What Makes the Bubbles Form When Water Boils?
When you boil water in a tea-kettle, you heat it to the point where it changes from a liquid to an invisible water gas called “steam.” Bubbles of steam rise through the water and escape into the air. This bubbling is called “boiling.”
By the time the steam has gone an inch or so from the teakettle spout, the steam cools and partly condenses into a cloud of tiny water droplets, much like those your warm breath makes on a cold day. If the water boils long enough, it will all turn to steam (or “water vapor” as it is usually called) and disappear into the air.
But first, let’s make clear that in the process of boiling a kettle of water you will actually observe two types of bubbles. First, just as the water starts to get hot, a lot of bubbles will form down the walls of your water container.
These bubbles are Air. Normally water has a lot of air dissolved in it. This is what allows fishes and other aquatic animals to breathe. The solubility of gases decreases when the temperature is raised, and that’s why the dissolved air bubbles out from the water.
Then, as the boiling point of water is reached (100 degrees Celsius or 212 degrees Fahrenheit), water vapor starts to form inside the liquid in the form of bubbles.
Remember that at boiling point water and its vapor are at equilibrium, which means that every molecule in the system has almost the same willingness to be in the vapor phase as in the liquid phase, so they very readily form bubbles inside the liquid.