Imagine a bottle 85% filled with water and the rest is just air, and the cap is closed. Here are now my questions.

Since the cap is closed completely, the air bubble is formed. Is that air bubble compressed? In other words, since air has the least force of attraction between its molecules, they are more free than the molecules of water. Now since water is creating a pressure against the air bubble, does it mean that the molecules in the bubble are now closely packed than usual?

Is it possible for the air bubble, if compressed a lot, to mix with water?

If the air bubble is able to hold itself even in water, why do we say that air has lesser force of attraction that water?

  • $\begingroup$ Please explain what you mean when you say : "the air bubble is formed" ! Which air bubble ? How and why is it formed ? What do you mean when you say that" the molecules in the bubble are closely packed" ? In a gas the molecules are not packed at all ... Explain ! $\endgroup$ – Maurice Mar 12 at 10:21
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    $\begingroup$ If you compress it, then it will be compressed, otherwise no. $\endgroup$ – Ivan Neretin Mar 12 at 11:38

Assuming that by air bubble you're referring to the pocket of air now sealed in the bottle above the water surface: Immediately after sealing the bottle, the air pocket is not at all compressed: it's at the same pressure as the surrounding environment (the air outside the bottle).

However, because you have a closed container, some water will evaporate, until an equilibrium is reached between the water vapor in the air pocket and the water in the liquid phase. This occurs when the chemical potential of the water in the two phases are equal.

Due to that vaporization, the total pressure in the air pocket (which now contains a higher amount of water vapor than the surrounding atmosphere outside the bottle) will be larger than the surroundings. This is a prominent effect when it's hot outside (have you ever left a sealed water bottle in a hot car?).

While this effect is usually small, as gases mix very well, it is non-zero and so yes, the air pocket is slightly compressed due to presence of water vapor.

As for whether the air pocket can mix with water: Assuming our container can tolerate such pressures and we were to externally compress it: liquids are much less compressible than gases, and so the air would (given high enough pressure) condensate, mixing into the liquid phase (but you wouldn't achieve this by hand!)

To address your final point: let's say we invert the bottle, so the air pocket travels as a bubble through the liquid phase to re-emerge at the other end of the bottle. One could ask, "why doesn't the air bubble fragment into little bubbles as it progresses upwards?"

To be fair, you do see a few bubbles splinter off, but for the most part, the bubble is intact. The air bubble stays largely intact due to the viscosity of water (the intermolecular attractions between water molecules is larger than the entropic benefits of mixing with the air bubble). So, to answer your question, the fact that the air bubble doesn't mix is not due to its intermolecular attraction, but rather it's due to water's intermolecular attraction.

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    $\begingroup$ thanks a lot for the detailed answer. I have another doubt now. Let’s imagine the same bottle with water and the air pocket. Now picture the molecules of air in the pocket (freely enclosed) and the molecules of water in the bottle (closely packet). Now since the air pocket has a defined boundary, I want to know if there is a force acting in between the two different forms of matter (air and water) that holds the air pocket together and does not let it mix with water. Also, the molecules part, what is present in between the molecules of air? $\endgroup$ – Deepalya Jha Mar 13 at 10:31
  • $\begingroup$ Assuming you just closed the bottle in typical surroundings, the initial composition of the air pocket will be identical to the surrounding air (4 parts N2, 1 part O2 by volume, with some water vapor in there too), and then the composition will start to become more humid (more water vapor) as equilibrium is reached between the water in the vapor phase (in the air pocket) and the water in the liquid phase. $\endgroup$ – jezzo Mar 13 at 13:37
  • $\begingroup$ There are quite a few effects acting on the air and water to prevent them from mixing: 1) gravity makes the more dense water “want” to occupy the region of the bottle with the lowest gravitational potential. While the air wants to do this too, it has a much lower density, so the water would push the air bubble out of the way (to the top) to minimize its energy. 2) As for why they don’t just mix: water has an intricate network of hydrogen bonds, giving it viscosity. This means the entropic effect of mixing with the air is overruled by the favorable water-water attraction. $\endgroup$ – jezzo Mar 13 at 13:44
  • $\begingroup$ 3) as for why the air doesn’t mix into the liquid phase so that they both get to occupy the bottom of the bottle: that would leave a very negative pressure at the top of the bottle, which would exert a much larger force on the system than gravity. $\endgroup$ – jezzo Mar 13 at 13:46

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