What Are the Bubbles in Boiling Water?

Chemical Composition of Bubbles in Boiling Water

The first bubbles that form when boiling water are air bubbles. Later, water vapor bubbles form.
The first bubbles that form when boiling water are air bubbles. Later, water vapor bubbles form. Dorling Kindersley / Getty Images

Bubbles form when you boil water. Have you ever wondered what's inside them? Do bubbles form in other boiling liquids? Here's a look at the chemical composition of the bubbles, whether boiling water bubbles are different from those formed in other liquids, and how to boil water without forming any bubbles at all.

What's Inside Boiling Water Bubbles?

When you first start to boil water, the bubbles that you see are basically air bubbles.

Technically, these are bubbles formed from the dissolved gases that come out of solution, so if the water is in a different atmosphere, the bubbles would consist of those gases. Under normal conditions, the first bubbles are mostly nitrogen with oxygen and a bit of argon and carbon dioxide.

As you continue heating the water, the molecules gain enough energy to transition from the liquid phase to the gaseous phase. These bubbles are water vapor. When you see water at a "rolling boil", the bubbles are entirely water vapor. Water vapor bubbles start to form on nucleation sites, which are often tiny air bubbles, so as water starts to boil the bubbles consist of a mixture of air and water vapor.

Both air bubbles and water vapor bubbles expand as they rise because there is less pressure pushing on them. You can see this effect more clearly if you blow bubbles underwater in a swimming pool. The bubbles are much larger by the time they reach the surface.

The water vapor bubbles start out larger as the temperature gets higher because more liquid is being converted to gas. It almost appears as though the bubbles come from the heat source.

While air bubbles rise and expand, sometimes vapor bubbles shrink and disappear as the water changes from the gas state back into liquid form.

The two locations you can see bubbles shrink is at the bottom of a pan just before the water boils and at the top surface. At the top surface a bubble can either break and release the vapor into the air or, if the temperature is low enough, the bubble can shrink. The temperature at the surface of boiling water may be cooler than the lower liquid because of the energy that is absorbed by water molecules when they change phases.

If you allow the boiled water to cool and immediately reboil it, you won't see dissolved air bubbles form because the water hasn't had time to dissolve gas. This can present a safety risk because the air bubbles disrupt the surface of the water enough to prevent it from explosively boiling (superheating). You can observe this with microwaved water. If you boil the water long enough for the gases to escape, let the water cool, and then immediately reboil it, the surface tension of the water can prevent the liquid from boiling even though its temperature is high enough. Then, bumping the container can lead to sudden, violent boiling!

One common misconception people have is believing bubbles are made of hydrogen and oxygen. When water boils, it changes phase, but the chemical bonds between the hydrogen and oxygen atoms don't break.

The only oxygen in some bubbles comes from dissolved air. There isn't any hydrogen gas.

Composition of Bubbles in Other Boiling Liquids

If you boil other liquids besides water, the same effect occurs. The initial bubbles will consist of any dissolved gases. As the temperature gets closer to the boiling point of the liquid, the bubbles will be the vapor phase of the substance.

Boiling Without Bubbles

While you can boil water without air bubbles simply by reboiling it, you can't reach the boiling point without getting vapor bubbles. This is true of other liquids, including molten metals. However, scientists have discovered a method of preventing bubble formation. The method is based on the Leidenfrost effect, which can be seen by sprinkling droplets of water on a hot pan. If the surface of the water is coated with a highly hydrophobic (water-repellent) material, a vapor cushion forms that prevents bubbling or explosive boiling.

The technique doesn't have much application in the kitchen, but it can be applied to other materials, potentially reducing surface drag or control metal heating and cooling processes.