Ask A Scientist Chemistry Archive

  
name         Amanda M.
status       student
age          14

Question -   Can you give me the names of some substances that
dissolve in water but do not form ions?  I know that salt ionizes and
sugar does not.  I want to compare the effect on water's freezing point
and boiling point of solutes which ionize and solutes which do not.  I
can find lots of compounds that ionize, but so far I can only come up
with sugar in the do not ionize column.  P.S. the substances need to be
solids.  Thank you.  Amanda M., 9th Grade, La Reina High School, 
California.
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Amanda,

I am sure someone else in the forum will answer your immediate question of 
what
compounds might be both soluble in water, not ionic, and a solid. I would 
like
to give you some information on how you can - in the future find this out
yourself.

In deciding a lot of such things, I find it useful to go back to "First
Principles".

First of all, what causes substances to ionize when dissolved in water? The
first principle is that such substances have some kind of ionic bond 
--such as
you find in salts, e.g. Na-Cl, K-Cl are ionic bonds, or in strong acids and
bases, e.g. H-NO3, Na-OH are ionic bonds.

Secondly, what causes substances to dissolve in water? The first principle is
that the substance must have a similar intermolecular force as water --either
ion-induced dipole, hydrogen bonding, or strongly dipole-dipole interaction.
Ionic molecules have ion-induced dipole interactions so that salts, strong
acids and strong bases tend to dissolve in water. Molecules that have O-H 
or N-
H bonds tend to have hydrogen bonding interaction. Alcohols, sugars, ammonia
have such a bond and dissolve in water. Molecules that have a net dipole can
have dipole-dipole interactions and so acetone dissolves in water.

Thirdly, what causes substances to be solids? This is a little bit more
difficult, but the first principles that apply are some kind of 
combination of
strength of intermolecular force (including size of the molecule) and 
symmetry
of the molecule.

Finally, in answer to your objective of comparing different substances' 
effect
on freezing point, you do understand that there is a colligative property
called freezing point depression, and it quantifies the effect of solutes
(both ionic and not) on the degree of freezing point depression. What the
mathematical formula denotes is that the freezing point is a function only of
the molality of the solution (moles of solute per kilogram of solvent)
multiplied by a constant (called the freezing point depression constant) that
is dependent only on the solvent. Some variants of this mathematical formula
actually puts in an additional constant which denotes the degree of 
ionization
of the solute - for such cases where the ionization is not 100% and molality
does not reflect the correct number of moles of species in solution.

Hope this has taught you something.

Greg (Roberto Gregorius)
=====================================================
Of the nonionic substances that come to mind, urea (CO(NH2)2) would be
my first choice. It is very soluble in water, low M.W. ~ 60 so the changes
in temperature are relatively large, non-toxic, readily available and
inexpensive. If you cannot get it through your school, garden shops and
farm supply stores will have it. Measuring the freezing point depression
will pose no problem; however, urea does decompose slowly over time at
elevated temperatures, so the boiling point elevation measurements should
be done fairly speedily, otherwise the boiling point may drift.

Vince Calder
=====================================================
I think you have noticed something real, Amanda.
There are rather few solid, non-ionic substances which are soluble in water.

Water is a polar solvent.  To be soluble in water requires a molecule to 
have partially (+) and (-) ends.
But just when the +/- strength is strong enough, it starts pulling apart 
H2O into ions, H(+) and -OH(-).
Almost the only thing that water likes a lot without ionizing or being 
ionized,
  is half a water-molecule hanging off the solute molecule:  the OH group.

The Alcohols have an OH group:
   Methanol  CH3-OH
   Ethanol   CH3-CH2-OH
   Isopropanol  CH3)2 CH OH
    CH3
     \
      CH-OH
     /
    CH3

Some things have 2 or 3 OH groups:
  Ethylene Glycol:  H HCOH HCOH H
      H
      |
    H-C-O-H
      |
    H-C-O-H
      |
      H

  Glycerine:  H (HCOH)3 H

The Ketone ("key-tone") group helps too:   CO, C=O

   \
    C=O   (The "=" means there are two bonds between the C and O.)
   /

The solvent Acetone (CH3)2 CO is entirely water-soluble.


But these are all liquids, so you do not like them for your experiment.

Sugar is water-soluble because it has OH groups all over, just like 
glycerine only bigger.

There is a water-soluble plastic called poly-vinyl-alcohol (PVA for short),
very non-toxic but a little hard to find,
which uses half as many OH groups:
(-CH2-HCOH-)n , where n is any large number, like 1000.

      H       H      H
  H   O   H   O   H   O
-C---C---C---C---C---C-
  H   H   H  H   H  H

more hydroxy-solids:
   Hexane di-ol and tri-ol are solid and soluble.
   1,6 Hexanediol is relatively inexpensive, $20/500gm.
   There is something called erythritol: H-(HCOH)4)-H.
   Pyrogallol, catechol, and hydroquinone fit, but they are a bit toxic.
     I wouldn't mess with those yet.

There is another water-soluble plastic: poly-vinyl-pyrolidone (PVP)
-CH2-CH[N(CH2)(CH2)(CH2)(C=O)]-

   H2C-CH2
    /   \
  H2C    CO
     \  /
  H   N
-C---C-
  H   H

It too is non-toxic, bio-degradable, even, but you would have to get it 
from a chemical company,
such as Alfa/Aesar or Fischer or JT Baker.  I think they will sell it to 
anybody.
However, N atoms tend to be basic, like ammonia (NH3);
they attract an H+, becoming (+) ionic points, and leaving an OH- in the 
water.
There are other little games the C=O and the N can play to give or take an 
H+, too.
So I do not know if you want to use this substance.
If you do, do not get "crosslinked" grade; it will not dissolve.

There is a class of molecules called which do not ionize water much,
but one end of itself ionizes the other end of itself.
End A gains a (+), end B gains a (-), and water likes that but is only 
slightly ionized in response.
These are water-soluble, but do they meet your specifications?
I think gelatin is in this class.
Amino Acids are in this class.
I wonder how water-soluble is the glucosamine in the health supplements 
you can buy in the store?

3,3 dimethyl 2-pyrrolidone looks like a soluble solid.

Dimethyl Sulfone, (CH3)2SO2, meets your criteria, if your supervising 
chemistry teacher will let you use it.
(not the same as sulfonyl, sulfoxide, sulfate, sulfolane.  Must be the 
right spelling.)

All these are a little exotic and expensive.
You may wish to change the premise of your experiment just a little bit:
i.e., "freezing point depression vs. degree of ionicity" or some such.

Hope you find a topic that works well for you .

Jim Swenson
=====================================================