Frezing of water at below 273 K (0 C)

[natasa_o]

Hello,

I made an experiment. On the bigger surface of the sample of porous material (size 0,5 m x 0,5 m x 0,1 m) thin layer of water was sprayed (25 g). After that the sample was sealed and the moistened surface was cooled to the temperature about 268 K (-5 C). I think most of water stayed at the surface where it was sprayed during the experiment. Measuring the heat flux trough the cooled surface we realized that freezing of water occurred at the temperature about 270 K (-3 C). Does anyone has an explanation for that? Where can I find something more about that?

It is very urgent. Thanks for your help.

 

[jayaganesh]

Basically the specific heat capacity of water is very high.Also it is obvious that water may cool at -3°C.This may because of impurities that act as catalyst to cool the water.

 

[natasa_o]

Thank you for your answer, but longer explanation would be very appreciated. Used porous material is rockwool thermal insulation.

Thanks for your help in advance.

 

[natasa_o]

I ve forgotten to mention, that after water starts freezing at -4C, the temperature of the surface near which freezing takes place rises for about 1 C. When the ice has formed, it drops again.

 

[M Quack]

The freezing temperature of water depends on many things. For example, if you mix it with salt (NaCl), it can drop as low as -21.3 C. Is is quite possible that your rockwool contains some chemicals that mix with the water and thus lower the freezing point.

http://en.wikipedia.org/wiki/Eutectic_system

The freezing transition is a first order (discontinuous) phase transition so that you can get supercooling, i.e. you can cool below the freezing point without any actual ice forming. Usually for this the water and the container need to be very clean, and you have to avoid any perturbations such as vibrations. I'd be very surprised if this could happen with water on rockwool.

http://en.wikipedia.org/wiki/Supercooling

The increase in temperature is normal and is due to the latent heat of the phase transition. Think of this as the opposite of how melting ice keeps your drinks cool.
For melting ice this is quite big.

 

[natasa_o]

M Quack, your explanation seems to be OK. I did not mention before that ice melts at 0 C. Does it change anything?

Can freezing temperature also be anyhow connected with surface forces?

 

[M Quack]

If the freezing and melting temperatures are different, then you have a case of supercooling.

If they are the same but different from 0 deg C, then your water is either impure, under pressure or something else has shifted the phase transition.

Freezing always starts at a nucleation center, the ice crystal(s) grow from there. That can be a sharp corner of the container, a grain of dirt or dust or something else. That's why super-clean water in a super-clean container can be supercooled very low.

I would think that in rockwool you have plenty of that, so I am surprised that you can get supercooling at all.

 

[natasa_o]

The sample of rockwool was seald with PVC foil. Probably the water was transferred to PVC and freezing did not take place in the rockwool, but in on the PVC foil. Is it possible there might be no nucleation centers on the PVC and subcooling was a consequence of that? I think there were milions of very small waterdrops on the PVC foil when experiment was finnished. Maybe subcooling is connected to high pressure in waterdorps, which is a consequence of surface tension.

Does anybody know if in the case of subcooling water vapor condenses to liquid water or resublimation takes place?

 

[M Quack]

The sample of rockwool was seald with PVC foil. Probably the water was transferred to PVC and freezing did not take place in the rockwool, but in on the PVC foil. Is it possible there might be no nucleation centers on the PVC and subcooling was a consequence of that? I think there were milions of very small waterdrops on the PVC foil when experiment was finnished. Maybe subcooling is connected to high pressure in waterdorps, which is a consequence of surface tension.

I guess it is possible. Size does matter in these things - even if most of the time you have to get down to nanometer sizes. I would not expect a single freezing temperature, though, but rather a distribution. Each droplet has a slightly different size, and probably a slightly different freezing temperature.

Does anybody know if in the case of subcooling water vapor condenses to liquid water or resublimation takes place?

Don't know.