Boiling Point before Melting Point?

[Abdul Quadeer]

How can an element boil before it melts?
I came across this- As - Melting Point 816 C and Boiling Point 615 C. Every other element given along with this element in my book has its melting point lower than boiling point which makes sense to me. I am curious to know what is different about As. What state does it exist at T< 615 and 615<T<816

[Borek]

Boiling point at what pressure? Melting point at what pressure?

Wiki lists 817 deg C (at 36 atm) as a triple point, not melting point. I guess we should refer to arsenic phase diagram to get what these numbers really say.

[Abdul Quadeer]

I am referring to J.D. LEE CONCISE INORGANIC CHEMISTRY.
At 38.6 atm, alpha As has a melting point of 816 C and boiling point of 615 C (sublimes)

[Borek]

Sublimation is not a boiling point.

[miteshsharma]

sir im very curious to know this?
when you are melting alluminium scrap what chemical will help separate hydrogen from alluminium

[Abdul Quadeer]

Sublimation is not a boiling point.

If it sublimes at 615C how can it melt at 816C?

[Borek]

Reread the thread, there is more parameters that can be changed than just temperature.

[Abdul Quadeer]

Sublimation takes place at 615C i.e. the solid changes into gas. Is it that change of state changes the pressure so that there is another melting point at the new pressure? What if we keep the pressure constant?

[symbolipoint]

Watch the meaning carefully. Sublimation is not boiling.

If a substance sublimes, it has not melted. Sublimation is a phase transition from solid to gas without a transition to liquid.

[Borek]

Sublimation doesn't take place at 615 deg C, it takes place always you have a solid and a gas above is below equilibrium pressure. Temperature given probably refers to the moment when sublimation pressure equals 1 atm.

As I signaled earlier, without phase diagram whole discussion is a moot. You are trying to force partial information into incomplete physical model. It won't work.

[Abdul Quadeer]

I found this Phase Diagram for As. I have no idea what info we can deduce from it.

http://books.google.com/books?id=F2HVYh6wLBcC&pg=PA119&dq=phase+diagram+for+arsenic&hl=en&ei=BwcuTa7BIsmQswbi-93gBw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCgQ6AEwAQ#v=onepage&q=phase%20diagram%20for%20arsenic&f=false

Look at page 120.

[Borek]

I found this Phase Diagram for As. I have no idea what info we can deduce from it.

Nothing. It doesn't show what happens at low pressures.

[karl-karl]

hi,
from abdul's original question, the boiling point of arsenic at room pressure (one atmosphere) is the given (the actual effect is sublimation as stated by borek above). arsenic will melt at about 40 atmosphere's (if i recall right). the table info you refer to is misleading (in my opinion). finding the phase diagram was an excellent recourse to your issue.

karl

[edit] i just checked wikipedia and it gave the triple point at 20 atmospheres. so at 20 or greater, the liquid state of arsenic is possible.

[dolkeen]

Usually as mentioned in previous discussion , arsenic sublime at 887K and melt at 1090K , which means that vapor of tetramer gas As4 " most stable arsenic gas " will be presented at 887K ,Arsenic requires high pressure to be converted into liquid phase 35.8 atm , the triple point Solid-Liquid-Vapor will be existed at 1090K

4As(s) to As4(v) : deta H = 144096 J/mol at 887 K
As4(v) to 4As(l) : delta H = 43452 J/mol at 1090 K
4As(s) to 4As(l) : delta H = 95392 J/mol 1t 1090 K

The standar value of energy of fusion As(s) to As(l) = 23848 J/mol

complete information about arsenic system found in N.A.GOKCEN –Bulletin of Alloy Phase diagrams vol.10 No. 1 1989