Ask A Scientist Chemistry Archive

name         Tom W.
status       student
age          16

Question -       Is it possible that after distilling a salt water
solution (by boiling it), that the 'purified' water will contain a small amount 
of salt?
And if it does, will the concentration of salt in the distilled water
depend on the concentration of the initial solution?
------------------------------------------------
Tom,

If the solution is distilled in such a manner that only vapor enters the
condenser, and no trace of water/salt aerosol is carried over by the vigor
of boiling, the liquid condensed from the vapor will be pure water and it
will contain no salt.

Regards,
ProfHoff 390
========================================================
Hi Tom,

Question:
Is it possible that after distilling a salt water solution (by boiling it),
that the 'purified' water will contain a small amount of salt?  And if it
does, will the concentration of salt in the distilled water depend on the
concentration of the initial solution?

Answer:
This is a very good question.  There are two possibilities.

1.)  Salt can take on water (hygroscopic).  So when you think you might have
exactly 1.000 g of NaCl, it might actually be less since the salt that you
weighed might actually be 0.99 g of salt and 0.01 g of adsorbed water.  You
now add some water to then distill it.  When you boil off all water you will
have anhydrous (without water) salt, around 0.99 g of it.  My point is that
you do not actually know the true molarity of the solution.

Solutions:
a.)  Boil off all waters attached to the salt (PRIOR TO WEIGHING IT) by
heating it on the stove.  After you are convinced that the salt is
anhydrous, immediately weigh it.  You can be assured that 1.000 g of salt
will be 1.000 g of salt with no water.

b.)  I do not believe that it is possible for a non volative solute to escape
the confines of the solution.  However, if you suspect that this is the case
you can measure the conductance, via resistance,  of all water samples in
question.  Take care to measure this conductance at nearly identical
temperatures.

Use a high accuracy ohm meter at least 4-5 sig figs.  Take the two probes
and tape them together.  This will ensure that the metal on each probe has
identical distances between each test.

b1.)  Measure the temperature of the water ( T tap ) used to dissolve the
salt prior to distillation, measure the resistance ( 1 / conductance )  Your
meter probably will still be off the scale, as if you had not event dipped the
probes in the water.  This is OK.
b2.)  After your experiment, let the distilled "pure" water you believe to
be harboring the salt cool down to T tap, then measure its resistance.

My guess is that the resistance of your tap water will be fairly high ( > 2
Million Ohms )
If you are correct in your assumption that salt is escaping to the
distillate you will see a very measurable difference in the resistance.  I
have done this experiment before and I was able to get the R (pure water)
down from 2 Million Ohms to R (salt water ) = 75,000 Ohms at one inch probe
spacing.

All this said, my gut still tells me that solution A will explain your
inability to account for the "loss" of salt.  If you do measure salt in the
distillate well than you sure have taken me back to school.  The Boltzmass
curves on the following web site might explain solution B:

http://www.chem.uidaho.edu/~honors/collig.html

I am intrigued by your question and experiment.  Thank you.  If it is
convenient for you, please forward your findings to us when you are
complete.
I hope this has helped.

-Darin Wagner
=========================================================
Properly distilled (i.e. boiled) water should not contain any inorganic
salts since the distillation (i.e. boiling) process involves converting
impure liquid water to water vapor and then condensing the vapor, which
contains no non-volatile compounds, to the liquid state again. The purity of
the distilled water can be determined sensitively by measuring its
electrical conductance (the reciprocal of its resistance) which is extremely
sensitive to small quantities of ionic impurities.

If one wants to get very picky about the question, water can dissolve small
quantities of ionic compounds in glass -- in fact -- under proper
conditions, water can dissolve detectable amounts of glass itself. But in
the common use of the terms, distilled water does not contain any
non-volatile salts such as NaCl, Na2SO4, etc.

Distilled water will also normally contain trace quantities of air, which
contains CO2. The CO2 reacts with water to produce H2CO3 == [ H+] + [HCO3-]
== [H+] + [CO3=].

Certain non-ionic substances can co-distill with water forming mixtures
called azeotropes. But the term "distilled water" usually excludes these
possibilities.

Vince Calder
=========================================================
Tom,

Indeed it can. That transferred by aerosol will be the same concentration as
the original salt solution because the transfer is a mechanical movement of
tiny "particles" of the parent solution. By the way, three distillations in
sequence should result in water that is very pure.

Regards,
ProfHoff
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