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Polar Molecules and Cohesive Forces
Name: Hannah
Status: student
Grade: 6-8
Location: IA
Question:
How do polar molecules affect the
cohesiveness of water?
Replies:
Hi Hannah,
I am a little puzzled by your question. The water molecule is itself
highly polar. What other polar molecules would you be thinking of in
this situation? I'm also unclear what you mean by "the cohesiveness of
water". Could you possibly elaborate on your question a little more?
Regards,
Bob Wilson
Hi Hannah,
The short answer is that the mixture properties depend on what, and how much
you add. There are many different kinds of polar molecules that have very
different effects. It is hard to make many accurate broad generalizations.
However, water really likes water, so most additives will reduce overall
intermolecular attraction ("cohesiveness").
If you want more detailed information, keep reading.
Terms like 'cohesiveness' and 'polar' are useful to illustrate basic
concepts, but they can be a bit too general/simplified when trying to
understand complicated subjects like mixture properties. 'Cohesiveness' is a
catch-all term that refers to many different kinds of attractions between
molecules. Each type of attraction acts differently in different
circumstances, so it is hard to accurately generalize overall 'cohesiveness'.
'Polar' is also a very general term. Molecules have different degrees of
polarity -- one molecule may be more polar than another, rather than simply
being classified as 'polar' or 'non-polar'. A molecule may be polar at one
end, but non-polar at another. For example, soaps have a polar end and a
non-polar end. One 'polar' molecule may have a very different effect than
another.
I am also not sure if you are asking if the water itself is changing, or if
the mixture properties are changing. A water molecule itself will not be
changed by being in a mixture or being with just other water molecules.
However, the mixture of water molecules and additives can change a lot
depending on what and how much you have mixed to it.
Think about substances as a collection of individual molecules. Each
molecule is surrounded by other molecules, and may be attracted or repelled
by them. There are many bulk properties (like vapor pressure, boiling point,
how much energy is required to evaporate [called heat of vaporization],
droplet contact angle, etc.) that depend on intermolecular attractions. Some
mixtures may have more self-attraction than the pure substance, while others
may have less.
The bulk properties we can observe all depend on how 'happy' (more
intermolecular attraction) the molecules are together. If they are 'happy',
it will take more energy to break them apart. If they are 'unhappy' (less
intermolecular attraction), it takes less energy to break them apart. Water
is highly attracted to other water molecules, so it is 'happy' -- and has
low vapor pressure, high heat of vaporization, and high boiling point.
Because water is so 'happy' by itself, most things you can add will
generally reduce the overall intermolecular attraction of the mixture. We
can observe changes in bulk properties that lead to this conclusion.
Alcohols are "polar" molecules, and low molecular weight alcohols are
miscible with water. An alcohol-water mixture typically has a lower boiling
point and higher vapor pressure, indicating less overall attraction.
However, the higher the molecular weight of the alcohol, the lower the
polarity and the less miscible with water. These indicate lower overall
intermolecular attraction. Alternatively, if you add a salt, e.g. sodium
chloride, that dissociates in water into a pair of charged ions, the boiling
point of the water-salt mixture rises. The vapor pressure changes in a
complicated way (it is higher than fresh water at hear-boiling, but lower at
near-freezing). This interaction is a little bit more complex than the
alcohol.
So hopefully, this shows you how complex the situation is. The results
depend on what specifically you add, and therefore overall generalizations
are hard to make.
Hope this helps,
Burr Zimmerman
This is difficult to answer, because it is not clear what the term
"cohesiveness" means. I do not know of a generally accepted definition of
the term. This needs clarification.
Vince Calder
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Update: June 2012
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