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Chemistry LibreTexts

Polarity of Solvents

[ "article:topic", "Fundamental" ]
  • Page ID
    3024
  • Chemical Concept Demonstrated

    • Differences in the polarity of solvents

    Demonstration

    • Reichardt's dye [2,6-diphenyl-4-(2,4,6-tripheynylpyidinio)phenolate, Aldrich 27,244-2] is placed into crystallizing dishes.
    • The following solvents are added to the dye.

     

    SOLVENT

    COLOR of SOLUTION

    methanol green-blue
    ethanol blue-violet
    2-propanol red-violet
    acetone orange
    acetone w/ water
    As the amount of water added increases, the color of the solution changes from blue-violet to violet to red-violet to red and then orange.

     

    Observations

    As the polarity of the solvent increases, the colors change from green to blue to violet to red to orange to yellow.

    Explanations

    When the dye used in this demonstration absorbs light, an electron is transfered from the negative end to the positive end of the dye, making the excited state less polar than the ground state.  Because polar solvents stabilize the ground state more than the excited state, the change in energy increases with the polarity of the solvent.

    As the solvents become more polar, the light absorbed by this dye shifts from the low energy, long wavelength (red) to the high energy, short wavelength (violet) end of the spectrum.  Remember, however, that the color we observe is the complement of the color of the light absorbed.  As a result, we observe a change from green to blue to violet to red to orange to yellow as the polarity of the solvent increases.  Because adding water to acetone increases the polarity of this sovent, the color of the acetone/water/dye mixture changes from blue-violet to violet to red-violet to red and finally to orange as the amount of water increases.

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