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What is meant by dissolving?

  1. Jun 25, 2013 #1
    Is it breaking of intermolecular forces or breaking of intramolecular forces?
     
  2. jcsd
  3. Jun 26, 2013 #2
    Intermolecular. When you dissolve table salt, the bonds between the sodium and chloride are being broken and the ions dispersed throughout the liquid. When you dissolve sugar (I think this are the two most popular substances used to illustrate dissolving), the bonds between sucrose molecules are being broken. The reason these substances are solid is because of the intermolecular bonding between molecules so these are the bonds that must be broken to take them to the aqueous state.
     
  4. Jun 26, 2013 #3

    Borek

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    Staff: Mentor

    I would not classify forces between sodium and chloride in NaCl as intermolecular.
     
  5. Jun 26, 2013 #4

    Borek

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    Staff: Mentor

  6. Jun 26, 2013 #5
    Though the attraction between the ions in ionic bond is stronger than the intermolecular forces between the molecules ,ionic compounds dissolve in water but not covalent compounds?can anyone explain??
     
  7. Jun 26, 2013 #6

    Borek

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    Glucose is a covalent compound, yet it nicely dissolves in water. Ethanol is a covalent compound that is miscible with water (in other words, it has an infinite solubility). Hard to say where and how, but you apparently have some misconceptions about solubility.
     
  8. Jun 26, 2013 #7
    Interionic would probably be a better term. Although, to quote Wikipedia,
     
  9. Jun 26, 2013 #8
    Water is polar, so its poles are attracted to the charged ions. If you try to dissolve a nonpolar compound in water, the attraction is weak and entropy favors maximizing water bonding to itself.
     
  10. Jun 27, 2013 #9
    So something like even though if the water tried to break open the non polar molecules it would relatively easy to do so, it doesn't want to as there is no attraction between them.

    But for something like NaCl, even though its really hard to break them apart water now has a incentive as its actually attracted to the charged Na+ and Cl- so it actively tries to break them up. So as a result NaCl is able to dissolve despite the stronger interionic/intermolecular forces?

    Is this correct?
     
  11. Jun 27, 2013 #10
    Well there are Van der Waals forces at work. The reason isn't that there's no attraction but that the attraction between the polar solvent molecules and themselves is stronger and would be disrupted by dissolving the solute. The reason oil forms droplets in water is because this minimizes the surface area of oil exposure to water and maximizes water's ability to hydrogen bond. This is known as the hydrophobic effect.

    And yes, water is strongly attracted to NaCl ions and would like to break it apart. Although the bond strength between single ions is stronger than that between a single water molecule and an ion, there are a lot more water molecules than ions and they basically gang up on the ions to overpower the ionic bonding.
     
  12. Jun 30, 2013 #11
    solvation or dissolution describes the association of a solute and a solvent such that the mixture reaches its lowest energy configuration (most desirable or comfortable arrangement). like the term implies, the association is based on similarities between solute and solvent. a polar solute will dissolve well in a polar solvent (like water or acetonitrile). on the other hand, nonpolar solutes will dissolve well in nonpolar solvents (like hexane or toluene).

    it is important to realize that there is no clear cut-off between polar and nonpolar solvents and these are just concepts based on properties such as dielectric constants or dipole moments which assign a numerical value to the polarity in or between solvent molecules. (you can google dielectric constant of solvents for lists and tables) water has a value of about 80 and toluene is about 2.

    so based on the assymetry in the structure of a molecule (or crystal) chemists can try and match proper solvents with solutes. (an assymetric molecule will be polar, like NaCl or H2O, a symmetric one will be non-polar, like O2, N2 or Ch3Ch2Ch2Ch3) this can also be computed ab inito if needed with pretty good accuracy.

    in practical terms, it always challenging to find solvents which will dissolve starting materials or products when novel synthetic reactions are carried out and the success of the synthesis is many times dependant on the solubility of materials in solvents.
     
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