Speaking of bubbling, what exactly are those bubbles you see in a pot of boiling water? Some people believe it’s air, since many bubbles you may be familiar with, such as soap bubbles, are indeed filled with air. Others believe it’s hydrogen or oxygen escaping as a result of a chemical change in the nature of water when it boils.
Neither of these is true, though. When you first pour water into a pan and begin to heat it, you’ll notice bubbles along the walls of the pan. These bubbles are indeed air. Most water has some air dissolved in it. As you begin to heat the water, this dissolved air escapes the water. These bubbles aren’t the bubbles associated with boiling water, though.
When water is boiled, it undergoes a physical change, not a chemical change. The molecules of water don’t break apart into hydrogen and oxygen. Instead, the bonds between molecules of water break, allowing them to change physically from a liquid to a gas.
You probably already know that water comes in three forms: solid, liquid, and gas. The solid form we know as ice. The liquid form is, of course, water. The gaseous form is water vapor. Water vapor exists around us in the air nearly all the time. We just can’t see it.
To convert a liquid to a gas via boiling, the liquid must be heated until its vapor pressure equals the atmospheric pressure. For water, this occurs at approximately 212º F (100º C). That’s why 212º F (100º C) is considered the boiling point of water. In reality, however, the boiling point of water can be higher or lower depending upon many different factors, such as altitude, atmospheric pressure, and other chemicals present in the water, to name a few.
When water is boiled, the heat energy is transferred to the molecules of water, which begin to move more quickly. Eventually, the molecules have too much energy to stay connected as a liquid. When this occurs, they form gaseous molecules of water vapor, which float to the surface as bubbles and travel into the air.
Instead of air, the bubbles in a boiling pot of water are actually made up of water — it’s just water in its gaseous state! What looks like a pot full of water and air is really just a pot full of water in two different physical states.
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