Anyone remember the ethanol question?
When you add 50 cm3 of ethanol to 50cm3 of water, the total
volume of the mixture is 97cm3. Ans: Ethanol molecules fit into the
gaps of water molecules
Why does the ethanol molecules fit into the gaps of water
molecules. Hell, I didn't know this was in my syllabus.
I read somewhere that ethanol pulls water molecules closer
together, because of the fact water molecules are polar. Is that
true?
Thanks for helping :D
Regarding ethanol "fitting into gaps between" water molecules,
that's only an approximate description for the actual process,
which is much more complex that can be handled at 'O' levels.
To simplify it - in the liquid state, particles can slide over
each other freely (unlike solid state), and hence the ethanol
particles can "fit into gaps between" water molecules. This would
not be possible in the solid state.
Ethanol is slightly polar (as you increase no. of C atoms,
polarity of alcohols decrease), just polar enough to dissolve in
water. Enough hydrogen bonding occurs between ethanol and water
molecules, that they are miscible (ie. can dissolve into each
other).
Hence this "fitting into gaps between" would also not be
possible if the alcohol was non-polar (eg. pentanol) and
could not dissolve in ("fit into gaps between") water. The
volumes would be more significantly conserved (eg. 99cm3) than is
the case with a polar alcohol that can dissolve ("fit into gaps
between").
Finally, don't worry too much. This is an MCQ qn. It's usually
obvious which of the 4 options is the best answer (even if two
options may be arguably correct, only one will still answer
the question best).