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Name: Alanna
Status: Student
Grade: 9-12
Location: IA
Country: United States
Date: September 2008


Question:
When you fill a glass with water, minuscule pockets of air (bubbles) form and rise to the top. Why is this? What causes it?



Replies:
When you pour water, it has air dissolved in it. Partly that is because most faucets have an aerator on them -- the aerator is not there specifically to add air to the water, but it has that effect nonetheless. The water is also colder than room temperature, and the solubility of air in water is higher when the water is colder (that is different than with solids like salt or sugar which have higher solubility at higher water temperature).

As the water warms up, the air's solubility drops, which means the air has to come out of solution. One way it does this is by forming bubbles. When there is too much of a solute dissolved in a solvent, it is called 'super-saturated'. Sometimes the solute molecules can cluster together to form 'nuclei' (be careful - 'nuclei' here does not mean the same as the 'nucleus' of an atom)-- in the case of gases like air, the nuclei are tiny invisible bubbles. In the case of solids like sugar or salt, the nuclei are tiny little crystals.

When these nuclei happen to find a happy place to lodge -- such as a crack or scratch in the glass -- they will immobilize and start to grow. Other air molecules will be 'happier' being in a bubble than they are in solution, so they will 'join' the bubble. In this way, bubbles form and grow. In saying 'happier', I am referring to a complicated thermodynamic dance between molecules and the probabilities of them being dissolved or not -- it is way too complicated for this forum, and it is an active topic of research today among scientists.

Air can also escape from the surface of the water without forming a bubble, but it can also re-dissolve into the water. Even if the water is not supersaturated, bubbles may still form and grow.

Because there is not that much excess air in tap water, the bubbles form and grow quite slowly. However, if you start with carbonated water, the same effect occurs, but because there is so much excess gas (CO2 in this case), the bubbles form and grow extremely rapidly. If you let carbonated water sit out and de-gas, it will approach the same behavior as tap water over time (slow bubble formation and growth).

Hope this helps,
Burr Zimmerman


Hi, Alanna. Most bubbles are dragged down into the fluid with the stream that is being poured. The area where the stream hits the surface of the water is very turbulent, and as the stream plunges down into the water, it 'entrains' air bubbles (drags air bubbles down with it).

High speed and/or high resolution photos can show you this process. A good way to confirm is to let water sit so any air trapped in it is released, then fill a glass with it and watch closely.

David Brandt


When you pour water into a glass air bubbles become trapped due to the turbulence of the water filling the glass. These bubbles may, or may not, become trapped on the walls of the glass or on particles in the water. The density of the air bubbles is much less than the density of water. This difference in density causes the air bubbles to rise because the force of gravity on the bubbles is proportional to the density of the medium. The same difference causes carbonated beverages to "foam". In zero, or micro, gravity things become more complicated because forces, such as surface tension, become important, that under "normal" circumstances are not important. In "zero" gravity, the bubbles coalesce into a single bubble in the center of the container!!

Vince Calder



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