When air is absorbed into water does the volume change.

We are performing a leak test in a closed pipe and are wondering when entrapped air is absorbed into the water, does the volume change.

Inspector Dude

To answer your question, yes I think the volume would change if a higher amount of air was diffused into the water (but not by much).

For diffusion to occur, there has to be a gradient, if you have water with no air in it, and some air surrounding it, the air will try to diffuse into the water, it's a nice place for the air to be.

As more air enters the water, some of the air thinks that maybe it is getting a bit crowded in the water, and so they diffuse back out.

This continues until the amount of particles wanting to go into the solution matches the solution going out. This is the case in most water that is left standing.

Of course, as air molecules have mass and volume, they will increase the volume of the liquid (only by a little).

You can change the equilibrium condition, which will increase or decrease the concentration of air in the water by putting work into or out of the system. The most likely way is through pressure. I realise you can get more air into the water by bubbling it through (think of fish tanks), but this quickly forces the air back out (not enough for it to bubble out), all it does is puts fresh air in, in order to make the old air diffuse out, thus increasing the oxygen content and helping your fish to not die.

As the pressure is increased, the air out of the water is compressed, and so the concentration of air out of the water increases, this means that the water (which does not compress nearly as much) looks like a nicer place to be, there is less air per unit volume, it's a better place to be. So more air will be absorped and the volume will increase very slightly.

Think of a can of coke, unopened it has a very high pressure inside, nearly all of the CO2 is dissolved in the coke, when you open the lid, it all comes out of the solution, as the air pressure decreases rapidly, the CO2 wants to be out of the coke as now the concentration out of the coke is much lower. Leave coke open for a while and it goes flat, as the gas in the coke and outside of the coke reaches equilibrium.

Now, if you pour out a can of coke and measure the volume, and then wait for it to go flat and measure the volume again, I doubt you will be able to tell the difference, but obviously the volume has changed, as the gas has bubbled out and escaped to the atmosphere.

I realise I have babbled on a lot, but I hope you can see that although there should be a very small expansion, the leaks are more much more likely to be due to high pressure rather than extra volume due to diffused air. (the only way to get the water to take on more air is to increase the pressure anyway).

Hope this was of help

Most of the time, the volume doesn't change.

Similar to CO2 in Coke or Salt in Water, it's the case of a solution.
Remember high school chemistry?
Volume can even decrease in some case, like salt in water. (Reduction of 2.5% of the volume)

Said quickly, it's the case of the molecules of air taking the empty spaces in between water molecules.
Simply put, you only have to remember that water in liquid form cannot be expanded or compressed.