What Are the Bubbles in Boiling Water?


The bubbles in boiling water are air and water vapor.
The bubbles in boiling water are air and water vapor. As dissolved gas escapes, bubbles become water vapor only.

When you boil water, you get bubbles. Have you ever wondered what’s inside the bubbles? Is it air, water vapor, hydrogen and oxygen, or what? Here’s a look at the chemical composition of the bubbles and how you can boil water without getting any bubbles at all.

Chemical Composition of the Bubbles

The chemical composition of bubbles in boiling water changes over time. The bubbles you see when you first heat water are tiny air bubbles. More generally, the chemical composition of early bubbles matches that of the atmosphere. So, if you boiled water in a carbon dioxide atmosphere, the bubbles would be carbon dioxide. In the normal atmosphere, the bubbles mostly consist of nitrogen, oxygen, and carbon dioxide (air).

As heating continues, water vapor bubbles form around the air bubbles and tiny imperfections on the side and bottom of the container. This happens when water molecules gain enough energy to make the transition from the liquid phase to the gas phase. By the time water reaches a full, rolling boil, the bubbles consist entirely of water vapor. Boiling water does not decompose it into its elements, so the bubbles do not contain hydrogen gas or oxygen gas (except from the atmosphere).

Both air bubbles and water vapor bubbles expand as they rise because there is less pressure on them near the surface. Sometimes water vapor bubbles appear to shrink and may even disappear. This happens at the bottom of a pan before water boils and at the top surface. The surface temperature of boiling water may be cooler than the liquid temperature because water molecules absorb energy when they change from liquid to vapor.

If you cool the boiled water and immediately reboil it, there isn’t any dissolved air in the liquid so the only bubbles that form are those containing water vapor. Because there are fewer nucleation sites for water vapor bubbles to form, the water can superheat beyond its normal boiling point and suddenly, explosively boil when the container is bumped. Under normal circumstances, boiling water never exceeds its boiling point, no matter how much heat you apply.

Boiling Without Bubbles

When you boil water in a pot or kettle, you see nucleate boiling. This is the type of boiling where bubbles form at nucleation sites formed by a slightly uneven surface or tiny particles within the liquid (usually air bubbles, in pure water). But, there are other forms of boiling. One of these is called “film boiling.” Film boiling is based on the Leidenfrost effect, which you can see if you flick droplets of water on a hot pan. The droplets skitter on the surface on a thin cushion of water vapor. The vapor layer has a low thermal conductivity and insulates the surface. While the water is boiling, it doesn’t bubble. Researchers have discovered boiling water on a highly hydrophobic surface produces the same effect. While film boiling doesn’t have a practical application for cooking, it may be useful to reduce drag on surfaces.

Bubbles in Other Liquids

Water vapor bubbles form in water. In other liquids, the same process occurs. Initially, there may (or may not be) bubbles of dissolved gas. Eventually, the bubbles consist of the vapor of the compound. So, boiling alcohol contains alcohol vapor bubbles and boiling gold contains gold vapor bubbles.

Boiling vs Evaporation

Boiling is the phase transition from the liquid to gas phase that occurs at a temperature called the boiling point. Boiling occurs when the vapor pressure of a liquid equals the force exerted on it by the atmosphere. Evaporation occurs at a temperature below the boiling point, when the liquid vapor pressure is less than the equilibrium vapor pressure. Aside from temperature, a key difference between evaporation and boiling is that evaporation only occurs on the liquid surface, while boiling involves the entire volume.

References

  • Clift, R.; Grace, J.R.; Weber, M.E. (1978). Bubbles, Drops and Particles. New York: Dover Publications. ISBN 978-0-486-44580-9.
  • Goldberg, David E. (1988). 3,000 Solved Problems in Chemistry (1st ed.). McGraw-Hill. ISBN 0-07-023684-4.
  • Vakarelski, I.U., Patankar, N.A.; et al. (2012) “Stabilization of Leidenfrost vapour layer by textured superhydrophobic surfaces.” Nature 489, 274–277. doi:10.1038/nature11418

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