[Wil"]

why increasing the pressure will decrease the freezing/melting point of water?
is it anything related to its density?

can anyone explain this in the qualitative view?

[azmanam]

start here

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/phase.php
(the picture on that page is NOT water)

then look at water's phase diagram here

http://encarta.msn.com/media_461541579/Phase_Diagram_for_Water.html

[Rabn]

I may be able to help you visualize this.   Imagine a box without a lid, this box is made out of a material that won't bend or break and pretty much is impervious and does not allow any energy to be transferred into or out of the box. Now there is a piston that fits in the box so exactly that there is no space between the piston and the walls of the box, the piston is made out of the same material as the box. Now put some water in the box and freeze it. Bring the piston down on top of the ice block. The water is arranged into a regular crystalline pattern, it also takes up more volume thatn the liquid did. Now increase the pressure that the piston exerts on the ice.  What happens is the piston stresses and bends the crystalline structure of water. Eventually enough pressure is added that the bonds holding the water together(hydrogen bonds) start breaking apart.  Once the bonds break, the water is back to being a liquid. Now imagine that you've added enough pressure that you've broken all the bonds that made the crystal, essentially crushing ice at the molecular level. The water is now all liquid. In order to make the liquid solid again, you have have to take energy away from the water, basically you added energy to water by increasing the pressure.  There is a catch here though, since the water can't expand when it freezes it needs to get to a level of energy, temperature, that allows for some of the water to form an irregular crystalline pattern, which the water doesn't like unless you take away enough energy from it. A human example, you're sitting on the couch watching t.v., one of the ones you have to get an walk to to change the channel,  and a t.v. show you dislike comes on. You generally would get up and change the channel. Now ask yourself how tired you would have to be to just sit there and watch the t.v. show, it is probably proportional to how much you dislike the show.  You'd have to be darn tired to watch a show you really hate.  Water doesn't like having crystalline defects, this is equivalent to your dislike of a t.v. show, and it's temperature, equivalent to your tiredness, determines how defective a crystal will be tolerated.