. Water vapor also is a big cause of the greenhouse effect. It is invisible, but makes visible water when it condenses.

Properties of water vapor

Hydrogen bonds between molecules of water

Water vapor is the gas form of water (H₂O). In this state, the water molecules are no longer stuck together by hydrogen bonds like they are in liquid or ice. Instead, each molecule moves around freely on its own. A single water molecule is made of two hydrogen atoms and one oxygen atom. These atoms are joined by covalent bonds, meaning they share electrons. The shape of a water molecule looks like a bent "V". It has an angle of about 104.5° between the hydrogen atoms. Oxygen pulls more on the electrons, making one side of the molecule more negative and the other side more positive. This makes water a polar molecule.

In water vapor, the tiny water molecules move quickly and randomly, like other gases. Each molecule is on its own, flying around freely. But because water molecules are polar (they have a positive side and a negative side), they can still weakly attract each other or other polar molecules in the air. These weak attractions can slightly change how water vapor behaves. This happens especially when there is a lot of water vapor or they are under high pressure. Still, in most everyday situations, water vapor acts almost like a regular or ideal gas.

People often think steam or mist is the same as water vapor, but that is not true. Real water vapor is actually invisible. You cannot see it with your eyes. What we call steam is actually made of tiny liquid water droplets floating in the air. These form when water vapor cools down and starts to condense. True water vapor, like the air we breathe, is a gas you cannot see. But even though it is invisible, we can still measure it using special tools. Water vapor has pressure, takes up space (volume), and has a temperature, just like any other gas.

In nature, water vapor is usually mixed in with other gases in the air. How much water vapor the air can hold depends on the temperature. Warmer air can hold a lot more water vapor than cooler air. That is why places near the equator are more humid. Water vapor is made of small, light molecules. Each water molecule has a mass of about 18 grams per mole, which is lighter than nitrogen (28) and oxygen (32), the main gases in air. This means that humid air is actually lighter than dry air. Because it is lighter, moist air usually rises up. This helps form clouds and plays a big role in weather.