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Author Topic: Alright water freezes at 0 degrees C..  (Read 9678 times)

Offline Simulated

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Alright water freezes at 0 degrees C..
« on: 31/12/2007 15:38:27 »
And I was told that it can only swich states, between ice and water, at 0 degrees C. So how come the colder it is outside say -10 degrees C the water freezes faster then it does at 0 degrees C?

Am I just missing something?


 

Offline lightarrow

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Alright water freezes at 0 degrees C..
« Reply #1 on: 01/01/2008 14:07:54 »
And I was told that it can only swich states, between ice and water, at 0 degrees C. So how come the colder it is outside say -10 degrees C the water freezes faster then it does at 0 degrees C?

Am I just missing something?
Water at T > 0 have to release its internal energy to the environment through a heat flow to it. The greater the T difference between water and env., the greater this heat flow, and so water freezes faster.
Of course the same for every other thing.
 

Offline Simulated

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Alright water freezes at 0 degrees C..
« Reply #2 on: 01/01/2008 18:03:48 »
Thanks lightarrow. I beileve I understand it better now.
 

Offline ukmicky

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Alright water freezes at 0 degrees C..
« Reply #3 on: 01/01/2008 21:27:11 »
Quote
And I was told that it can only swich states, between ice and water, at 0 degrees C.


Am i right in thinking that the freezing point of water changes at different pressure's and the lower the pressure the higher the freezing point . If so would that men the freezing point of  water is relative.
In other words if there were people living on another planet who had a temperature scale like ours, there 0 degrees freezing point wouldn't tally with ours as there planet may have different air presure at sea level.

Or am i totally cuckoo, be nice I'm a Mod  ;D ;)
« Last Edit: 01/01/2008 22:14:11 by ukmicky »
 

Offline turnipsock

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Alright water freezes at 0 degrees C..
« Reply #4 on: 01/01/2008 22:02:10 »
The lower the pressure, the lower the freezing point.

Another thing is that water boils at a lower temperature at higher altitudes (lower pressure), which helps to explain the lack of tea shops on Everest.

 

Offline ukmicky

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Alright water freezes at 0 degrees C..
« Reply #5 on: 01/01/2008 22:16:41 »
So therefore the 0 degrees freezing point of water is just an earth thing
 

Offline lightarrow

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Alright water freezes at 0 degrees C..
« Reply #6 on: 02/01/2008 17:28:31 »
The lower the pressure, the lower the freezing point.
No, it's the opposite for water:
 

Offline Simulated

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Alright water freezes at 0 degrees C..
« Reply #7 on: 02/01/2008 19:11:25 »
Thanks guyys
 

Offline opus

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Alright water freezes at 0 degrees C..
« Reply #8 on: 02/01/2008 19:15:45 »
isn't 0 also the melting point of ice? If so how does the water know whether to freeze or melt at 0?
 

Offline lightarrow

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Alright water freezes at 0 degrees C..
« Reply #9 on: 03/01/2008 12:43:26 »
isn't 0 also the melting point of ice? If so how does the water know whether to freeze or melt at 0?


It's not enough that water reaches 0C to change phase. If it's in the solid form (ice) it have to acquire a specific amount of heat to become liquid (melting enthalpy); if it's in the liquid form, it have to release that same heat to the environment; the last could be, depending on the situation, easier or more difficult than the first.

Examples:

1. you have a big sphere of ice (T = 0C) inside a room which temperature is 25C; the spere melts very slowly. You take now a thin pane of ice (T = 0C), the same mass of the previous sphere, inside a room which temp. is 10C and it melts completely in a few seconds, why? Because the heat flow from room to ice is much greater in the second case.

2. you put a glass of liquid water (T = 0C) inside a thermos and then you put the thermos inside the fridge, which T = - 30C; it freezes only after many hours. You pour 100 litres of liquid water (T = 0C) on an aluminum pane which temp. is -1C and the water freezes immediately, why? Same reason as up, just the heat flow is in the opposite direction.
 

Offline opus

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Alright water freezes at 0 degrees C..
« Reply #10 on: 06/01/2008 17:58:10 »
Thankyou Lightarrow- I think I understand now.
 

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Alright water freezes at 0 degrees C..
« Reply #10 on: 06/01/2008 17:58:10 »

 

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