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Let's say you have a spherical container in a zero gravity environment filled completely with water. When you heat the sphere uniformly how would the water boil?

My intuition is that the water would start boiling at the very outer layer of the sphere first. So you would have a ball of water inside the sphere that gets smaller and smaller as more water turns into gas. Thus the pressure rises and the boiling point goes up and the layer of water vapour gets bigger and bigger, which means the water in the middle will take longer and longer to boil.

Is this correct or am I missing any important concepts here?

macco
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  • Is the spherical container rigid? What would be the initial pressure of the liquid water in the container before you start adding heat? Is the heating very gradual, or do you have a very high heat flux to the water at the surface of the sphere? – Chet Miller Jun 02 '17 at 19:08
  • Yes, the container is rigid and the liquid water would be at room temperature (20 Celsius). I assumed gradual heating, but if it behaves differently with high heat flux that would be of interest to me too. – macco Jun 02 '17 at 19:12
  • Well, for gradual heating, you could assume that the system is always at thermodynamic equilibrium at the new higher temperature. What makes you think that any vapor would form? You are aware that the specific volume of liquid water increases with increasing temperature at constant pressure, correct? Do you know how to use the Steam Tables? – Chet Miller Jun 02 '17 at 21:43
  • If the sphere is large enough or the heating fast enough I don't think we can assume that the system is always at thermodynamic equilibrium? I think vapor would form because as the water is heated it would change to a gas. If heated enough the rigid container would have to explode sometime? (Given that the container doesnt melt). I don't know what steam tables are. – macco Jun 02 '17 at 21:57
  • Yeah sorry, I meant when heated fast. – macco Jun 02 '17 at 22:02
  • The container won't explode if you say that say that it is strong enough that it won't. Do the problem for gradual heating first, and then start thinking about how to do it for rapid heating. Why? Because, if you can't solve it for slow heating, you certainly won't be able to solve it for rapid heating. – Chet Miller Jun 02 '17 at 22:05
  • I did try giving my intuition from what I know what would happen, in my question. What concepts is my explanation missing? Or is it just wrong altogether? – macco Jun 02 '17 at 22:15
  • that last statement seems off. Why would it be colder inwards? –  Jun 02 '17 at 22:33
  • even if it were, the vapour would not appear inside first because the middle can only be as hot as the outside –  Jun 02 '17 at 22:35
  • You are missing that the liquid water wants to expand when you heat it, and this increases the pressure imposed by the sphere. – Chet Miller Jun 02 '17 at 22:40
  • At 100 C, if the liquid water in the sphere were forced to be contained within the same volume as initially, its pressure would rise to 1000 bars. So no vapor would be formed. – Chet Miller Jun 02 '17 at 22:58
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    The only way that this experiment has a chance is if the thermal expansion of the container is greater than the thermal expansion of the water. Otherwise, the pressure of the liquid water will increase until the container ruptures. And yes, @macco, you are missing some important concepts here. The heat of vaporization goes DOWN as the water temperature goes up, until it becomes zero at the critical point. This means that the vaporization rate of water goes up as you add heat at a constant rate. However, the heating rate would go down as the water heats, assuming a constant hot temp. – David White Jun 03 '17 at 02:21
  • Continuation of last comment: The competing factors of thermal expansion, decreasing heat of vaporization, decreasing heating rate, lack of convection while in free fall, and possibly other factors, would make this a difficult problem to quantify. – David White Jun 03 '17 at 02:23
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    If the container were not completely filled with liquid water to begin with, but instead contained both liquid water and a certain minimum fraction of water vapor at the equilibrium vapor pressure, then, as you increased the temperature, some of the liquid could evaporate and form more vapor at the new higher equilibrium vapor pressure. – Chet Miller Jun 03 '17 at 05:24

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Given your conditions (a strong rigid container filled with water) it would never boil no matter how much heat you add. The lack of gravity would not change this answer.

Boiling causes water to change from a liquid to a gas. As a liquid, the molecules are always touching (but they are not rigidly connected as they are in ice). So as water they take up little space. In a gas, as steam, the molecules are flying around freely and spend relatively little time bumping into each other. They take up a lot more space as a gas. The pressure of the gas on the walls of a container arises from the constant bombardment of the molecules hitting the walls of the container. If there is less space for the steam then there will be more bumping into the walls so there will be more pressure.

What would happen in your question is that as you add heat the water would try to expand a little. Finding no room for expansion, the pressure would rise dramatically. The boiling point of water is very dependent on the pressure, so as the pressure goes up so does the boiling point.

At some point it will pass the "critical point" of water, beyond which there is no phase change between liquid and gas. The critical point of water is known to be about 374° C and 3212 PSI, which is well over 200 atmospheres.

Now, if you change your conditions to an open container in no gravity, but in a space craft with normal atmospheric pressure, some of the water might gradually float out and form balls because of surface tension. But this is not boiling. It would be hard to heat the water this way.

If you change the conditions again so that the open container is exposed to the vacuum of space, then the vacuum means the boiling point of water pretty much matches the freezing point of water. Water at 20° C would way above the boiling point and so it would instantly (perhaps explosively) boil until the cooling effect of doing so brought the temperature of any remaining water down to near freezing.

At 0.01° C and just under 1% of an atmosphere of pressure, you have reached the triple point of water. This is where ice, liquid water and steam can exist in equilibrium. But in the vacuum of space you have zero pressure and there can be no liquid water. At zero pressure, water sublimes (changes directly from ice to steam with no liquid state, like dry ice) at about -60° C.

Search "critical point of water" and "triple point of water" for more info, Wikipedia articles and YouTube videos.

phr
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