Re: Methanol and water...


Marcus Fox wrote:
I have an experiment that involves mixing 50 cc of methanol with 50 cc of
water. However, I need to find an explanation why the volume of the combined
liquids would not be as some students may expect. I am able to use Google to
find a detailed explanation, involving thermodynamics, but this is way above
the level that I need to explain it at. Do I simply say that the water
dissolves in the methanol, or the methanol dissolves in the water, or some
other explanation? And is this experiment safe enough for a group of pupils
to do on their own instead of a teacher demonstration? It seems to be
described as so in books at this level, but I always remembered methanol as
being quite hazardous.

Many thanks for all comments,

Marcus

Hello Marcus:

An explanation involving strength of forces between molecules should be
understandable to the students.

Three kinds of forces that can be considered in dissolving processes:
solute-solute, solvent-solvent, and solute-solvent.

If solute-solute forces are very strong (e.g., AgCl in water) the
solvent can't pull apart the solute species, and the solute doesn't
dissolve. If solvent-solvent forces are very strong (wax in water) the
solute can't force itself between the solvent molecules; no dissolving.

When all three forces are of comparable magnitude, the solute can get
in between the solvent molecules, and dissolving can occur.

However...if the solute-solvent forces are *slightly* stronger than the
other two, dissolving still occurs, but the molecules of solvent and
solute are pulled together a bit more tightly than they are in the pure
solvent and pure solute. The volume of the solution is accordingly
reduced compared to solute and solvent alone.

Best -- Terry

.

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