Boiling Water using Heat vs. Vacuum

[KrisOhn]

If you bring a pot of water to boil, you will notice that the bubbles form on the bottom surface of the pot. This seems to make sense intuitively as this is the hottest surface, so this would be where the boiling action starts.

When I was in school during a thermo lab we pulled a vacuum in a chamber filled with water to induce boiling. I noticed that when you do this the bubbles also form from the bottom surface of the chamber, leading me to think that this is maybe a phenomena caused by the pressure of the water, not from a heat source.

Can anyone shed some light on why this happens? Why does the boiling action not start from a random point in the vacuum chamber? Why does it not start from the surface?

 

[anorlunda]

What is the pressure at the bottom compared to the top?

Does a submarine experience more pressure as it dives deeper?

 

[russ_watters]

Can anyone shed some light on why this happens? Why does the boiling action not start from a random point in the vacuum chamber? Why does it not start from the surface?

My guess:

In order to continuously boil water you have to heat it, and that heat has to come from the metal pot. As the pressure drops, water boils and its temperature drops. Then it transfers heat from the pot, to heat the water and cool the pot.

 

[KrisOhn]

What is the pressure at the bottom compared to the top?

Does a submarine experience more pressure as it dives deeper?

I think Russ already answered my question, but I'm curious as to the result of yours.

Are you implying that a submarine does not experience more pressure as it dives deeper?

 

[anorlunda]

Are you implying that a submarine does not experience more pressure as it dives deeper?

The opposite; it does.Here's your question.

Why does the boiling action not start from a random point in the vacuum chamber?

If the water is all at the same temperature, then you subject it to a vacuum, the water with the least pressure (i.e. near the surface) will boil first. That is not a random point.

 

[hutchphd]

You should also notice that the bubbles in a carbonated beverage typically rise from the bottom of the glass. My guess is that the most important factor in all of these cases is the presence of, and detachment geometry from, nucleation centers.

 

[Baluncore]

Why does the boiling action not start from a random point in the vacuum chamber? Why does it not start from the surface?

When pulling a vacuum, the water should start boiling at the top, since the hydrostatic pressure is greatest at the bottom, and least at the top.

Before the water boils the dissolved gasses will be released and escape. They will form as gas bubbles at imperfections and at sites of contamination on the container surface. That will include the scratched bottom of the container.

For shallow containers scratches may also influence the point where you first see water boiling. In a deep container, such as a 10 metre barometer, the water will boil at or close to the surface.

 

[OmCheeto]

When I was in school during a thermo lab...

Since you don't mention how long ago that was, I can only guess that:

a. you remember incorrectly​

b. your pressure flask had a dirty and/or rough bottom. as mentioned in the last two posts,​

c. things happened too fast to accurately describe what really happened, or​

d. a combination of the above​

In the following video, starting around 2 minutes, nucleation points can be seen from top to bottom of the thermometer, an 'artificial nucleation' device.



When I slowed the video down to its minimum speed, and zoomed in on the left hand side of the thermometer, I noticed that during the initial stages of boiling, the bubbles near the top were swept away first, leaving a blank area. At normal speed, it looks as if they aren't even there! Hence the illusion that the boiling starts at the bottom.

 

[KrisOhn]

If the water is all at the same temperature, then you subject it to a vacuum, the water with the least pressure (i.e. near the surface) will boil first. That is not a random point.

When pulling a vacuum, the water should start boiling at the top, since the hydrostatic pressure is greatest at the bottom, and least at the top.

This is intuitively what I would expect to be correct.

b. your pressure flask had a dirty and/or rough bottom. as mentioned in the last two posts,
c. things happened too fast to accurately describe what really happened, or
d. a combination of the above

I expect it's a combination of the above. I can remember that when I was in the lab and observed the behaviour I asked the instructor about it, but he never had a good answer for it.

Thanks for all the answers everyone.