Subject: What factors determine whether water will for ice or slush?

Question: What determines whether water, when cooled, will form slush or 
ice? Is slush the point between water and Ice?

Observation: I like to put water in drink bottles (plastic evian type) and 
put them in the freezer so that they are colder than refrigerated water.  
Sometimes, when I go to take a swig I'll see that thin sheets of ice have 
formed on the top surface of the water. Other times, I'll go to drink it and 
as I tilt the bottle back, it turns to slush as it tries pout out of the 
openning.  

My Hunch:  I think it has something to do with how cold the freezer is when 
I put the water in it.  If it's really cold, it'll form thin sheets of ice 
on the top surface and around the insides of the bottle.  If I were to keep 
it in longer, it would eventually turn solid layer by layer, from the outide 
in because the oustide cools faster than the inside.  If the freezer isn't 
as cold....all the water in the bottle will slowly cool at the same time and 
form a slush at first and then a form a solid cube of ice.

Conclusion??? Am I right or are there other factors involved that I'm not 
aware of.

Re: What factors determine whether water will for ice or slush?