# Why is boiling water bubbling?

#### Khashishi

What's the difference between a column of gas and a column of liquid?

#### rogerk8

An interesting question.

My thought is that a liquid is incompressible while a gas is highly compressible.

So taking a "slice" of water would not matter if there is water next to it.

Taking a slice of gas and the gas will spill all over the place.

Please observe that this i mainly me playing around a bit while seriously trying to understand basics.

Best regards, Roger

#### rogerk8

The normal process of boiling can be looked upon as an exaggerated form of convection. It's something that can't happen in zero g. If you have a heating element immersed in water in a space ship, you will just get a volume of vapour round the element (effectively insulating it, thermally) and the pressure can increase catastrophically without heating the liquid that's quite near to the element.

It is common for bubbles to collapse as they rise (normal g) because the surrounding liquid is cooler and the vapour can condense. That can be a noisy process and is sometimes referred to as Bumping. They put 'anti bumping granules' in the water in laboratory distillation equipment (many nuclei will prevent single, large bubbles forming).
Very interesting to know!

Referring to the first paragraph, what do you mean by catastrophically?

Do you perhaps mean that the thermal insulation by the vapour is so good that pressure can become extreme?

And all this due to zero g, right?

Yet, I don't understand pressure in this context.

"Column of fluid-pressure" (or Gravitational Pressure) I now understand but this kind of pressure (J/m^3) I simply do not understand.

Best regards, Roger
PS
I will now study convection further.

#### rogerk8

The most interesting part was that due to gravity, hotter less dense fluids rise above colder ones which fall downwards creating a circulation.

This is called a convection cell.

So, sophiecentaur's example without gravity is an interesting example of the extreme.

That is, without gravity, there is no convection and pressure can become "catastrophical".

Is this correctly understood?

Best regards, Roger
PS
It is also interesting to hear that the vapour itself is actually insulating the element thermally from the liquid. How come?

#### sophiecentaur

Gold Member
what do you mean by catastrophically?
pressure can become "catastrophical
My "catastrophe" was more to do with what can happen to a heating element that is designed to heat water, when it suddenly finds itself in an insulating gas and gets much hotter than the designer expected. I wouldn't expect the pressure necessarily to rise particularly high. I would expect the small amount of gas around the heater to expand proportionally to the absolute temperature and the pressure would depend more on the pressure in the cabin (?) if the change were not to quick. (you wouldn't expect to be boiling water in a pressure vessel, would you?) That temperature would depend upon (be limited by) other forms of heat loss. I guess it could be quite high

#### rogerk8

First, I feel that have to correct my stupid self.

Consider two hollow glass tubes of say 1cm diameter (even though it doesn't matter) and 1m of length.

Both are sealed air-tight at one end while integrating a pressure sensor.

The first tube is then filled with water.

The second tube is "empty" (i.e air of normal pressure resides inside).

Putting these tubes vertically gives

$$P_{water}=\rho gh\propto 1000*10*1[Pa]$$

and

$$P_{air}=\rho gh\propto 0.001*10*1[Pa]$$

So the pressure differs about a million between water and air.

Noteworthy is that a 1m "column" of water means a tenth of normal air pressure.

That is, pressure while diving increases by one atm at each ten meters.

Best regards, Roger

#### rogerk8

My "catastrophe" was more to do with what can happen to a heating element that is designed to heat water, when it suddenly finds itself in an insulating gas and gets much hotter than the designer expected. I wouldn't expect the pressure necessarily to rise particularly high. I would expect the small amount of gas around the heater to expand proportionally to the absolute temperature and the pressure would depend more on the pressure in the cabin (?) if the change were not to quick. (you wouldn't expect to be boiling water in a pressure vessel, would you?) That temperature would depend upon (be limited by) other forms of heat loss. I guess it could be quite high
But what if I would?

I'm hopeless with pressure so just let me use the existing formulas (while not actually teaching me anything):

$$P=n_{mol}RT=nkT$$

This is the ideal gas law (igs) without the Van der Wall equation of state (eos).

Yet it is a gas (and not a liquid?) law.

So what happens if we where about to increase the pressure of the vessel?

There is no vapour yet so can we really use igs?

If we can, higher pressure should just mean that temperature should have to be higher.

But this is by simply inspecting igs.

It tells me absolutelly nothing about what's really going on!

Best regards, Roger
PS
You may also look at igs and say that, well if pressure in the vessel rises, temperature of the fluid rises. But this has to be nonsense.

#### rogerk8

1) Pressure is a scalar (which means it has no direction).
2) Since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume.
3) Gauge Pressure is relative to normal air pressure (atmospheric pressure).
4) In a static gas, the gas as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion.
5) Considering a bucket with a hole in it, the speed of liquid out of the hole is [PLAIN]https://upload.wikimedia.org/math/0/8/3/0835a4bbe607438986f2a8705e3dab96.png, [Broken] where h is the depth below the free surface.
6) Interestingly, this is the same speed the water (or anything else) would have if freely falling the same vertical distance h.

Point 2 gives J/m^3 as unit but is not totally revealed because what makes the pressure?

What is the mechanism?

It sounds good but what is "potential energy stored per unit volume" really?

Best regards, Roger

Last edited by a moderator:

#### sophiecentaur

Gold Member
It is also interesting to hear that the vapour itself is actually insulating the element thermally from the liquid. How come?
.. . . and many others of your questions.
It seems to me that probably you need some more basic Physics if you are to go further. one can't run before walking.
There are many sources of info on the web but they will not all suit your particular level. I found this one which may help you. Try to read a lot of it before coming up with questions.

