Difference between crystallization and vaporization?

  1. If a solution is heated to dryness, a powder will be left. But under evaporation, a crystal will form. So what's the difference between crystallization and vaporization? How is the process of crystallization?

    Are all the opaque (or milky) solutions contain a precipitate? And why some liquids are obpaque while some are transparent?

    Here is a beaker. Inside is some oil on top of water. If the water is heated, so it becomes less thick and thus less dense, will it has a density lower than oil and so flow on the oil?

    What is the general definition of a 'salt'?

    -thx-
     
  2. jcsd
  3. chem_tr

    chem_tr 613
    Science Advisor
    Gold Member

    Hello

    I will try to answer your questions briefly.

    Well, the difference is that the first will form amorphous materials, while the second one will be crystalline. In order to obtain good quality crystals, you should boil the solution until a clear solution is obtained, then let it cool unattended, in ambient temperature. Crystals will form, since you give enough time to allow solid phase formation. Amorphous materials occur instantly, they don't have much time to facilitate crystallization.

    They are rather called suspensions, and if you allow some time, a process known as "sedimentation" will occur, that is, the upper liquid becomes clear, and the insoluble solid precipitates to the bottom of the flask. Opaque liquids have some insoluble material, so they are suspensions or emulsions, depending on the physical state of the insoluble material.

    I think the solubility of oil increases with increasing temperature, so the layer becomes thinner. I don't think that densities would change greatly with a simple heating process.

    A salt is basicly a compound, formed between a base and and acid. The anion of the salt comes from the acid, while the cation is from the base.
     
  4. Borek

    Staff: Mentor

    IMHO the question is faulty - if you heat NaCl solution to dryness crystals will form. They will be smaller and not so nice, but they will form.

    Look for so called Tyndall effect.

    The simplest answer is oil is heated too so it becomes even less dense at the same time.

    Besides, what is an 'oil'? It can be anything, from edible to car lubricant and I have no idea what range of densities is possible for substances that can be called 'oils'.
     
    Last edited by a moderator: Aug 13, 2013
  5. Monique

    Monique 4,700
    Staff Emeritus
    Science Advisor
    Gold Member

    When a solution precipitates very rapidly, there is not enough time for molecules to find their place in the lattice. For a crystal to form there first needs to be a 'seed', whose formation is energetically unfavourable.
     
  6. Borek

    Staff: Mentor

    While this is generally true it is all a matter of kinetics - which process is faster. I have never heard about amorphous form of strongly ionic compounds, like - say - KCl. Then, there are many things I haven't heard of :wink:
     
    Last edited by a moderator: Aug 13, 2013
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