[hairygorillaz]

I have a quick question about AlCl₃. I'm wondering if there is a reason why the sublimation point is lower than the melting point. Also, once the compound has been sublimed, how can the melting point be determined?

[mjc123]

The vapour pressure of the solid reaches 1 atm at a temperature below the melting point. Draw a phase diagram with the triple point at a pressure > 1 atm, and then draw a line at P = 1 atm.

[hairygorillaz]

Arh. So am I right to say sublimation temperature and melting points (at least here) are theoretical constructs and not something that is measured? Because I was wondering how at 1atm, you could measure the melting point when the compound would have sublimed.

[AWK]

If you look well on the internet, you will find information that AlCl₃ melts at a pressure of 2.5 atm. Unfortunately, this information is not available in English wikipedia.

[mjc123]

So am I right to say sublimation temperature and melting points (at least here) are theoretical constructs and not something that is measured?

No, I was merely trying to help you visualise how the melting point could be above the sublimation point. Both are measured quantities (m.p. at pressures > 1atm).

[hairygorillaz]

If you look well on the internet, you will find information that AlCl₃ melts at a pressure of 2.5 atm. Unfortunately, this information is not available in English wikipedia.

Ah. I tried looking in the CRC Handbook and I didn’t find this piece of information.

[hairygorillaz]

So am I right to say sublimation temperature and melting points (at least here) are theoretical constructs and not something that is measured?

No, I was merely trying to help you visualise how the melting point could be above the sublimation point. Both are measured quantities (m.p. at pressures > 1atm).

I see. Thanks a lot!