Why don't ice cubes spontaneously form in a cup of water

In summary, the conversation discusses why ice cubes do not spontaneously form in a cup of water and whether it is possible for the rest of the universe's entropy to increase more than the decreased entropy caused by the ice cubes forming. The second law of thermodynamics and the concept of playing odds in statistical mechanics are also mentioned. It is concluded that freezing is spontaneous at room temperature if the Gibbs free energy of the water phase is significantly increased compared to that of the ice phase. The possibility of ice forming and melting rapidly at a small scale is also considered.
  • #1
Lymitra
6
0
Why don't ice cubes spontaneously form in a cup of water? Someone was explaining to me that for ice cubes to spontaneously form, the water molecules would have to go to a more ordered state. I know about the spontaneous symmetry breaking that occurs when water is at 0'C, but I was wondering if it's possible for the rest of the universe's entropy to increase more than the decreased entropy caused by the ice cubes forming. That way, the Second Law of Thermodynamics is not violated, and it still allows the ice cubes to form. Is this a question of chance then, that we'd have to wait a long enough cosmological time span for the entropy in all other areas of the universe to suddenly increase to such an extent as to make up for the ice cubes forming?

Thanks for answering.
 
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  • #2
Before thinking about the second law of thermodynamics, you need to think about the first. Heat flows from a body at higher temperature to one at a lower temperature. So your cup won't cool down below the temperature of the ambient air.
 
  • #3
Vanadium 50 said:
Before thinking about the second law of thermodynamics, you need to think about the first. Heat flows from a body at higher temperature to one at a lower temperature. So your cup won't cool down below the temperature of the ambient air.

Actually the second law places the constraint on the direction of heat flow, not the first. :smile:

CS
 
  • #4
You're right. :eek:

My brain is clearly broken.:blushing:
 
  • #5
Vanadium 50 said:
You're right. :eek:

My brain is clearly broken.:blushing:

Just a Freudian slip I'm sure! :wink:

CS
 
  • #6
They would sometimes if you waited long enough - statistical mechanics is just a matter of playing the odds.
 
  • #7
mgb_phys said:
They would sometimes if you waited long enough - statistical mechanics is just a matter of playing the odds.

31 days 9 hours. No ice observed. Will keep posted february.
 
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  • #8
Lymitra said:
I was wondering if it's possible for the rest of the universe's entropy to increase more than the decreased entropy caused by the ice cubes forming.

You are describing exactly the process of cooling to 0°C and freezing. Thermal energy and entropy leave the water and move to the surrounding area; of course, the process is only spontaneous if the surroundings are at <0°C, as Vanadium 50 notes. Energy is conserved, and some extra entropy is produced because of the temperature gradient.

Freezing would be spontaneous at room temperature if you could increase the Gibbs free energy of the water phase significantly more than the ice phase. For example, a large enough increase in pressure will make the liquid phase energetically unfavorable because its specific volume is larger than that of the solid phase.

EDIT: The first line of the last paragraph should read "if you you could increase the Gibbs free energy of the water phase significantly more than you increase the Gibbs free energy of the ice phase." You only need [itex]G_\mathrm{water}>G_\mathrm{ice}[/itex] for spontaneity.
 
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  • #9
Maybe the ice is forming and melting rapidly all the time, and it's just happening too fast and at too small a scale for you to see?
 

1. Why don't ice cubes spontaneously form in a cup of water?

Ice cubes do not spontaneously form in a cup of water because the temperature of the water must be lowered to its freezing point in order for ice to form. This requires an external source of cold, such as a freezer or ice cube tray, to lower the temperature of the water enough for ice crystals to form.

2. What is the freezing point of water?

The freezing point of water is 0 degrees Celsius or 32 degrees Fahrenheit. This is the temperature at which water transitions from a liquid to a solid state.

3. Can ice cubes form in water at room temperature?

No, ice cubes cannot form in water at room temperature. The temperature of the water must be lowered to its freezing point for ice to form. At room temperature, the water molecules have too much energy to form into a solid state.

4. Why do ice cubes form on the surface of lakes or ponds?

Ice cubes form on the surface of lakes or ponds because the temperature of the air is lower than the temperature of the water. This causes the water at the surface to lose heat and reach its freezing point, forming a layer of ice on top.

5. How do impurities in water affect the formation of ice cubes?

Impurities in water, such as minerals or particles, can lower the freezing point of water. This means that the water will need to be colder in order for ice to form. Additionally, the impurities can act as nucleation sites, providing a surface for ice crystals to form on and promoting the formation of ice cubes.

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