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Poured Fluid and Trapped Air
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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Update: June 2012
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