In this technique Vacuum is created in the chamber and water is placed in the chamber. As the pressure decreases so the boiling point also decreases and water start boiling and evaporation starts. leaving behind the solid ice. My question is that how ice is made. Is it like that water takes energy to boil from itself, leaving behind the solid ice?
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$\begingroup$ @Joshua "Vacuum boiling isn't any different" Really? (1) Here, one is reducing the surrounding pressure rather than applying a heat flux. (2) Because evaporation/boiling requires energy, the water can be expected to cool down rather than heat up or remain at ~100°C, as is the case with cooking pasta. (3) Because certain modes of heat transfer don't occur in a vacuum, the cooling could be expected to reduce the temperature substantially. As a result, it would not be remarkable to ultimately obtain ice from water exposed to a vacuum. It would be remarkable to obtain ice when boiling pasta. $\endgroup$– ChemomechanicsAug 1, 2017 at 19:12
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$\begingroup$ When you boil water on the stove, the heat required for boiling comes from the flame that is under the pot. Because that heat is continuously supplied to the pot, and because pressure remains constant, boiling occurs without a temperature drop. When you boil water with a high vacuum, the heat to boil the water comes FROM THE WATER, so the temperature of the water must decrease. If the vacuum is high enough, and the water temperature is low enough, the water freezes. $\endgroup$– David WhiteAug 1, 2017 at 19:46
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$\begingroup$ This is so obvious to me now, my bad. Thanks for pointing these things out. $\endgroup$– NeutronStarAug 1, 2017 at 20:06
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$\begingroup$ Very well explained by David White. Thank you sir. $\endgroup$– HusnainAug 3, 2017 at 9:34
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$\begingroup$ @Husnain, you're welcomed. $\endgroup$– David WhiteAug 5, 2017 at 0:53
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2 Answers
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water takes energy to boil from itself, leaving behind the solid ice
Yes, that's essentially what happens. If it feels strange, remember that the process of evacuation is removing energy from the chamber.
Why this is so is easy to see in the traditional example of a moving wall (or a piston) increasing the volume of an adiabatic chamber: the gas in the chamber performs work on the wall, $$ W = \int F\,\mathrm{d}x = \int pA\, \mathrm{d}x= \int p\, \mathrm{d}V > 0\,\,\text{ (since $p>0$ and $V$ increases)},$$ i.e., the moving wall forces the gas to transfer energy to the environment, and it can only do so by cooling itself. EDIT: That's an equilibrium thermodynamics description that is not very relevant here. See bellow.
What is going on, considering the concrete case of a chamber, is:
- initially there is air and some liquid water at room conditions (1);
- the chamber is then evacuated and pressure drops below the boiling point;
- the water looses energy through ebullition and evaporation, and freezes (2).
Original picture source: Cmglee, https://commons.wikimedia.org/w/index.php?curid=34865054
Some important points:
- There is a cool Youtube video demonstrating this phenomenon.
- That's an out-of-equilibrium process, so thermodynamical variables are at times undefined, and some common assumptions might be unjustified;
- The drop of pressure in the chamber is not the drop experienced by the water, due to surface tension.
- A big chamber (with respect to the amount of water) or the continuous removal of water vapor might be important for freezing to occur.
- This nice answer to the question Water in vacuum (or space) and temperature in space provides some calculations, and qualitatively corroborates the reasoning above.
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$\begingroup$ I do not understand, how your explanation is related to my question. $\endgroup$– HusnainAug 3, 2017 at 9:35
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$\begingroup$ My answer to your question (As the pressure decreases [...] Is it like that water takes energy to boil from itself) is just the part "Yes, that's essentially what happens". The remaining gives an intuitive explanation of why the reduction of pressures leads the water to behave in this way. $\endgroup$– stafusaAug 3, 2017 at 9:50
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$\begingroup$ I don't understand how this is a (complete) answer to the question either. The question is about a phase change. Maybe you could add a phase diagram? The question describes a juxtaposition of boiling water and freezing water, things which at atmospheric pressure happen at very different temperatures. $\endgroup$– uhohAug 4, 2017 at 7:25
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Actual Phenomenon behind the vacuum freezing is low pressure boiling. When the pressure is lowered the boiling point of water decreases. At very low pressure the water start boiling and takes heat of vaporization form water itself. Thus, decreases the temperature of water and eventually water is converted in to ice. More or less it is similar to the phenomenon occur in the evaporator of air conditioner, where refrigerant boils at low pressure and takes this energy of boiling from the room to be conditioned thus decreasing the temperature of the conditioned room. Hope this answer is helpful for all. Thanks
edit: This phase diagram for water from here shows that by moving from atmospheric pressure at 100 kPa down to about 600 Pa, the freezing point and boiling point of water converge to the same temperature. Near this point, the boiling of the water removes enough heat to cool the rest of the water enough for it to freeze. That point at which all three phases can exist at the same time is called the triple point.
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$\begingroup$ More about water can be found at Prof. Martin Chaplin of London South Bank University's amazing site Water Structure and Science. $\endgroup$– uhohAug 4, 2017 at 7:45
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$\begingroup$ To sublimate water completely, attaining triple point is compulsory or is there any other way? $\endgroup$– HusnainAug 4, 2017 at 15:39
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$\begingroup$ It's a good answer, I just added one of my favorite diagrams. As long as the partial pressure of the water vapor above the ice is below the vapor pressure (or equilibrium pressure), it will continue to sublimate. $\endgroup$– uhohAug 4, 2017 at 15:41
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$\begingroup$ As in the phase diagram we can see that at triple point must be 0 degree centigrade but vacuum freezing occur at ambient temperature. Can you please explain. Thanks $\endgroup$– HusnainAug 4, 2017 at 15:42
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$\begingroup$ If you put a thermometer into the ice you will see that the ice itself is not at ambient temperature, it is literally "ice cold!" This is usually done inside a glass vessel, and the glass walls are good thermal insulators. Also note that the triple point is not precisely at 0C, although it is close. $\endgroup$– uhohAug 4, 2017 at 15:46