How does removing air from a vessel of water create bubbles?

Question

I recently started reading Richard Feynmans 'six easy pieces'. I did physics and chemistry combined in secondary school but that was 3 years ago and thought it would be a nice introduction back into it.

It's been really interesting so far but I've a question about a statement made on page 12/13.

The statement is referring to water in a vessel and is as follows: '...oxygen and nitrogen molecules will work their way into the water and the water will contain air. If we suddenly take the air away from the vessel, then the air molecules will leave more rapidly than they came in, and in doing so will make bubbles...''

I dont find this very intuitive. Cause from my naive perspective bubbles in water are air and if we removed the air how would any bubbles remain. Never mind create them.

I'd really appreciate someone maybe rephrasing this or expanding on it so maybe I could get a better grasp of the idea.

Thanks :)

Answer 1

Basically, oxygen and nitrogen, like carbon dioxide, are soluble in water. The higher the pressure in the water, the more soluble they are.

Remove the pressure and they come out of solution, in the form of bubbles.

Remove air from a closed vessel and the pressure will drop. So bubbles will form. The behavior with oxygen and nitrogen is exactly the same as with soda pop when you open the can.

Answer 2

I think you're unsure about what Feynman specifically means when he states If we suddenly take the air away from the vessel.

Not having read the entire article myself, I suspect he's referring to the removal of the surrounding air. In other words, if we had a glass of water in a sealed room, we then evacuate the air in the room and not the air that is inside the water itself.

As explained by Tim, removing the air from the room results in a pressure drop, and because of this the surrounding temperature drops and the boiling point temperature of the water continues to decrease until it's at or below the temperature of the water, at which point the water boils releasing it's oxygen and nitrogen content into the room.

You are correct in your assertion that

if we removed the air ... [sic] ... no bubbles remain

since if we could remove all the air$^1$ that is in the water, then there would be no air to form bubbles.

$^1$ This happens, for example, where hydroelectric plants use boiling water (steam) to spin turbines to generate electricity. This used boiled water is therefore severely depleted of its air content, and when disposed of in rivers, lakes etc., this has a deadly effect on fish that live in these bodies of water.

Answer 3

When a liquid is exposed to a gas, like water in a glass exposed to air, some of the gas molecules (O2, N2 etc) that hit the liquid will not bounce off but get inside the liquid instead. At the same time, some of the molecules inside the liquid leave and return to gas state.

At equilibrium state, the flow of gas entering the liquid is the same as the flow of gas leaving the liquid, so the amount of dissolved gas inside the liquid remains the same per unit volume. This is the solubility of this gas in this liquid.

The exact value depends on the gas, the liquid, temperature, pressure, etc. Some have more affinity towards each other and some less. Here are some curves for oxygen and other gases. Some gases, for example CO2, have high solubility in water, in this case because CO2 turns into carbonic acid HCO3 which is soluble in water. This is how your body transports CO2 in the blood to get rid of it in the lungs. Since HCO3 is an acid, the body measures blood acidity, uses that as a proxy for blood CO2 concentration, and adjusts breathing speed accordingly.

Gas solubility is strongly dependent on gas pressure above the liquid. To understand this intuitively, imagine gas pressure both pushing gas into the liquid and making it harder for the gas to leave the liquid. So higher pressure which shifts the equilibrium towards more gas in the liquid, ie higher solubility, and lower pressure shifts it to lower solubility.

The experiment described in the question is about removing the gas above the liquid, for example by putting a glass of water into a vacuum tank and sucking the air out. It's not about removing the gas from the liquid, because... how would you do that?

In fact, you can do this experiment without any fancy lab gear, by opening a bottle of sparkling water. When the bottle is closed, it is at equilibrium, and pressure inside is quite high. When you open it, gas pressure inside the bottle drops to atmospheric pressure, and the gas dissolved inside the liquid escapes as bubbles. In this case, the bubbles contain CO2, not air.

The same happens if you do anything that lowers gas solubility. For example if you put a pot of water on the stove, you will notice before it boils, bubbles form in it. This is not water boiling, as temperature would be too low. This is the air dissolved into the water escaping, because warm water has lower gas solubility than cold water.

Putting a container of liquid inside a vacuum chamber is often done to get rid of bubbles when making molds or resin inclusions. In this case the mechanism is different: it isn't about solubility. Rather, there are bubbles already present in the liquid from mixing, but they can't escape because the liquid, usually silicone or resin, is viscous. The lower pressure makes bubbles expand, giving them more buoyancy, so they rise to the top and escape, and you get a nice bubble-free silicone mold.

Answer 4

bubbles in water are air

No, bubbles in water could be any gas. But let’s set that aside for a moment, because the quote says:

air molecules will leave … and in doing so will make bubbles

So clearly the bubbles of interest here are air.

if we removed the air how would any bubbles remain

As joseph h’s answer says, the reasonable interpretation here is that only the air above the water is removed.

With that established, the quote basically explains itself: air will obviously not come into the water as rapidly as it did previously, but it will still leave as rapidly as it did previously. Therefore, it will leave more rapidly than it will come in, making bubbles.

Now, let’s address the larger question in the title:

How does removing air from a vessel of water create bubbles?

Removing the air above the water also creates a different type of bubble, consisting of gaseous water. How does this work? Of course, the process that forms such bubbles is called boiling, and normally results from heating the water to its boiling point. But another way to create boiling is to reduce the boiling point to the current temperature (or below), and this can be done by reducing the pressure of the gas above, in this case to zero.