#### rogerk8

In yet another attempt in saving my face I need to revise my pressure calculations above.

In the air case I am just measureing ambient (which I prefere to call it) pressure.

Nothing else.

This came clear to me while thinking a widening of the tube diameter and its impact on pressure.

I could just widening it as much I want while considering the pressure outside of the border/tube.

The pressure outside of the tube will of course be the same as inside of the tube.

And it doesn't matter how high the tube is.

You will still get the ambient pressure (at the bottom to be extremely thorough).

In other words, Gauge Pressure is zero in this case.

When it comes to water I am however measureing a total pressure (ambient + gauge).

While water pressure is so much higher than ambient pressure I may however say that I am measureing the water (gauge) pressure only.

Best regards, Roger
PS
Now that I know this I may actually calculate the hight of the atmosphere.

$$p=\rho gh\propto 100kPa$$

Using the approximate density of air (which however probably varies with hight) from above we get

$$h=\frac{p}{\rho g}\propto\frac{10^5}{10^{-3}*10}=10^7m=10^4km$$

Hmm, I don't think the atmosphere is 10000km :D

So something is wrong here.

Still I think that within a couple of thousand meters above the surface of Earth, the density is about the same.

And I do think that the hight of the atmosphere is somwhere around 100km.

#### rogerk8

.. . . and many others of your questions.
It seems to me that probably you need some more basic Physics if you are to go further. one can't run before walking.
There are many sources of info on the web but they will not all suit your particular level. I found this one which may help you. Try to read a lot of it before coming up with questions.
Thank you very much for that link.

Please correct me if I'm wrong but isn't the whole purpose of a forum to ask questions?

And cut some corners?

I have studied more than you might believe and I'm tired of theory!

I want to understand.

To understand my own way.

Not by reading a lot of formulas about everything which often actually teaches you nothing.

Nothing that makes you really understand, anyway.

So I'm using this nice forum to "blog" about my thinking.

Hope you don't mind

Best regards, Roger
PS
Please do not take my questions too seriously. Sometimes I ask a question just to get it on the table while being only lightly insecure about the answer.

In any case, I refere to my signature.

Last edited:

#### rogerk8

sophiecentaur, I wish to add a link in my signature would you mind helping me?

I can't seem to find out how to do that.

Best regards, Roger

#### rogerk8

I take back my former statement, your link seems to lack equations almost totally so it is just educational.

Thanks!

Best regards, Roger

Last edited:

#### sophiecentaur

Gold Member
I'm tired of theory!
As far as I am concern ed, "tired of theory" is the equivalent of "tired of Physics".
If, as you say, you want short cuts then you run a massive risk of getting such a limited view of the subject that you will get totally the wrong picture.
If you don't like equations then you are also limiting your potential understanding. Maths (and I have made this point so so many times on PF) is the language and If you don't want to use it, you just can't get it. Someone else on PF likened it to the process of trying to study French poetry without learning the French language.

I wish to add a link in my signature would you mind helping me?
Sorry. I have no idea. You could try one of the computer based sections of PF.

#### rogerk8

Interesting.

You misunderstood me completelly.

I love equations, but reading litterature often gives you equations while you at the same time do not fully understand the basics behind them.

What I'm saying is that you need to have a kind of abstract understanding of the subject before you are being served equations.

Otherwise the equations just look nice.

Like the ideal gas law which I still do not understand.

But I'm beginnig to grasp the concept of (gas) pressure in the terms of J/m^3 with the thankful aid of Wikipedia who told me that a system under pressure has the potential to do work on its surroundings which means that pressure is a measure of potential energy stored in a unit volume.

I can relate to this but actually I do not understand this at all!

Best regards, Roger

#### rogerk8

Today at gym it suddenly struck me that

$$P=\rho_s \frac{dv}{dt}...[N/m^2]$$

This while only thinking unit substitution in the familiar

$$F=m\frac{dv}{dt}...[N]$$

But this also makes me kind of begin to understand pressure which is my quest.

It tells me that gas pressure has to do with collective behaviour of a gas.

Because it has mass within a unit area (i.e surface density).

It seems like force isn't relevant within a gas and that it's counterpart is pressure.

This is obvious from the way I derived this.

But don't think I'm ready with my quest.

Far from it!

Let's view the acceleration part that is

$$a=\frac{dv}{dt}$$

How do we know it?

Imagening a particle of gas hitting the cannister wall.

If we for simplicity consider totally elastic collision.

The impulse change is then (normal angle)

$$\Delta p=2p$$

or with regard to speed, 2v due to same mass.

So we have a maximum speed change of 2v.

Okey, lets consither this and set

$$dv=2v$$

Now, during how long time did this speed change happen?

Because if we doesn't know that, we do not know what the pressure is.

Even though we might be able to calculate it knowing density and temperature, but that is cheating

So what limits the rate of speed change?

Because throwing a steel ball into a steel wall will give the same speed after as before but during what time span?

Wait a minute, wouldn't the steel ball just fall to the floor right next to the wall?

I understand nothing

Best regards, Roger
PS
I should have said "I'm tired of reading" and not "I'm tired of theory", sorry.

Also, I think it is important to fully understand all the variables before any equation really makes sense.

Last edited:

#### rogerk8

If you take a column of water and throw that at a window at a certain speed, the window will shatter at a certain length of that column.

But if you take a column of gas and throw that at a window at the same speed, the window will shatter at a certain area of that column.

The first statement is due to liquid incompressibily which makes force a matter of column length.

The second statement is due to the gas being compressible and thus force is not dependent of column length but on the integration of surface density.

How far off am I now?

Best regards, Roger

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving