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Homework Help: Is this true? NaCl dissolving

  1. Jan 11, 2005 #1
    considering the Na+ and Cl- in it's ionic lattice.

    the formula, electrostatic force
    F = Qq/(4.pi.e0.r^2)

    where e0 is the permittivity of free space. However in water e0 is 80 times bigger, so
    F= Qq/(4.pi.80e0.r^2), therefore electrostatic force between the ions is smaller, it simply dissolves and doesn't react.
  2. jcsd
  3. Jan 11, 2005 #2


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    The problem is ill posed.Water is not an "ether" (it's the weakest acid :tongue2: :tongue2: ),it's made up of molecules,just like sodium chloride is made up of atoms...The second formula u used cannot be put in that form,but in tha same form with the first,because,even in the presence of water molecules,the space between chlorine and sodium atom would still be vacuum,with the relative permitivity "+1".

  4. Jan 11, 2005 #3

    Andrew Mason

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    This is an interesting point. Water's permittivity is low because it is a polar molecule. In an electric field, polar molecules line up to reduce the electric field.

    So, would one not have to know the distance between the Na+ and Cl- ions in solution? This would be a function of concentration. The electric potential between the molecules would be:

    1. the coulomb potential or force between them (1/r^2), less
    2. the opposing field from the water molecules between them

    The latter is related to the number of water molecules between the Na+ and Cl-. When this number drops to 0, you will start to have precipitation.

    So wouldn't permittivity of water be a factor?

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