[natasa_og]

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 occured 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.

[Borek]

Explanation for what? For the fact that water didn't freeze at zero?

How pure was the water?

[natasa_og]

Yes, I want to know why water did not freeze at 0C.

It was water from a tap, and the porous meda was rockwool thermal insulation. When 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. Ice melts at 0 C.
Regards

[Wald_ron]

At what altitude are you?
Did you run water through the porous media to eliminate potential impurities?

[Borek]

Tap water never freezes at 0°C, taking into account it could leach some soluble substances from the rock wool freezing point could go even lower.

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

[Arkcon]

Yes, I want to know why water did not freeze at 0C.

It was water from a tap, and the porous meda was rockwool thermal insulation. When 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. Ice melts at 0 C.
Regards

This part here is really interesting for me.  You give very precise measurements, of things that I wouldn't expect to be measured so precisely.  How are you doing it?  Do you have a calibrated lab thermometer, or maybe you're using a thermocouple?

[natasa_og]

Hi,

I wanted to make discussion on an easy example, that s why I simplified the description of my experiment. Maybe to much. I ll try to describe it once more - this time more precisely.

I took a rockwool sample of the size 0,5 m x 0,5 m x 0,1 m and sprayed some water on it. Than I sealed the sample with the PVC and put it into the testing setup. Testing setup consisted of 2 plates (size 0,5 m x 0,5 m), temperature of which can be regulated (heating/cooling between -8C and 40 C). I have put the sample of moistened and sealed rockwool between both plates, so that temperature gradient could be established in the sample. I have set the temperature of plate 1 to 40 C and temperature of plate 2 to 20 C and waited (1 day) until all the water evaporated near the plate 1 and condensed near the plate 2. Vapor transfer occured by vapor diffusion. Than I have set the temperature of plate 2 to 40 C and temperature of palte 1 to 20 C and waited until all the water evaporated at the boundary 2 and condensed at the boundary 1... and so on...
After some cycles, when water condensed near boundary 1, the temperature of that boundary was cooled down to -8 C...I realised that water was freezing at -4C. When the boundary was heated, water was tawing at 0C. During the experiment I measured temperature (with termocouples) and heat flux.

Looking forward to hearing from you,
Regards

[vex]

I still don't know if I understand your experimental setup, but is it possible that the thermal insulation is interfering with your temperature measurements in some way? If you had a temperature probe at the surface of the insulation, would you see the same result?

Also, I think freezing point depression of -3 °C is unlikely, but for consistency's sake your experiment should be repeated (if possible) with purified water and adequately cleaned surfaces.

[djt]

As Borek says, Colligative properties if you are using tap water.

Tap water never freezes at 0°C, taking into account it could leach some soluble substances from the rock wool freezing point could go even lower.

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

Could you repeat the experiment with distilled/de-ionised water, or even better, if you have access to a MilliQ water system which is about the purest water you can get.

If you wanted to get really fancy, use MilliQ water and contaminate it with NaCl (or similar) at different concentrations and you should see a linear relationship between the freezing point and concentration of NaCl.

[natasa_og]

I am not a chemist, but... when water evaporates (at 40 C) near boundary 1 of my sample and diffuses trough rockwool to the boundary 2 (20 C) of my sample, where it condenses, destilation actually takes place. Or am I wrong?

So it does not matter that I moistened the sample with tap water, because destilatin takes place every time when I change boundary temperature.

Similar experiments are presented here (Page 20 - materials and method):
http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/nrcc30889/nrcc30889.pdf

, but they did not cool the boundary down to sub zero C, when water condensed near the boundary.

[Borek]

If I understand you correctly, water is all the time in contact with the rockwool and it condenses on the rockwool, so no, it is not distilled.

[natasa_og]

But then tawing temperature would also be -4 C, but it is not...

What if water, which is in contact with rockwool evaporates and condenses on the PVC boundary? Is that destilation? We are talking about very small mass of water (100 g/m2)

[natasa_og]

Hi,

I am pretty convinced water is supercooled in my case. Arguments:
- tawing temperature remains at 0 C
- surface temperature raises up to 2 C during freezng

Doubts:
- I used taped water

I am still not sure if water gets clean enought for supercooling during my experiment or not.

[Borek]

In the contact with any rough surface water won't get supercooled.

[natasa_og]

But in my case there is no bulk water, but water consists of tiny waterdrops. 100 g water per m2 is only 0,1 mm water on average.

Do you think water gets clean enought during evaporation - diffusion - condensation to get supercooled?

I am not trying to convince you, but in my opinion that is only reasonable explanation of the proces.