• Please review the updated member agreement. Included is a new statement supporting the scientific method and evidence-based medicine. Claims or statements about disease processes should reference widely accepted scientific resources. Theoretical medical speculation is encouraged as part of the overall scientific process. However, unscientific statements that promote unfounded ideological positions or agendas may be removed.
  • Free admissions webinar for pre-vets! “Apply Smarter” Webinar

Sonyfan08

10+ Year Member
7+ Year Member
Dec 4, 2008
148
0
Status
Pre-Medical
How can a compound/element have 2 different solid phases?
 

PingPongPro

7+ Year Member
Jan 28, 2010
590
15
Status
Medical Student
I think this is a result of a compound having different possible lattice structures.
 

collegestud2013

Probationary Status
10+ Year Member
Jun 28, 2010
388
92
Status
Resident [Any Field]
Diamond and graphite are the classical example of this; diamond is more stable high temperature and pressure, while graphite is more stable at STP. Of course the activation energy for conversion of diamond to graphite at STP is so high that it would take millions of years to occur.
 
Jun 26, 2010
412
11
Status
Diamond and graphite are the classical example of this; diamond is more stable high temperature and pressure, while graphite is more stable at STP. Of course the activation energy for conversion of diamond to graphite at STP is so high that it would take millions of years to occur.
Your example is slightly misleading, since it only applies to elements and networked covalent solids like diamond and graphite are really just single molecules.

For compounds, the picture is different. They may become a solid, if the intermolecular forces are sufficiently large enough to allow the individual molecules to settle together in a crystalline structure. Depending upon the method for cooling (e.g., isobaric, adiabatically, et.al.), different lattice structures become more likely - my guess is that entropy and lattice energy work together to determine which configuration is more thermodynamically favorable. Quite a different story than diamond and graphite.

Water is probably the most studied compound on this subject - there are many different thermodynamically stable forms for ice. Here's a relatively recent discovery of another one:

http://arxiv.org/pdf/0906.2489v1
 
About the Ads