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Hello. I am 9 years old. I did a test to see which would freeze first, ocean, tap or pool water. I did it twice and both times the ocean water froze first. It isn't suppose to. Can you help me answer why this happened? I used 8 oz of each water and put them in plastic water bottles. I checked on them every 30 min. I noticed the the ocean water got thick and slushy. It formed a top layer of ice then it seemed to freeze from inside out. The pool and tap water froze from the top down first then the sides and bottom started to freeze. My science fair is coming up and I can't explain why this happened. Thank you!
Question Date: 2015-01-25
Answer 1:

What an interesting result from your experiment! You are right in thinking that the ocean water should freeze after the tap or pool water. This should happen because by adding chemicals to pure water, such as the chlorine in pool water or salt in the ocean water, the freezing point of the water decreases. I would expect that the tap water freezes first, then the pool water, and then the ocean water.

However, there is an interesting thing that can happen to relatively pure water. When you put it in a clean container it can become "supercooled". Supercooled means that the temperature of the water can go below the freezing point before it actually freezes. In this case, it takes some extra action to get the water to freeze such as shaking the bottle or slamming it on the table.

There is a video I found that shows this supercooled freezing taking place

supercooled freezing

This is my best guess as to what is happening during your experiment. I think in the ocean water, there is more salt and other objects for the ice to start crystalizing against, even though it must be at a colder temperature to start freezing. Maybe you could try leaving the water in "rougher" containers in the freezer such as maybe paper cups or try shaking the containers during the freezing process. It may be that the pure water is already cold enough to freeze!

I hope this explanation helps and is potentially the reason for your experimental results. Good luck!

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