Water

[clew]

What temperture does water freeze at?

[mongos bird]

zero degrees centigrade

[ll_billym]

You are going to wish you never asked this one.

[silly moo]

What height above sea level are you ?

[Grunty]

And how much salt is in the water?

[wayneo]

try boilin point

[JOEYSHABADO]

Ice melts at 0 degrees Centigrade, under pressure conditions of 1 Atmosphere. Water freezes normally a fraction of a degree below this. The melting point of ice and the freezing point of water are not the same point.

Furthermore, if the water you are using is completely pure, in a completely clean beaker, you can supercool the liquid whereby the water will be liquid at a few degrees below zero. The ice crystals need somewhere to form. This only lasts a little while though, at which point the water molecules bump together and the whole lot freezes at once.

So, under normal conditions, water freezes at about -2 degrees C, and ice melts at 0 degrees C.

[volvo179]

Whatever may be the precise temperature why does water which is frozen have a volume about 10% more that before it froze?

[magicdice]

it's to do with all the hydrogen bonding in it - when H2O becomes solid, it turns into a lattice structure which is less dense therefore same mass requires a larger bonding

[lara_maesteg]

following on from magicdices post - this is why there will always be some liquid water at the bottom of a frozen pond. As water reaches its maximum density at around 4�C, the ice that forms is less dense, and floats on top of the water.

[Gef]

Sorry but in his first and third paragraphs Joey is wrong. The freezing point of pure water is exactly the same as the melting point of ice made from pure water.

[JOEYSHABADO]

Ok, let's go heads up on that one. What's your evidence?

[newtron]

The way I understand it, at zero deg C and 1 atm pressure, ice and liquid water can coexist.  Energy being added or extracted from the system causes the phase change instead of changing the temperature of the system.  So if your are removing heat from the system, the temperature will remain zero deg C untill all the water has turned to ice.  Then the temperature of the system will begin to decrease below zero deg C.  If you are adding heat to the system, the temperature will remain at zero deg C untill all of the ice has melted.  Then the temperature will begin to increase above zero deg C.   

[JOEYSHABADO]

Thanks Newtron, but where's the evidence that for pure water, the temperature of the water will not drop below 0 degrees C until it has turned into ice?

[newtron]

http://www.chemistrycoach.com/Phase_diagram.htm

Check out this phase diagram.  The boundary lines represent the freezing/melting/vaporization points for a given pressure and temperature.

[JOEYSHABADO]

I don't see how that demonstrates what you're saying.

My point is that freezing is a more complex process than people imagine: it's the formation of ice crystals, and this can only occur under certain conditions. Pure water, or water that is fairly pure but in a very small quantity (like in the cells of an arctic animal), cannot freeze at 0oC, or even -1oC, because the ice crystals cannot form. When I spoke of 'normal conditions' above, I meant 'pure water". In fact, I think you'll find that normal run of the mill tap water, especially in a smooth-lined beaker, will not freeze above -1oC. If you want, I can dig out the docs. But very interested in what you have to say.

I would say that I'm very well qualified to speak about this because I live in Scotland and it's bloody freezing. All the time. But the water certainly isn't pure.

[Gef]

joey I live in Scotland too but that's no excuse. Under the same conditions, and I know these very rarely exist in the natural world. water and ice can exist in equilibrium at 0C as newtron says. Once something is dissolved in the water the freezing/melting point is less than 0C but still the same for both components.

[JOEYSHABADO]

Water and ice can coexist at lots of temperatures, from less than -100oC to over 40oC, depending on the pressure. The liquid has to go through a process to become solid, ie crystalization.

If the water is completely pure, the freezing point will be well below zero, at normal atmospheric conditions.

And finally, the melting point and the freezing point will always be slightly different.

[brachiopod]

I'm with Gef and newtron on this one. For a given pressure the freezing point and melting point are the same temperature.

Don't confuse either thermal inertia or super-cooling effects to mean that the melting point is slightly lower.

[JOEYSHABADO]

Not sure about your last paragraph at all. Is that what you mean?

Anyway, "supercooling" can just be used to cover a situation where you have pure water. We are talking about pure water here right? In that case, cooling it below zero will always be 'supercooling' it...way down past zero.

[newtron]

According to thermodynamics, the freezing point and melting point are the same, and it is the temperature and pressure at which liquid water and ice can coexist in equilibrium.  At one atmosphere pressure, pure liquid water is not stable below zero deg C.  Super cooling as brachiopod mentioned is where the thermodynamically stable phase is not achieved on the account of the kinetic stabilization that occurs in the absence of nucleation centers.  I think this is what you are talking about JOEYSHABADO.  As Brachiopod said, don't get this effect confused with the strict definition of freezing/melting point.  Also, as Gef said, when you start dissolving stuff in water, there is a depression of the freezing point (this  is a colligative property).  I think this is the primary reason why tap water probably freezes at a temperature slightly below zero deg C.