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

#### 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?

#### 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.

#### 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.

#### 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
« Last Edit: 01/01/2008 22:14:11 by ukmicky »

#### 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.

#### 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

#### 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:

#### Simulated

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

#### 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?

#### 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 0°C 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 = 0°C) inside a room which temperature is 25°C; the spere melts very slowly. You take now a thin pane of ice (T = 0°C), the same mass of the previous sphere, inside a room which temp. is 10°C 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 = 0°C) inside a thermos and then you put the thermos inside the fridge, which T = - 30°C; it freezes only after many hours. You pour 100 litres of liquid water (T = 0°C) on an aluminum pane which temp. is -1°C and the water freezes immediately, why? Same reason as up, just the heat flow is in the opposite direction.

#### 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 »