# Where do the bubbles in a glass of water come from?

This weekend I accidentally left a glass of water on my kitchen table for a few days while I was out of town. Unsurprisingly, bubbles formed on the walls.

As I understand from the last time I attended a chemistry class quite a while ago, this happens due to water self-ionizing (at about a ratio of $10^{-14}$ if I recall correctly).

$$\ce{H2O + H2O \rightleftharpoons H3O+ + OH-}$$

However, for some reason, the bubbles only formed up to a certain depth:

What is the reason for this? Does self-ionization happen uniformly in the glass? Do bubbles start forming from the top down and would they have reached the bottom if left untouched for longer?

• Nucleation sites in the glass? – Evan Carslake Mar 23 '15 at 18:44

Gases are less soluble in warm water than cold water.

The bubbles are likely from dissolved gas coming out of solution as the water warmed.

The bubbles are unrelated to self-ionization of water. The self-ionization does vary somewhat with pressure, so at an extreme depth in the ocean you might need to consider what Kw is, but not in a glass of water.

The solubility of gases increases with pressure, so on this basis it is more likely that you would have bubble appear near the top. However, the pressure is only 1-2% greater at the bottom of the glass compare to the top. There may be fewer nucleation sites (such as tiny scratches) near the bottom of that particular glass. Also, if the bubbles move up along the wall sometimes, it would be logical that there are less at the bottom.

• So there would be no / fewer bubbles if I left a hot glass of water to sit and cool, correct? – Nit Mar 23 '15 at 21:21
• I wouldn't expect any – DavePhD Mar 23 '15 at 21:24

I've always assumed that the bubbles formed because they're coming out of solution after the water pressure drops. Inside your plumbing, the pressure is high and the water can hold a lot of gas, and then after you pour it into the glass, the pressure is lower and you have a supersaturated solution, and the gas gradually comes out of solution at the nucleation sites on the glass walls.

Suggested experiment: take such a glass of water after it's been standing a few days, and pour the water into a new glass. See if new bubbles form.

Another thing to try: seal the water off from the atmosphere, while leaving it at approximately atmospheric pressure. Maybe you could invert a glass full of water in a tub, taking care not to trap a bubble? I'm going to do this right now!

Update:

The inverted glass gathers about as many bubbles as the upright glass, so the gas seems to be coming from "inside" the water, and not diffusing there. I didn't notice any bias toward the top of the glass at my first check after a few hours, but now, after two days, the inverted glass has some more, and larger, bubbles near the top, where they get trapped. I'm speculating that that's just gravity doing it's thing: occasionally a bubble will detach from the walls and simply float up, although I don't have the patience to see if they ever reattach to the walls rather than simply always continuing to the top.

• yes, that's a good point, the pressure in the plumbing would be an important factor. Could be as high as 80psi for city water. 30 psi is common for well water – DavePhD Mar 24 '15 at 14:19

You are mixing things up. The self ionization of water is not related with bubbles at all. The ionization is an equilibrium process which goes on forever under constant conditions meaning thereby the concentration of ions does not increase under constant conditions. Also ions are produced in aqueous phase not gaseous phase

$$\ce{2H2O~(l) -> H3O^{+}~(aq) + OH-~(aq)}$$

The drinking water is not pure, it is a solution of many minerals and gases. Changing temperature and pressure of the surroundings can affect the solubility of the gases. Gases are less soluble at high temperature. So as water absorbs heat gases come out in form of bubbles. About the particular depth, solubility varies inversely with pressure. As pressure increases with depth so bubbles appear only upto a particular depth.