Melting Point and atmospheric pressure

[orangetea]

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So this was a question from TPR chemistry book

and it asks that how is water's melting point effected in place of higher altitude.

I am just confused because I put no change.. thinking that the melting point is not effected significantly as is the boiling point.. but because this is water and the Solid-Liquid line has a negative slope the melting point increases?

=/

 

[Lunasly]

That's a really good question. To the best of my knowledge, increasing or decreasing altitude does not affect the vapour pressure of a substance, yet it still affects its boiling point. At higher altitudes, the atmospheric (external pressure) is lower, thus water boils at a temperature lower than 373 K.

I found this on the internet, but I'm not sure how to explain why an increase in pressure would result in a slight decrease in MP. This is a very difficult topic for me, too.

"Water expands a lot when it boils: one kilogram of water is one litre of liquid water, but it becomes about 1700 litres of steam at atmospheric pressure. This means that even modest increases in altitude can measurably reduce the boiling temperature. Some people complain that this affects cooking and even the taste of tea at altitude.

It is also true that pressure changes the melting temperature. However, because the volume occupied by a kilogram of liquid is not much different from that occupied by a kilogram of solid, this effect is very small unless the pressures are very large. For most substances, the freezing point rises, though only very slightly, with increased pressure.

Water is one of the very rare substances that expands upon freezing (which is why ice floats). Consequently, its melting temperature falls very slightly if pressure is increased."

 

[el_duderino]

 

[DrknoSDN]

I put no change.. thinking that the melting point is not effected significantly as is the boiling point.. but because this is water and the Solid-Liquid line has a negative slope the melting point increases?

Your initial assumption of no change was correct. If the book says otherwise they are splitting hairs. For normal environmental conditions increasing elevation does not change pressure enough to significantly alter the freezing/melting point of water. At that point you would need to decipher if the question want to know if there is any detectable change or if there is a significant change.

Technically it does change but that's like saying liquids are compressible. Technially yes, but for MCAT the answer should be no.

The img from jonnythan shows the tiny 0.01C maximum change up to the triple point, but that's a pressure diagram, and not elevation. Normally when you think elevation you are looking at earth ground level ranges which would be even smaller. Pressure at the top of everest is closer to 0.33 atm vs 0.00603

 

[el_duderino]

Yup. It is different, but insignificantly so. Another example of the reasons I don't like TPR/TBR/ExamKrackers/whatever.