What is the effect of pressure on melting point?
It depends upon whether, when the solid melts, it either expands and so becomes less dense, or it contracts and becomes more dense.Most solids do the former, such that the solid is more dense than the liquid and therefore sinks within it. But, a few, with ice being the most important example, actually expand on melting, meaning that the solid is less dense than the liquid, and floats.For the majority, increasing the pressure will increase their melting point, in accordance with LeChatelier’s principle. Since they expand upon melting, an increase in pressure tends to prevent it from melting, therefore raising their melting point. With water, it contracts upon melting, so an increase in pressure is encouraging melting, and so, its melting point decreases.
Effect of pressure on liquid boiling point?
what do you mean by vacuum? 0 pressure? < 1 atm pressure? from the clausius clapeyron equation... ln(P1/P2) = (dHvap / R) x (1/T2 - 1/T1) the idea is that if you measure P and T at one point (like the normal boiling point), then given a P2, you can calculate T1. example.... water.... normal boiling point = P1, T1 = 1 atm, 373K dHvap = 40680 J/mole R = 8.314 J/moleK right? so... at P2 = 0.5 atm, T2 = 354K. that is the boiling point of water at 0.5 atm at P2 = 0.01 atm, T2 = 276K.. bp of H2O at 0.01 atm you tell me what happens when P2 = 0 atm
Estimate the freezing point of water at 1.5 atm.?
use a phase diagram like this one to estimate http://images.encarta.msn.com/xrefmedia/... my estimate is about -.1 *C we know water freezes at 0*C at 1atm and a little more pressure should decrease the freezing temp very very slightly the difference between 225atm and 1atm could have less than 5*C difference in freezing temp as a side note Generally increasing the pressure increases the freezing point (not decreases). This is because generally solids are more compact than liquids, so increasing the pressure makes it harder to melt as the heat has to work against the compression to expand the substance. Therefore, you have to raise the temperature higher to melt it. Therefore the freezing point is higher. The best known *exception* to this rule, is water. Water ice forms crystals with more empty space than water, so it is less dense than liquid water. Therefore, increasing the pressure tends to force it into a more dense liquid state. Yet more pressure (6175 atmospheres) forces it to freeze again, into a new, more compact type of ice called ice VI. There are at least eight types of water ice (ice I thru ice VIII) that form at various different temperatures and pressures.