# Dissolving a gas in water

I have a few simple questions about the process of dissolving a gas in water, if anyone could give me any help it would be appreciated.

1. How do I know the gas has dissolved? i.e. if I "bubble" gas into the liquid how do I know it hasn't just "bubbled" out? Or would this only occur once the liquid has been saturated? How do I know when the water is saturated?

2. If I measured the amount of gas that I put in how would I calculate the gas concentration of the liquid?

3. What would the liquid become? i.e it would no longer be pure water, please could someone give me an example of what the liquid would become for an example gas.

4. Is there molecular bonding or do the water molecules stay seperate from the gas molecules?

Apologies if these are stupid / basic questions as I'm not a chemist but trying to get an idea as to what is happening and what the result is when you introduce a gas to a liquid.

Last edited:

1. You don't know, except you test it with titration (for example Winkler method for oxygen) or spectroscopic methods. But every compound is a bit soluble so you can say that when the gas stayed in contact with the liquid, there is at least one molecule or atom which is solvated. The bubbling out and the uptake of gas in the liquid is occuring ever because dissolving is a dynamical chemical equilibrium. This means that at equilibrium point the number of gas molecules bubbling out is equal to the number of gas molecules taken up by the liquid.

2. When you can measure the amount you solvated in the liquid and you get it as a volume unit like litre (L) you have to divide by the gas density (kg/L) to get the mass of gas in kilogram (kg) this mass you have to divide by the molar mass of the gas to get the molar amount of gas in the liquid and this is divided by the Volume of the whole mixture (maybe you have 1 L of Water and you injected 500mL NH3, the resulting mixture hasn't a Volume of 1.5L maybe it's 1.1L or lesser so you have to calculate with 1.1L), to get the concentration of the gas in mole per litre (mole/L)

3. The example gas is oxygen. Without oxygen the water molecules completely build hydrogen bonds. 4. There is no direct bonding but there are attractive forces and no separated areas of different structure. The picture has a mistake, because the two hydrogen atoms of the water molecules never get so close. Maybe here you find a better picture. http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/

Sorry for the not exact values in point 2, but i just want to illustrate the principle.

Last edited:
rwooduk
Borek
Mentor
Your tap water contains dissolved gases. Once you boil it, gases are lost (but they start to dissolve back on cooling). Have you ever spotted any difference between fresh and boiled water? No? That's OK, because without a proper equipment it is quite difficult to check. The difference between water saturated with gases and without gases is - for most applications - negligible.

rwooduk
Thank you for the replies, they are extremely helpful!!

2. When you can measure the amount you solvated in the liquid and you get it as a volume unit like litre (L) you have to divide by the gas density (kg/L) to get the mass of gas in kilogram (kg) this mass you have to divide by the molar mass of the gas to get the molar amount of gas in the liquid and this is divided by the Volume of the whole mixture (maybe you have 1 L of Water and you injected 500mL NH3, the resulting mixture hasn't a Volume of 1.5L maybe it's 1.1L or lesser so you have to calculate with 1.1L), to get the concentration of the gas in mole per litre (mole/L)
Got a little lost here to be honest. Is this related to Beer's Law?

http://www.chem.ucla.edu/~gchemlab/colorimetric_web.htm

3. The example gas is oxygen. Without oxygen the water molecules completely build hydrogen bonds.
Would an example of the be distilled water? And what would heppen if you introduced say Xenon to distilled water?

Your tap water contains dissolved gases. Once you boil it, gases are lost (but they start to dissolve back on cooling). Have you ever spotted any difference between fresh and boiled water? No? That's OK, because without a proper equipment it is quite difficult to check. The difference between water saturated with gases and without gases is - for most applications - negligible.
Thank you. That's a good way of looking at it. For my PhD I will be using ultrasound to test solutions' reactions that are saturated with various gases, so trying to brush up on my basic Chemistry as I have only done Physics thusfar.

With ultrasound you can't fully degass liquids. Look at this link http://depts.washington.edu/eooptic/linkfiles/Freeze_Pump_Thaw.pdf. This is an protocol for the full removement of gases. And for Xenon and all other gases it's the same except the ones with dipole moment. They build a so called hydrate sphere. And my calculation is only theretical nature. It works only when you know the exact amount of gas volume which is dissolved but there is no experiment to get this value.

rwooduk
With ultrasound you can't fully degass liquids. Look at this link http://depts.washington.edu/eooptic/linkfiles/Freeze_Pump_Thaw.pdf. This is an protocol for the full removement of gases. And for Xenon and all other gases it's the same except the ones with dipole moment. They build a so called hydrate sphere. And my calculation is only theretical nature. It works only when you know the exact amount of gas volume which is dissolved but there is no experiment to get this value.
Many thanks for the further info and the link it may come in useful! The purpose of the ultrasound is not to degas the liquid, it will be to facilitate a reaction between solvant gas and the liquid. I haven't started the project as yet but it will follow on from my 3rd year project of sonoluminescence, I'm hoping it will relate quite heavily.

SteamKing
Staff Emeritus
Homework Helper
3. What would the liquid become? i.e it would no longer be pure water, please could someone give me an example of what the liquid would become for an example gas.
Ever heard of soda water? Soda pop? Seltzer? Carbonated beverages? Champagne? Sparkling wines?

All are examples of carbonated beverages, where carbon dioxide is dissolved in the water, often under pressure to increase the amount of gas dissolved.

rwooduk
Nidum
Gold Member
Depends on whether gas is reactive with water .

Bubble air in and nothing much happens . Relatively easy to get most of air back out of solution again .

Bubble HCl in and you get a strong acid solution . No easy way of getting HCl back out of solution .

An interesting one is water vapour - bubbles out and condenses back in all on its own just with changes of temperature/pressure .

rwooduk