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B The physical properties of diamonds: no melting point?

  1. Nov 3, 2018 #1
    I had heard an opinion from my high school teacher, but I can't understand??

    "An experimental record of a French scientist.
    He heats the diamond and sublimates it. After cooling, it turns back to solid barbecue carbon.
    In this case, although there is a physical transition, the substance seems to be a chemical change.
    This is just my personal opinion: the physical properties of diamonds are not suitable for describing as melting point"

    What???
    No melting point how it transition??
     
    Last edited by a moderator: Nov 4, 2018
  2. jcsd
  3. Nov 3, 2018 #2

    Charles Link

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    Sublimation means it went directly from solid to gaseous state. There was no liquid state of the carbon in this case. Upon cooling of the gaseous carbon, it didn't make a diamond crystal, but rather simply charcoal. ## \\ ## I think there might exist a liquid form of carbon, but it would require enormous pressures to exist in the liquid state.
     
  4. Nov 3, 2018 #3
    get it,but is it good for describing diamond as no melting point?
     
  5. Nov 3, 2018 #4

    Charles Link

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    See my added comment above about the liquid state. At 1.0 atmosphere, there is only a solid to vapor transition. The same is true for carbon dioxide. I believe ## CO_2 ## requires about 5 atmospheres in order to produce a liquid state. Let me see if I can get a phase diagram of ## CO_2 ##: https://chem.libretexts.org/Textbook_Maps/General_Chemistry/Map:_General_Chemistry_(Petrucci_et_al.)/12:_Intermolecular_Forces:_Liquids_And_Solids/12.4:_Phase_Diagrams ## \\ ## See also https://en.wikipedia.org/wiki/Carbon It lists the triple point of Carbon as 10.8 MPa which is about 100 atmospheres of pressure. For pressures below this, there is no liquid phase of carbon.
     
    Last edited: Nov 3, 2018
  6. Nov 4, 2018 #5
    okay get it all,thanks!
     
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