Is Dissolved Air in Water Still Considered a Gas?

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Discussion Overview

The discussion revolves around the nature of dissolved air in water, questioning whether it is still considered a gas and how it affects the compressibility of water. Participants explore the implications of dissolved gases in various contexts, including theoretical and practical applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether dissolved air is "physically dissolved" or merely trapped between water molecules, seeking clarity on its impact on water's compressibility.
  • Another participant asserts that "dissolved gas" refers to gas trapped within water molecules, citing that fish can extract oxygen from water without a chemical reaction.
  • A participant raises the question of whether dissolved air exists as tiny bubbles and how this relates to compressibility.
  • One response maintains that dissolved air is still considered a gas, comparing it to carbon dioxide in soda and discussing how gas solubility changes with pressure.
  • A participant emphasizes a definition of solution as a single-phase homogeneous mixture, arguing that air dissolved in water is in the liquid phase and affects compressibility.
  • Another participant proposes a mathematical model for calculating the compressibility of a liquid based on the fraction of dissolved air and the compressibility of air and water, while acknowledging potential issues with using gas compressibility values.
  • One participant challenges the notion that air is liquid, questioning the conditions under which air can be considered a liquid and discussing the fluid nature of gases and liquids.
  • A later reply suggests using partial molal compressibility for a more accurate model, indicating that the compressibility of dissolved air differs from that of gaseous air.
  • Another participant questions the validity of assuming that dissolved air has the same compressibility as air in the atmosphere for modeling purposes.
  • A response strongly disagrees with this assumption, highlighting the significant differences in compressibility between gases and dissolved substances.

Areas of Agreement / Disagreement

Participants express differing views on whether dissolved air should be classified as a gas and how it affects the compressibility of water. There is no consensus on the definitions or implications of these concepts, indicating ongoing debate.

Contextual Notes

Participants acknowledge various assumptions and limitations in their discussions, including the need for specific conditions regarding temperature and pressure, as well as the complexities of measuring compressibility in solutions.

strid
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I have a question regarding a project I am working with currently...

Air that is dissolved in water (there always is around 0,004% I think, in normal pressure and 20 degress celsius), is it "physically dissolved"? I mean is the dissolved air still gas or is just air-molecules "trapped" between water molecules?

The essential reason for my question is to know whether the dissolved air affects the compressibility of the water, as air-bubbles definitely do, but I don't know about dissolved air.

I can give more information about the project if it's neccesary to fully understand my question...
 
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From what I know, "dissolved gas" means gas trapped within the water molecules. Despite the general chemical definition of the solute having to be ionized.

I believe this is true because fish are able to extract oxygen from water, and they don't perform and chemical reaction with the water in order to retreive this oxygen. In addition, the recent diffusers that are able to extract air from water work on henry's law, which is nothing chemical.
 
but is it still gas? is it tiny tiny bubbles?

and then, would it affect the compressibility...
 
Yes, still as a gas. The same way you have carbon dioxide in your coke. And in a reciprical manner, the same way you have water vapor in air. Well, I don't know about that, but actually as you try compressing the liquid, it will be capable of dissolving larger quantities of gas. As you decompress it, you will see gases bubbling out as a result of a lower soluability.
 
Let's not make up definitions of "solution" as we go along, shall we? Solution: a SINGLE phase homogeneous mixture of two or more components. There are NO constraints regarding ionization.

Air dissolved in water is in the LIQUID phase. Got it?

Does it affect compressibility? Yes. ALL solutes affect the compressibilities of all solutions.

What's the sign of the effect? Depends upon the partial molal volumes of the solutes and solvents; net decrease in volume forming the solution favors increasing solubility with increasing pressure.

Isn't there a net decrease in volume when dissolving air in water at STP?
Couldn't tell you --- the partial molal volume of air in aqueous solution isn't something I know off the top of my head --- it could be positive, it could be negative, or it could be so close to zero as to be negligible. It is possible to dissolve gases in solvents, increase pressure on the liquid phase and have bubbles form as the gas is driven from solution.
 
so you say that the air is liquid? Sounds interesting... But doesn't the temperature of air has to be quite low in order to be liquid??

Also, would it be valid to do the following:

Lets say we have the compressibility of air (C.a) and the compressibility of water (C.w)... If we are given that the fraction of air (R) dissolved in a glass of water would this equaton be valid?

Compresbility of the liquid = R * C.a + (1-R)* C.w

The slight problem in this formulation is that the value of C.a is when air is gas, which might affect...

EDIT: and of course, we are asuming that the air is evenly spread out in the water...
 
Last edited:
Bystander said:
Air dissolved in water is in the LIQUID phase. Got it?

Well that makes sense, each substance has various conditions depending on its density, gas or liquid or solid...

But gases are liquids are considered fluids..So air can dissolve into water or liquids or substances in a liquid state.

Like when oxidating a metal in a liquid state for example..
 
strid said:
(snip)Compresbility of the liquid = R * C.a + (1-R)* C.w

The slight problem in this formulation is that the value of C.a is when air is gas, which might affect...
(snip)

Use partial molal compressibility, or some other composition variable, and the composition, and your equation is correct. The trick, of course, is getting values for compressibility. For water and air, one measures the compressibility of the solution, compares it to that of pure water, and backs out the compressibility of air, or more specifically, of the various components. No, they are not the same as those of the gases at the same T, P.
 
so how wring would it be to assume that the air in the glass has the same compressibiliity as the air around us. Would it be valid for making a simplified model..?
 
  • #10
Very, vveerryy, vvveeerrryyy wrong. Compressibility of an ideal gas, decent approximation for air at conditions you say interest you, is -RT/(VP2), or 1/P (in atm); compressibilities of liquids far from the critical point, again, your conditions of interest (the state of dissolved air), are measured in tens of parts per million per atm, some 10-100 thousand times less than for gases.
 

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