Science Buddies: "Ask an Expert"

I read through the FAQs for supercooling and snap freezing and don't see an answer applying to our issue. We are comparing the ability to supercool across four different water types: distilled water, tap water, well water, and salt water. We finessed our procedure to produce supercooling conditions, but they occurred in our tap, well and salt water solutions and NOT the distilled water trials where 3 of the 4 trials froze above -1 deg C (the quickest for all our water types). We did have one trial supercool to -6 deg C., but we could only achieve supercooling this one time. We used the same plastic container, cleaned the container beforehand and dried it to make sure no water dropped into the cup during the cooling phase. We were also careful not to jiggle the table. In addition we used three other types of distilled water thinking the jug was contaminated from the plastic in the jug lining. All distilled water sources produced the same results, no supercooling. The procedure says a thin plastic cup. Is it the plastic cup leaching impurities into the water contaminating our "pure" water? We were expecting the distilled water treatment to supercool the best being their was no impurities in the water to act as zones for ice nucleation. Are we misunderstanding what the expected findings are? Thank you of any help!

Hello Potts,

I'm assuming that you're working on the project described here:

https://www.sciencebuddies.org/science- ... ng#summary

Materials dissolved in water will decrease the freezing point. So you'd expect that impurities leaching into your distilled water from the plastic cup would lower the freezing point. The observation that the distilled water froze above -1 deg C in 3 of 4 trials suggests that this was not the case. Perhaps the 3 trials that froze at -1 deg C were cooled too quickly. Or a dust particle or other debris may have fallen into the water and started ice crystallization. You might want to attempt some additional trials with distilled water. And you might want to try using a glass container for the water instead of plastic.

The observation that your other water types (tap, well, salt) appeared to super cool might be explained by freezing point depression. Tap, well and salt water would be expected to have lower freezing points than distilled water due to dissolved materials. To be certain that your “impure” samples were super cooled in your experiments, you'd need to know that the freezing points of the samples were higher than the temperatures you found in your experiments.

I hope this helps. Please post again if you have more questions.

A. Norman