Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fats and solvents

  1. Mar 16, 2004 #1
    i recently read in my biology notes that fats, namely triglcerides , are insouble in water but soluble in organic solvents...any ideas why?
  2. jcsd
  3. Mar 17, 2004 #2
    This is basically a chemistry question.. I'll try and answer it..

    Whether a compoud can dissolve in water, depends whether it can bond with water. Now water is polar i.e it is charged.

    You should know that opposite charges attract... so inorder for something to be soluble in water, it must be charged, or some parts of it charged.

    Looking at the structure of triglceride, there are no polar groups (things like -OH, NH, nor any ions), so it does not dissolve in water. I think the easiest way to visualise it, is that the water will bond stronger with itself, and the fat will bond with itself, so the two never mix.

    This compound does in organic solvents, because there are no charges. The triglceride will be able to bond to the solvent via dispersion forces.
  4. Mar 17, 2004 #3
    watz dipersion forces?
  5. Mar 17, 2004 #4
    An easy way to remember this is that likes disolves likes.

    Water disolves polar groups.

    fats disolves fats or non-polars disolve in non polar.

  6. Mar 17, 2004 #5
    Dispersion forces (London forces) are difficult to explain, unless you have a chemistry back ground.

    But, I will try to make it simple. Every molecule have a temperary dipole moment (or tempary positive and negative end) With dispersion forces the temporary dipoles line up (although they are not stationary) this lining up creates an attraction between the molecules, although it is an extremely week attraction.

  7. Mar 17, 2004 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Interesting is the definition of lipid according to the Dictionary:
    Lipids are thus categorized by a physical property rather than by structural features.

    Water is extremely polar, and will thus have a strong tendency to exclude non-polar substances, organic solvent usually have long apolar carbon chains (which makes them organic) that facilitates the solvation of substances like lipids.
  8. Mar 17, 2004 #7
    wait doesnt dispersion forces make all molecules polar then? even for juz a moment?
  9. Mar 17, 2004 #8
    Exactly, everything is in motion, electrons are whirling or waving depending on which theory you beilieve. So, yes for moments non-polars are polar, but the net movement is non-polar.

  10. Mar 18, 2004 #9
    then why are some molecules non-polar if at any moment they are polar?
  11. Mar 18, 2004 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is about the dipole moment, and how strongly a molecule pulls electrons towards itself in a bond. Polar substances have such a defined dipole moment, the dispersion force would be something more flexible and adaptive I'd think.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook