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
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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