Experiment to prove that water has air

Question

In elementary school, there was an experiment to show that water has dissolved air, which involved boiling some water and observing the drops at the surface of the container, and this proved that water has air. Now, in senior high school, I was thinking about the same, and I think this conclusion is wrong. Aren't the drops formed by the conversion of liquid water to steam due to evaporation? Or is it really the dissolved air that is escaping? What are your thoughts?

Answer 1

There are two ways that I interpret the question you're asking, and I'll try to address each in turn.

1. If your teacher boiled a container of water and held a piece of glass above it to collect drops of liquid water, he or she may have been trying to show that water doesn't disappear when it boils, that instead it turns to a gas that can move through the air and can be condensed back into a liquid. Essentially, $\ce{H2O}$ is $\ce{H2O}$, regardless of the state of matter it occupies.

2. If instead the teacher wanted you to observe the bubbles of gas forming in a container of hot or boiling liquid, he or she might have been on the right track, but either failed to convey the correct message to you or had the wrong idea in the first place.
   
   If I take water from the tap and heat it strongly, bubbles will form on the inside of the vessel I'm heating it in prior to it boiling. These are bubbles of gas that are no longer soluble because the temperature of the liquid is too high (and its pressure has decreased as compared to the tap.) The gas is dissolved air (mostly carbon dioxide, I suspect) with a large portion of water vapor. In any case, they are bubbles of gas that were dissolved but are no longer. You can see this happen any time you boil a pot of water on the stove.
   
   Once the pot reaches boiling, any bubbles are almost exclusively gaseous water formed via vaporization.
   
   An easier, if slower, way to see this is to run a cold tap until the water is as cold as it will get, then pour a glass. As it sits (perhaps for an hour or two), bubbles will eventually form on the walls of the glass. These eventually will dissolve again and the gas will escape through the top of the glass, so it's not something you can set up at night and come back to the next morning. The bubbles are air that was able to dissolve in the high pressure/low temperature water in the pipes but not in the low pressure/high temperature water in the glass.

Answer 2

A German scientific TV show once showed a great experiment (alas, I cannot find any copy of it on Youtube). They had two pots of water. In each of them there was a large water-filled closed-top glass cylinder that reached out of the water so that its upper part was freely visible. They then boiled the water in both pots for some time; part of the gas was collected in these cylinders and made the water level in them drop.

Then, they let the water cool down again (this was recorded in fast motion). In one of the two cylinders (calling it cylinder 1) the water level rose back to the top, the other one kept a cushion of air at the top of the cylinder (cylinder 2).

The explanation: The water in cylinder 1 and its corresponding pot had been pre-boiled. Since gases are better soluble at low temperatures, boiling and re-cooling effectively removed all dissolved gases from the water of cylinder one before the experiment. Thus, the only gas to be observed at the top of cylinder 1 was water vapour.

Cylinder 2’s water had not been pre-boiled. There were still gases dissolved in the water which again escaped upon boiling. These gases were collected at the top of cylinder 2 along with water vapour. When the setup was cooled down, the water vapour recondensed (to a significant fraction, at least) but the previously dissolved gas remained on top.

Answer 3

Your description is somewhat vague ("boiling some water and observing the drops at the surface of the container"), but it does seem unlikely that such an experiment demonstrates that air dissolves into water. If by 'drops' you mean gas bubbles, then yes: the gas bubbles in boiling water are substantially all water vapor, not air. You are correct: any drops of liquid forming on the inner surface of the container would be from re-condensed water vapor, itself evaporated from the boiling liquid, and would have nothing to do with air having been dissolved in the water.

What I would do to attempt to demonstrate that air dissolves into water would be to rig up a 2L clear plastic (NOT GLASS) soda bottle so that I could connect it to an air compressor. (I'd have to think a bit how to accomplish this.) Then:

1. Fill the bottle about half full with plain water
2. Cap the bottle & connect to the compressor
3. Wearing safety glasses and being very careful (the bottle could burst, or turn into a rocket if the cap comes loose), pressurize the bottle to 100psi (~700 kPa),
4. Let the bottle stand, pressurized, for a while -- thirty minutes, maybe? -- periodically swirling it gently.
5. CAREFULLY vent the pressure over 5-10 seconds, like when opening a bottle of soda that's been shaken.
6. If no bubbles appear, tap the side of the bottle several times.

The amount of air that dissolves in water increases with the pressure. The hope would be that enough additional air dissolves under 100psi air, that when you release the pressure in step 5 and the amount of air the water can hold then drops back down to normal, the excess air will form a bunch of bubbles throughout the water.