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What everyone tells me is that hot water transfers its energy to air. But I have also heard of water freezing down in the vacuum of space. How does that happen?

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  • $\begingroup$ I think in vacuum without air pressure, the boiling point of water becomes so low that it boils even at room temperature. $\endgroup$ Commented Apr 19, 2020 at 17:26

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It won't freeze though at the beginning - quite the opposite, it will boil because of zero pressure in space.

You can see in this diagram (by Wikimedia Commons User Cmglee) if water is vapour or solid, given temperature and pressure:

You can see in this diagram (by Wikimedia Commons User Cmglee) if water is vapour or solid, given temperature and pressure.)

It starts out at 20 °C (assuming it has room temperature in your spaceship and you "throw it out of the window") and low pressure (1 Pa is low enough for current purposes) as vapour and then cools down (going left in the plot), becoming solid at some point.

It will cool down to approx. 3 Kelvin (that's approx. -450 °F/-270 °C). That's the temperature of the so-called Cosmic background radiation that "surrounds" matter even in vacuum and interacts with it. Water can't cool any further because the radiation would heat it back up to 3 Kelvin.

Some sources say that this cooling process happens very fast:

When the astronauts take a leak while on a mission and expel the result into space, it boils violently. The vapor then passes immediately into the solid state (a process known as desublimation), and you end up with a cloud of very fine crystals of frozen urine.

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  • $\begingroup$ I have two more questions (1) Can we calculate the time it takes specifically for water to radiate all its heat? (2) Heat is illustrated as kinetic energy of molecules, so is it possible for any moving object in space to lose its kinetic energy through radiation? $\endgroup$ Commented Apr 19, 2020 at 17:38
  • $\begingroup$ (1) It's possible to calculate the time it takes until the water has reached e.g. 3.1 Kelvin - assuming that the heat only escapes via thermal radiation, this should be governed by the Stefan-Boltzmann law - which states how much radiation leaves the object - and the heat capacity of water. You can find more details in this question. I think this happens much slower than the vaporization, so I think the latter is negligible. $\endgroup$
    – zonksoft
    Commented Apr 19, 2020 at 19:05
  • $\begingroup$ (2) Mostly, the kinetic energy will be lost until the object is cooled down to 3 Kelvin. $\endgroup$
    – zonksoft
    Commented Apr 19, 2020 at 19:07
  • $\begingroup$ @zonksoft, water ejected into the vacuum of space will boil until it freezes, then the ice will sublimate. $\endgroup$ Commented Apr 19, 2020 at 19:20
  • $\begingroup$ David, very interesting, I didn't know about the dynamics! Do you have a link to some literature about this? $\endgroup$
    – zonksoft
    Commented Apr 19, 2020 at 19:23
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Everything radiates till it comes in a thermal equilibrium with its surrounding.Now coming to your question and neglecting 0 pressure in space, water will radiate it's heat to surrounding space till it comes in thermal equilibrium with surrounding. Note:temperature of interstellar space is ~3K so if keeping only temperature in mind and neglecting other fact water will reach at most 3K.

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  • $\begingroup$ I have two more questions (1) Can we calculate the time it takes specifically for water to radiate all its heat? (2) Heat is illustrated as kinetic energy of molecules, so is it possible for any moving object in space to lose its kinetic energy through radiation? $\endgroup$ Commented Apr 19, 2020 at 17:37
  • $\begingroup$ Bring realistic it will be very difficult to calculate (this is what I think!) because if keeping in mind that pressure in space is zero then water in space will first boil then it will freeze, process is called Desublimation!...so you must have got what I am saying.. $\endgroup$
    – Barry
    Commented Apr 19, 2020 at 17:49
  • $\begingroup$ Coming to ur second question,i dont really understand that you want to ask about process of heat loss through radiation or that heat loss through radiation in space is possible or not? $\endgroup$
    – Barry
    Commented Apr 19, 2020 at 17:51
  • $\begingroup$ In fact radiation is only mean by which a body can lose/gain heat to space in space and how do this occur is the same process that happens on earth or anywhere. $\endgroup$
    – Barry
    Commented Apr 19, 2020 at 17:52
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hot water transfers its energy to air

Yes it does. But that isn't compulsory for water to cool down. Anything hot will cool down overtime, whether or not its in contact with something like air, as long as its in cooler surroundings. This happens via thermal radiation.

I have also heard of water freezing down in the vacuum of space

A glob of hot water flowing around in space will freeze because the temperature (~ few K) and pressure(~$10^{-10}$mbarr) conditions of space only allow for the frozen phase of water.

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