trouble with temperature changing quickly inside test tubes

[kathleenensor]

Today we tried the solution of 5.8 g NaCl + 100 g H20. The thermometer in the test tube read -9 Degrees Celsius before the solution started to freeze. Then the solution in the tube would freeze up with ice crystals forming quite rapidly. While the ice crystals formed the temperature rose to the more predicted level of around -4 Degrees Celsius. So we are not certain whether to record -9 C or -4 C as the freezing point of the solution. Is there something we are doing incorrectly? We are stirring the solution with the thermometer and we were very careful to measure the water and the salt. Thank you for your help!

[Ray Trent]

I'm not entirely sure why that's happening, but it sounds like you're experiencing "supercooling", which happens when your surfaces are so smooth that ice crystals have nothing to form on. It's a very interesting phenomenon, and I'd definitely record that it's happening in your experiment, as this is a somewhat unexpected event.

Now, you could think about how to test if this is the problem (hint, it involves providing sites in the tube for crystals to form, in the form of imperfections in the glass or insoluble granules.

Here's a start on understanding supercooling, so you can see if you can design an experiment to prove whether this is happening or not: http://en.wikipedia.org/wiki/Supercooling

[kathleenensor]

Thank you for your reply and help. I did read the supercooling info on Wikipedia. I'm not sure that's what is happening because the temp inside the test tube rarely goes below -10 degrees Celsius. Tonight we tried the 2.9 NaCl solution again, and once again we had a repetition of the same pattern. The temp in the test tube would drop to around -7 degrees C, then the ice crystals would be noticed. Then the temperature of the ice crystals in the tube would rise to -2.5 degrees Celsius, which is much closer to the expected result than the origianl -7 degrees Celsius. The ice crystals would form very rapidly and build up vertically inside the tube in front of our eyes.

We kept the tube in the ice bath and the thermometer in the tube while we watched this happen over and over again. We don't know which is the freezing point due to this change in temperature of the solution while it remains in the ice bath!

[kathleenensor]

Thank you Ray. I did look up the information on wikipedia and I also checked out videos on YouTube. After watching the YouTube video demonstrations, I see that what is happening is indeed supercooling (something I was unfamiliar with). Also I failed to mention we chose to use distilled water. I can see that the distilled water can cause this to happen, especially when there is nothing else for the ice crystals to use to begin their formation.

Thank you for your help! It is much appreciated and we'll record this in my daughter's project.