[iamstupid]

WHY water can be frozen by dropping its pressure ?

[Phlogiston]

WHY water can be frozen by dropping its pressure ?

It depends on the temperature too.  You should take a look at the phase diagram of water.

[iamstupid]

ya, but i really don;t understand why pressure drop water can be frozen. i thought pressure drop is more to vapour.

[Phlogiston]

ya, but i really don;t understand why pressure drop water can be frozen. i thought pressure drop is more to vapour.

For most substances, you are correct- lowering the pressure will cause a liquid to vaporize.  However, for water, the slope of the liquid-solid line is negative, so that if you are at a temperature lower than the triple point of water (273.16 K), and you are currently considering a pressure where liquid is the state, then if you lower the pressure you will cross the solid-liquid line and it will solidify.

Why is the liquid-solid line's slope negative?  Because the density of liquid water is higher than that of solid water.

[iamstupid]

but then it decrease it boiling point mean its boiling point can be very low and boiling point can be negative tempature, so how it frozen since it boiling point so low while decreasing the pressure . feel like contradiction with these two statement.

[Phlogiston]

but then it decrease it boiling point mean its boiling point can be very low and boiling point can be negative tempature, so how it frozen since it boiling point so low while decreasing the pressure . feel like contradiction with these two statement.

The boiling point will never be below the freezing point.  Also, it's better to think of temperature in Kelvin so that there are no negative temperatures.  Look at it this way- does it make sense to you that when you take solid water (ice), and raise the pressure, that it melts?  Because what we're talking about is just the opposite process.

[iamstupid]

yup u raise pressure ice melt but in certain temparature

[Phlogiston]

yup u raise pressure ice melt but in certain temparature

Exactly!  Now, imagine you just did that, and now do the opposite process- lower the pressure- and the water freezes.