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Q & A: what type of water freezes first?

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Most recent answer: 01/25/2015
Q:
Hello. We conducted an experiment to see which froze faster: ocean, pool or tap water. We did it twice and both times the ocean water froze first. Wierd. We used 8 oz of each water in sealed plastic water bottles. We checked on them and noticed the ocean water got slushy and thick as it froze. It looked like it was freezing from the center out. The pool and the tap water seemed to freeze from the too down. I don't know why this is. Can you explain what I might have done wrong? Thank you! L
- Lulu (age 9)
Hartford, CT, USA
A:

I don't think you did anything wrong in the experiment. The world is complicated, and sometimes those complications make the results of experiments surprising. 

The true freezing temperature of your ocean water is a bit lower than the purer waters because it has salt dissolved in it. (We discuss why that is on other answers, e.g. .) So that's why it surprising that the other waters didn't freeze first when they all were cooled down together. Why did they instead freeze later?

Here's what I think happened. It's hard for freezing to get started, especially in water that doesn't have little bits of dirt in it. The ocean water does have those, so as soon as it got cold enough it started to freeze. By that time, the purer water was already below its freezing point, but it probably didn't have any good place to start freezing sit "supercooled". Lots of our readers have seen supercooling. (see .) 

We'd expect the ocean water to get slushy, because as some forms ice, it leaves behind more salt in the rest. That lowers the freezing point of the remaining liquid. Even in a fairly cold freezer, the saltiest part left won't quite freeze.

 It's no surprise that your purer waters started to freeze at the top, which was probably the coldest point. Maybe also the surface made it easier for the freezing to get started there. I don't know why the ocean water started to freeze in the middle, so there's still a little mystery.

How can you test the supercooling idea? There are various ways. You could try tapping the bottles every few minutes, since a sharp tap can help start the freezing. You could put just a little of the ocean water in each of the other bottles, since it doesn't take much dirt to help start the freezing. You could leave the bottles open, and every few minutes put a few tiny ice shavings in each. Nothing beats a little bit of ice to help start making more ice.

Mike W.

posted without vetting until Lee gets back

 

 


(published on 01/25/2015)

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