What temperature does water freeze at?

Autopilot

Distinguished Member
Ah, but before everyone shouts "zero degrees Celsius, duh" - is it?

Tonight's gadget show question is this, with the obvious answer being zero Celsius.

But i thought zero was the temperature that ICE MELTED at, not the same thing. In fact i seem to remember this also being on IQ with freezing at zero being the common misconception. Am i wrong?
 

tonyrees687

Well-known Member
I thought it was how long it takes. Water will not freeze at O straight away.
 

Inferno

Distinguished Member
Pure water freezes at 0 degrees centigrade.
 

Confucius

Active Member
Pure water freezes at 0 degrees centigrade.

Indeed, it's how the Celcius scale is defined.



The not quite freezing mentioned earlier referred to the triple point of water - the point at which all three states can coexist in thermodynamic equilibrium.

Anyhoo, water reduced to 0C will not freeze until more energy is removed, it will stay at 0C until frozen. Same in reverse; when energy is put in to ice it will reach 0C, melt while remaining at 0C, then the water will rise in temperature.

Obviously this must be accomplished in controlled conditions, a std household freezer will have (relative) hot and cool spots, parts of a body of water, or ice, will have different energy states.

(It's been several years since my A-levels but I'm pretty sure I remembered the theory closely enough. It might have been during the O-level syllabus in fact)

I seem to recall that it took almost as much energy to melt a given quantity of ice (that remained at 0C) as to heat that same body of H2O from 0C to 100C.

EDIT: Useful page of info... http://chemistry.about.com/od/workedchemistryproblems/a/Heat-Capacity-Phase-Change-Example-Problem.htm
 
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johntheexpat

Distinguished Member
It gets interesting when you have super-cooled water. If you get a spotlessly clean test tube (or any other glass receptacle) it is quite easy to cool the water to well below freezing and have a super-cooled liquid. If you then add something that the water can freeze around (a seeding agent) then you can watch it rapidly turn to ice in front of your eyes. You can do this at home!

To get really cold water, you will need to get a mixture of salt and well crushed ice, mixed really well, which will have a temperature of well below zero. Then take your test tube of pure water (distilled is best, but deionised should do) in a very very clean glass container. (Test tube or brand new tumbler, or similar) and then cool the pure water in the glass inthe ice-bath. After a while the temperature of the pure water drops below zero. When it is as cold as its going to get, add a spec of dust, a piece of soil or rub the side of the glass with something hard and you should see the pure water turn to ice before your very eyes. In a couple of seconds, more or less. Its really very impressive as a party trick.
 

DPinBucks

Distinguished Member
...I seem to recall that it took almost as much energy to melt a given quantity of ice (that remained at 0C) as to heat that same body of H2O from 0C to 100C....
Latent Heat of Fusion of water (ice <-> liquid) = 334 KJ/Kg

Heat required to raise water by 100K = 4.18 x 100 = 418 KJ/Kg

So it's not far off: about 3/4 in fact.

A pool of clean water in an air temperature of 0C will not freeze: as you say, there is no temperature difference, and therefore no heat transfer and therefore no freezing. But lower that temperature by even a tiny amount and it will eventually freeze, depending upon air currents taking the heat away.
 

saintie1

Active Member
But what is really amazing is that the solid form of water (ice) floats. Now if it wasn't for this fact life on earth would be dramtically different!
 

pandemic

Well-known Member
That's not really amazing the ice form just has a lower density than the liquid form.

What I find cool is how much the behaviour changes when a slight 'impurity' is added to the water. i.e. our tap water, the last time I tested it had boiling point just above 100 Celsius.

Or that if you put hot water in a ice cube tray and water straight out the tap in a tray, put them both in the freezer. The warmer one freezes first.
 
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shodan

Distinguished Member
I'm definately going to try to get some of this info into some normal conversations today....
 

DPinBucks

Distinguished Member
That's not really amazing the ice form just has a lower density than the liquid form....
Yes, but the fact that it does really is amazing.

