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Dr. Christopher S. Baird

Why does the air only contain water vapor and not other kinds of vapors?

Category: Earth Science      Published: June 11, 2022

Public Domain Image, source: Christopher S. Baird.

The air does indeed contain many different vapors. Any time you smell something, it is because a strong concentration of that particular vapor has entered the air and then entered your nose. For instance, when peppermint oil is sitting out, some of it naturally evaporates. Peppermint oil in the vapor state then enters your nose and you smell it. If properly done, just about anything can be vaporized and dissipated into the open air.

Even though a common substance like vinegar is in the liquid state at room temperature, some of it still turns into a vapor and dissipates into the air. How is this possible? The key is that not every molecule in a liquid has the same energy. The temperature of a cup of a liquid does not tell us the exact energy of every single molecule. Rather, the temperature gives us an idea of the average energy of all of the molecules. Because of the random, jostling nature of thermal motion, some molecules in a liquid end up with far more energy than the average, and some molecules end up with less energy than the average. The molecules with more energy than the average often have enough energy to break free of their liquid-state intermolecular bonds and shoot off into the freedom of air. In this way, some amount of a liquid will always evaporate from its surface, no matter the temperature of the liquid.

You may have learned in school that a liquid in a closed container at equilibrium will have no net evaporation. However, even in this situation, some of the liquid still evaporates (but is still stuck in the container as a vapor). Because there are as many molecules in the vapor condensing onto the liquid per second as there are molecules in the liquid evaporating, there is no net evaporation, but there is still evaporation. However, this concept only applies to closed containers. A liquid in an open container or not in any container at all is not a closed system and cannot reach equilibrium. Given enough time, a liquid exposed to the open air will eventually completely evaporate away.

The word "vapor" in science means a substance that is in the gas state. Therefore, oxygen in the air is actually a vapor. In addition to the vapors in the air that have a strong odor, the air contains many other vapors. The most abundant vapors in the air (by mole fraction) are (1) nitrogen, (2) oxygen, (3) argon, (4) water vapor, and (5) carbon dioxide, in that order. Amazingly, these five vapors make up about 99.997% of the air. Therefore, the most common chemical processes that involve air include one or more of these five vapors. Note that at the local level, the amount of water vapor in the air is highly variable. It can be more abundant than argon or much less abundant than carbon dioxide.

Nitrogen in the air comes in the form of molecular nitrogen (N2) and is fixed by microorganisms in the soil, thereby establishing the most common source of nitrogen for plants.

Oxygen vapor in the air is used by animals to help unlock energy from food through cellular respiration. When you breathe in air, it is the oxygen that your body uses. Oxygen in the air also causes metals to rust. Additionally, oxygen enables common chemical combustion, such as occurs in everyday fires and in the burning of common fuels. Oxygen in the air takes the form of common molecular oxygen (O2) and ozone (O3). Ozone in the upper atmosphere helps block and protect the earth from the harmful ultraviolet rays in sunlight.

Water vapor in the air often turns into clouds, fog, rain, snow, hail, and dew. For this reason, the water vapor content in the air is a huge driver of weather effects. The majority of the water vapor in the air comes from the evaporation of oceans, rivers, and lakes.

Argon is a noble gas and does not participate in any chemical reactions at ambient temperatures. This actually makes argon very useful in processes where you want to suppress chemical reactions. For instance, argon is the gas typically used in incandescent light bulbs, where suppressing chemical reactions makes the light's filament last longer.

Carbon dioxide in the air is used by plants to help convert the energy in sunlight to chemical energy through photosynthesis that is stored in sugars and starches. Higher-than-normal amounts of carbon dioxide in the atmosphere is what traps more-than-normal amounts of heat and is the main driver of global warming.

Of the five main vapors in air, four of them (nitrogen, oxygen, argon, and carbon dioxide) stay in the gas state at the temperatures that are common in the atmosphere. In contrast, water typically wants to be in the liquid state at these temperatures. Therefore, of the five vapors that make up about 99.997% of air, water vapor is the only one that is constantly trying to turn to a liquid. For this reason, the water vapor content in the air - which we call humidity in everyday life - has a huge effect on the weather, on how the air feels to the skin, on how fast things dry, and on other everyday processes.

Under normal conditions, all of the other vapors in the air besides these main five vapors only make up about 0.003% of the air. Therefore, these other vapors have little effect except in phenomena that work at very low concentrations, such as the act of smelling. However, abnormal conditions can arise that put other vapors into the air in high concentrations. The vapor that most commonly builds to harmful levels in abnormal situations is carbon monoxide. Under normal conditions, carbon monoxide in the air is at a very low concentration and is safe. However, appliances such as water heaters and furnaces that are not vented properly can cause unusually high concentrations of carbon monoxide vapor to form in the air, which can be deadly to humans. For this reason, everyone should have a carbon monoxide alarm in his or her home.

Although water vapor in the air naturally wants to condense down to liquid, the other four common vapors can be forced to become a liquid using machines. Liquid nitrogen is used commonly as a cryogenic fluid, i.e., a fluid that can quickly deliver extremely cold temperatures. Liquid oxygen is commonly used as the oxidizer that helps the fuel in rocket engines burn. Liquid argon is commonly used as a cryogenic fluid in applications where liquid nitrogen is too reactive. At atmospheric pressure, carbon dioxide cannot form a liquid at any temperature. When cooled enough at atmospheric pressure, carbon dioxide instead forms a solid commonly called dry ice. Dry ice is used in fog machines and to carbonate beverages. As you can see, there are many vapors in air and they participate in many different phenomena.

Topics: air, atmosphere, gas, oxygen, vapor, water vapor, weather