Re: When I keep a bottle of water in the refrigerator for a long time, it

Your refrigerator must be colder than most.  It sounds like the water has 
become what is known as *supercooled*.  Supercooled water is water at a 
temperature below the freezing point which has not frozen yet.

As you probably know, water freezes at 0 degrees Celsius (32 degrees 
Fahrenheit).  But freezing doesn't necessarily happen immediately when it 
gets that cold.  Because the structure of ice is pretty different from 
that of water, the liquid water structure has to get broken up and 
rearranged into a solid ice structure.  Once the first bit of ice forms, 
that acts as a "nucleus" for more ice to grow on.  But the process 
of "nucleation", forming that first nucleus of ice, can take a very long 
time if the water is left still.  If you are very careful, it is possible 
to supercool water 20 degrees (Celsius) or even a little more before 
nucleation happens.  But when you disturb the supercooled water by shaking 
it or putting something into it, that gives enough of a jolt to the liquid 
structure for ice to start forming.

By the way, the same thing can happen at the other end of the liquid range 
with boiling.  That is one reason why you should be careful heating water 
in a microwave.  It is possible to heat up the water beyond its boiling 
temperature but for it to stay liquid if it is not disturbed.  But then 
moving it (like to take the mug out of the microwave) can be enough of a 
change to cause bubbles of vapor to nucleate, and you can get a sudden 
release of boiling steam.

Supercooled water is not just a curiosity; it is actually quite important 
in meteorology (science of weather).  Because water in clouds is not in 
contact with any solid surface, it is very common for it to get 
supercooled.  Dust and pollution (and "cloud seeding") can then have an 
impact on the weather by providing surfaces to help the water to nucleate.

Allan Harvey, NIST
"Don't blame the government for what I say, or vice-versa."