# what's do special about 4 degrees water?

Hi everyone! I have this question related to water and its lowest temperature when it's liquid. Our professor told us that 4 is the lowest degree water can reach before coming ice, not 0, 1, 2, or even 3. Could anyone please tell me what's so special about this temperature and how did scientists reach this conclusion and based on what. I really appreciate your help!

3 months ago

See Steve's response. Either your instructor misspoke, or you misunderstood.

As you cool water, it becomes more and more dense (the absolute increase is small but measurable), while remaining liquid. At 4 C, at normal atmospheric pressure, the density reaches a maximum. When you cool from 4 to 0 C, water's density _decreases_, as the molecules start to "lock" themselves into the open hexagonal crystal structure of ice. But water is *NOT* frozen during that process; it is still liquid.

As Steve said, it is possible to cool water, very carefully, below 0 C. For example, if you put it into a perfectly clean container (preferably glass), with no scratches or defects on the surface, you can put it into the freezer and it will still be liquid once it cools down. If you disturb it, though, it will suddenly freeze all at once. This is called "supercooling", and is a fun subject for home science experiments.

3 months ago

I wanted to see a graph showing this alleged density maximum, as a function of temperature, and Google(r) Images returned:

I'm still not sure if I actually believe this story about liquid water changing density as a function of temperature, but there are the graphs, with a max at 4C. Anyway, I figured I might as well share those with the rest of the class.

3 months ago

Yeah, but look at the scale -- the difference between 4 C and liquid water just before freezing is less than one per mil. You need accurate and precise measurements to verify it.

caarntedd3 months ago

3 to 4 degrees C is the perfect temperature for beer. ; D

steveastrouk3 months ago

You need to go back and ask your "professor" what he's talking about. Its possible for water to be very much colder than zero C and still liquid. 4 deg C water is at its point of maximum DENSITY.