What is Water Vapor? - Definition, Pressure & Formula

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about vapor pressure and how this affects cloud formation and boiling water. We will also look at a formula to measure the water vapor at a given temperature.

Water Vapor Definition

When we boil a kettle of water, we can see the steam coming out of the spout. Or, when we look up at the sky, we see beautiful shapes in the clouds. We know that both of these things happen due to water evaporating. But did you know that evaporated water (water in the gaseous state), called water vapor, is invisible? So, how do we see the steam coming out the kettle and the clouds in the sky? What we are seeing is actually liquid water suspended in the air by water vapor.

Clouds are formed when water vapor floats into the sky and then condenses back into water

Water vapor still has the same chemical formula as typical water - H2O - but the water molecules in vapor interact less with one another and are not as structured as they are in water and ice. Whether or not water will be in liquid or gas form is dependent on pressure, temperature, and relative humidity.

Temperature and Pressure

We typically refer to the boiling point of water as 100 degrees Celsius. But really this is only the boiling point at sea level pressure. As pressure decreases, we need lower and lower temperatures in order to evaporate water. If the pressure is low enough, we could even get water vapor directly from ice!

Water vapor chart

Water will also evaporate in order to balance out dry air. The pressure caused by air comes from a combination of all the particles in the air. When looking at the pressure from only one type of particle, water vapor in this case, its pressure is measured using partial pressure. Water will evaporate to try to ensure that enough water vapor is present in the air in order to reach a point of equilibrium.

Equilibrium refers to a specific level of water in the gas phase versus the liquid phase needed to create an ideal partial pressure. This ideal partial pressure changes based on the temperature of the water. Once water reaches equilibrium, the air has reached saturated vapor pressure.

There are several different formulas that can be used to determine the maximum vapor pressure, called saturated vapor pressure, to describe equilibrium at any given temperature. The most accurate is the Buck equation, where temperature (T in the equation) is in degrees Celsius and saturated vapor pressure (P in the equation) is in kPa:

buck equation

As the water vapor pressure increases in the air (with an increase in temperature), we see an increase in humidity. Since temperature is in the exponent portion of the equation, the maximum possible percent humidity increases exponentially for every degree increase in temperature.

Real-Life Examples

We have talked about several different terms, and looked at the formula, but what does this actually have to do with the water cycle? Let's look at a large lake, such as Lake Erie. Let's say that it is 25 degrees C. We can plug this into the Buck equation:

buck equation calculation

Rounding to the nearest hundredths place, we get about 3.18 kPa for the saturated vapor pressure. This means that the water will evaporate until the partial pressure of water vapor equals 3.18 kPa in the air. The water vapor will continue to float in the air until it reaches a lower temperature where the pressure does not need to be as high, and it will condense back into water. If this happens in the sky, we'll see clouds form. Eventually, enough water will collect and will fall as rain or snow. And the cycle continues.

What about boiling water? If we were to simply leave a pot of water, with no heat, there would be some water that would evaporate, due to water trying to reach equilibrium, but not very much (just like in the lake). Now, if we add heat, the temperature is increasing. When we look at the Buck equation, we see that for every degree increase in temperature the vapor pressure needed to reach equilibrium increases.

Water boils when it reaches a temperature where all of the water wants to be in the gas form
Boiling pot of water

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