Question for one problem in Perterson/cracker book

[Ayae]

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HI,
I just finished one of the practice test on Princetonreview/examcracker. One of the answer keys provided by the website seem wrong.
Let me lay out the answer key for the question: The melting point of water at the lower pressure will be higher than that of it at the higher pressure.
From my understanding, since water boil at low temp at lower pressure condition, I am expecting the melting point of it will be lower at the lower pressure as well.
Please feel free to correct me if my understanding for concept is wrong.

Thanks. I am taking exam in July and frantically studying for it now.

 

[anchoi]

No the BP is always opposite with the vapor pressure. So the lower vapor pressure will have the higher BP, and vice versa. But I am not sure about the melting point.

 

[xtsukiyox]

Here is my understanding, backed up by the super-scientific source, wikipedia: 😉

Boiling Point
Boiling point is the temperature at which the vapor pressure of a liquid equals the vapor pressure of the surrounding environment. Normal boiling point is defined as the boiling point at 1 bar (roughly equal to atmospheric pressure at sea level). This means that at low pressure (i.e., a vacuum), the boiling point will be depressed (lower) than normal, and at high pressure, the boiling point will be higher than normal.

If you think about how temperature relates to vapor pressure and what is actually happening on a molecular scale when you heat up a liquid - and when a liquid changes to a gas - this relationship sort of makes sense.

http://en.wikipedia.org/wiki/Boiling_point

Melting Point
The relationship between melting point & pressure depends on whether there is a volume increase or decrease between the solid & liquid states. If the substance decreases in volume when it melts (as water does), high pressure causes a decrease in melting point. Again, consider this from a molecular standpoint and it makes sense. Most substances are more dense in solid form than liquid, so the reverse (high pressure means an increase in melting point) is usually true.

It is important to note that the difference between the volume in solid and liquid states differs quite a bit less than the difference between the volume in liquid and gaseous states, so pressure has a much lower influence on melting point than on boiling point.

http://en.wikipedia.org/wiki/Melting_point

I hope that helps. 🙂