To my knowledge, no other solid is less dense in its liquid phase. The fact that ice floats is fundamental to life's survival, because it prevents lakes and ponds from freezing solid and it traps a thin layer of air.

=== EDIT ===
In fact, I've now found quite a few substances which float when they solidify. But water is far and away the most significant.
 
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mjn

Distinguished Member
It gets interesting when you have super-cooled water. If you get a spotlessly clean test tube (or any other glass receptacle) it is quite easy to cool the water to well below freezing and have a super-cooled liquid. If you then add something that the water can freeze around (a seeding agent) then you can watch it rapidly turn to ice in front of your eyes. You can do this at home!

To get really cold water, you will need to get a mixture of salt and well crushed ice, mixed really well, which will have a temperature of well below zero. Then take your test tube of pure water (distilled is best, but deionised should do) in a very very clean glass container. (Test tube or brand new tumbler, or similar) and then cool the pure water in the glass inthe ice-bath. After a while the temperature of the pure water drops below zero. When it is as cold as its going to get, add a spec of dust, a piece of soil or rub the side of the glass with something hard and you should see the pure water turn to ice before your very eyes. In a couple of seconds, more or less. Its really very impressive as a party trick.

Didn't Mythbusters do something a little similiar with beer in a fridge? Using a certain temperature they could make it freeze by simply tapping the edge of the bottle?
 

nheather

Distinguished Member
I've had similar experience with bottles of lager. I put them in the freezer for a few hours after buying them in the hope of getting them cold quickly.

I left them in for an hour or two. When I took them out they were still liquid.

Popped the top off one, and I could see the beer crystallise before my eyes - it took a few seconds and you could plainly see the crystallisation 'moving' through the bottle.

It didn't freeze solid but sturned into a beer slush puppy - sounded nice at first but was a horrible sensation to drink.

Also noticed that if I let it melt back to liquid which only took a few minutes the beer was absolutely flat, no gas at all.

Cheers,

Nigel
 

kav

Distinguished Member
I've had similar experience with bottles of lager. I put them in the freezer for a few hours after buying them in the hope of getting them cold quickly.

I left them in for an hour or two. When I took them out they were still liquid.

Popped the top off one, and I could see the beer crystallise before my eyes - it took a few seconds and you could plainly see the crystallisation 'moving' through the bottle.

It didn't freeze solid but sturned into a beer slush puppy - sounded nice at first but was a horrible sensation to drink.

Also noticed that if I let it melt back to liquid which only took a few minutes the beer was absolutely flat, no gas at all.

Cheers,

Nigel

I've had this a few times too - you check the bottle before opening it and it seems to be fine, then as soon as you open it it turns to slush. Horrible!
 

DPinBucks

Distinguished Member
I've had similar experience with bottles of lager. I put them in the freezer for a few hours after buying them in the hope of getting them cold quickly.

I left them in for an hour or two. When I took them out they were still liquid.

Popped the top off one, and I could see the beer crystallise before my eyes - it took a few seconds and you could plainly see the crystallisation 'moving' through the bottle.

It didn't freeze solid but sturned into a beer slush puppy - sounded nice at first but was a horrible sensation to drink.

Also noticed that if I let it melt back to liquid which only took a few minutes the beer was absolutely flat, no gas at all.

Cheers,

Nigel
Strictly speaking, that's not supercooling. A liquid under pressure will have its freezing point reduced, so in fact even in the freezer it's still above it. Take away the pressure and the freezing point rises so the liquid freezes. I've even seen it happen to a bottle of white wine, which is pressurised slightly when the cork is hammered in.

Supercooling is when it's still liquid below its freezing point.

I've no idea why the beer was flat. The gas must have gone somewhere.
 

har42

Novice Member
The triple point of water is 0.01C at this point it can be a gas, liquid or a solid, i tend to say water freezes at 0C as lots of factors can affect it and most people look at you oddly if you say 0.01C
 

imightbewrong

Distinguished Member
So the lesson there is - make sure your beer is above zero before you open the can :)
 

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