# Can water pressure ever be high enough to trap gas bubbles or keep them from surfacing?

Can water pressure ever become high enough to trap gas bubbles and/or keep them from surfacing?

• I think a bottle of soda is a good example of how that is done ;)
– TMS
Commented Jul 7, 2013 at 6:23

The highest pressure in the ocean is at the bottom of the Mariana trench, where the pressure is 1 086 atmospheres. Using the online calculator for the properties on nitrogen at 4 °C and 1 000 atmospheres the density comes out as 602 kg/m³, which is still less than water. So a bubble of nitrogen would rise even at the deepest point in the ocean.

Response to comment:

In principle we can continue increasing the pressure and the nitrogen should get denser. However at temperatures above 0 °C and the sorts of pressures we are talking about, nitrogen is a supercritical fluid so it does not obey anything like an ideal gas law. Calculating at what point the density would exceed the density of water is far from easy.

The effect of pressure on water is straightforward. At sea bottom temperatures (about 4 °C) the density of water increases only slowly with pressure to about 1 050 kg/m$$^3$$ at 6 000 atmospheres, at which point the water freezes to form ice V. So the question is whether the density of nitrogen exceeds 1 050 kg/m³ below a pressure of 6 000 bar.

I can’t find any figures for the density of nitrogen at these sorts of pressures and temperatures, though I did find this paper that gives a Mie-Grüneisen type equation relating the density, pressure and temperature. Unfortunately the preview only shows two pages and the rest of it is behind a paywall. However using the figures they give and waving my arms around a bit I find the density of nitrogen rises to 1 050 kg/m³ at around 4 000 atmospheres.

So, it might just be possible to get a nitrogen bubble that is denser than water and will sink instead of floating. But I don’t know whether the equation from the paper I cited is accurate at these sorts of pressures and temperatures, and it’s possible the nitrogen will solidify before the water does (though I’d guess not).

• But, under artificial compression, is it possible to trap the gas bubbles somehow? Commented Jul 6, 2013 at 17:48
• @josephminor: I've edited my answer to respond to your comment Commented Jul 7, 2013 at 10:49
• But wouldn't another gas possibly be dense enough at lower pressure? Commented Jul 7, 2013 at 23:02

In case your question stemmed from seeing a similar phenomenon yourself, what you saw might have been an antibubble. An antibubble is a droplet of water, encased in a thin shell of air, suspended in water. These are pretty unstable in nature, so they're rarely observed unless artificially induced.

An antibubble will have roughly neutral buoyancy, being composed almost entirely of the medium it is suspended in, and will therefore accelerate neither upwards nor downwards (unless disturbed). This means it can easily be mistaken for an anomalous gas bubble that is "trapped" in place.

If you fill a glass tube with water, and then pull a vacuum on it, at some point bubbles will start to form. Increase the vacuum, and the bubbles will rise. Reduce the vacuum and the bubbles will disappear.

What's going on there is that the water is evaporating to form the bubbles. There is a continual motion of water molecules across the bubble interface, moving from vapor to liquid. The process is highly dependent on pressure. If more molecules are becoming vapor than are becoming liquid, then the bubbles grow until equilibrium is reached.

Something similar can happen with dissolved CO2 or dissolved nitrogen in water, which explains why soda in a closed bottle does not keep making bubbles, and why divers need to ascend slowly from great depths to avoid "the bends": formation of nitrogen bubbles in their blood and tissues. Solubility of CO2, nitrogen, and other gasses is highly pressure-dependent.