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In the study of water, its physical properties are determined by the highly polar...

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lilian-0716 | Student | (Level 1) Salutatorian

Posted August 15, 2013 at 10:59 AM via web

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In the study of water, its physical properties are determined by the highly polar nature of the molecule.

If I am going to choose water's ability to dissolve other substances, (1) what experiment can I carry out that will demonstrate how the intermolecular forces effect this property?

(2) Explain why water has this distinct property and (3) how it can be changed by interfering with this force.

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ncchemist | eNotes Employee

Posted August 15, 2013 at 1:53 PM (Answer #1)

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Many liquid solvents have intermolecular forces that affect how the solvent behaves and what kind of substances it can dissolve.  The main intermolecular force acting on an extremely polar substance like water is called hydrogen bonding.  The partial positive hydrogen atoms in one molecule of water are electrostatically attracted to partial negative oxygen atoms on neighboring water molecules.  The result is a complex network of electrostatically favorable hydrogen bonds within the water.  This hydrogen bonding is what gives water such a high boiling point for such a low molecular weight compound.

As mentioned before, water is a highly polar substance.  Since the general rule of thumb is that "like dissolves like," we can set out to test that water will dissolve polar substances but not nonpolar substances.  This can be easily tested in an experiment.  Sucrose, or ordinary table sugar, is a sugar molecule that consists of lots of polar hydroxyl groups (OH).  It is a polar compound.  We would expect it to dissolve readily in a polar solvent like water.  Dissolving some sugar in water shows that this is certainly true.  On the other hand, we would expect a nonpolar substance to not dissolve in water.  Oil is made up of nonpolar hydrocarbons (long chains of carbon and hydrogen).  Take a few drops of oil and mix it with water and you will find that none of the oil dissolves in the water and will simply float on top of it.

It we interfere with the hydrogen bonding of water, it will affect its  polarity and its dissolving properties.  We cannot really change the hydrogen bonding of water but we can make a substitution.  Isopropyl alcohol is similar to water in that it has an OH group, but whereas water has a second hydrogen atom, isopropyl alcohol has an isopropyl group (CH3-CH-CH3) instead.  This substitution means that isopropyl alcohol does have hydrogen bonding properties but not nearly as strong as water.  If you take an equal volume of table sugar that we used before with water and try to dissolve it in isopropyl alcohol, you will find that it does not dissolve nearly as readily as it did in the water.  Likewise, a very small amount of oil can dissolve in isopropyl alcohol whereas none dissolved in water.  These observations are all directly related to the hydrogen bonding properties of water.

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lilian-0716 | Student | (Level 1) Salutatorian

Posted August 19, 2013 at 6:51 AM (Answer #2)

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thank you :)

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