Ask A Scientist Chemistry Archive

name         Ty M.
status       student
age          11

Question -   Can you tell me in simple words what makes liquids
immiscible?
------------------------------------------------
I'm 11 years old, and do not understand some of the words used in Newton's
database question.  I am learning about immiscible liquids for my science
fair project this year, but do not understand the meaning of
intermolecular bonds or dielectric constants.
------------------------------------------------
Your questions are not easy, but here is a try.
1. Immiscible liquids are liquids that do not dissolve in one another. Some
combinations of liquids mix in all proportions, e.g. alcohol and water, mix
at some compositions but not at others, or hardly mix at all, e.g. gasoline
and water.

For two liquids, A and B to mix, the energy holding liquid A together must
be overcome, as does the energy holding liquid B, and these energy "costs"
must be regained by the energy released when the solution of A and B is
formed. The more similar two liquids are chemically, the more likely will
this energy balance will be favorable. The chemical rule of thumb of this
statement is , "Likes dissolve likes." But of course that is a an
oversimplification.

The theories that attempt to "explain" the mutual solution of two liquids
are beyond your present level of chemistry. And in fact are not very
general.

I think that an attempt to explain the other terms "intermolecular bonds"
and "dielectric constant"  would require more space than a forum like this
allows.

Vince Calder
=========================================================
Ty,

Sometimes, if two different kinds of liquid molecules are very different
from each other in composition or size or shape, they will not mix with each
other. Consider how blocks of wood and wooden balls might  pack together
inside a container. The blocks can be made to fit together rather well.
Likewise, the balls will arrange themselves in a neat pattern when placed in
the container. If you mix the blocks and balls and then pour them into the
container, you'll discover a very disordered arrangement. They do not fit
together very well at all because their shapes are very different.

Something similar happens between different (immiscible) liquids. The liquid
with the greater density will sink to the bottom and the lighter liquid will
float on top. It's a little like salad oil and water. These molecules are
very different from each other in shape, size, and composition. As a result,
they don't mix with each other. If you shake the oil-water mixture together
and then allow it to stand undisturbed, the oil and water molecules will
slide past each other. In time, they will separate from each other -- with
the oil  floating atop the water. The oil molecules feel an attraction for
each other, but very little attraction toward the water. Water molecules
bond with water molecules and not at all well with the oil molecules.

Intermolecular bonds are just connections between molecules. If the
connections are strong, the molecules hang together tightly. Oil and water
(two immiscible liquids) show very little intermolecular bonding toward each
other.

Dielectric constants are hard to explain until you lean more chemistry. For
now, you can think of the dielectric constant as being related to both the
shape and electrical properties of molecules.

Regards,
ProfHoff 370
=========================================================
The underlying reasons making different liquids immiscible with each other
are complex and subtle.  In fact, I believe that it is not yet possible to
predict from first principles (knowing only the molecular structures of the
different liquids) whether any two liquids will be miscible or not.

That said, the direct reasons for miscibility or its lack are fairly
straightforward.  Several interactions work to help or hinder mixing, and
the eventual result depends on which interactions dominate.

The main feature that promotes mixing is that if two liquids are miscible,
then their molecules can occupy any region of the shared space.  For
example, if you combine 1 liter each of water and alcohol, each water or
alcohol molecule will be able to range over 2 liters of total volume.  If
the liquids were immiscible, the water and alcohol molecules would be each
confined to only one liter of volume.  Why does this matter?  Let's imagine
the simplest case, where individual molecules aren't attracted to any of the
other molecules.  Any single molecule, if it is free to travel throughout
the entire 2 liter volume, will be in any half of these 2 liters exactly
half of the time.  Two molecules will be in the same half exactly 1/4 of the
time; three molecules exactly 1/8 of the time, and so on.  Now, a liter of
water contains about 3.3 x 10^25 molecules; a liter of alcohol (ethyl
alcohol here) contains 1.0 x 10^25 molecules.  So, the chance that all
ethanol molecules will be in one half of the volume and the water molecules
will be in the other half is vanishingly small.  There is a tremendous
statistical drive for the two substances to mix.

Of course, this isn't the whole story.  The rest of the story is that
molecules are NOT isolated individuals - they interact with each other.
Basically, we have three types of interactions to consider:

  water-water
  water-alcohol
  alcohol-alcohol

If, say, water-water interactions are especially stabilizing, much more so
than water-alcohol interactions, then the water molecules will tend to keep
together and neither go into the alcohol nor let alcohol molecules into the
water.  In the case of alcohol and water, though, water-alcohol interactions
aren't much worse than water-water interactions, so the tendency to mix
dominates.

In the case of water and oil, however, the water-water interactions are MUCH
more favorable than water-oil interactions.  The loss of the favorable
water-water interactions that would result if the two phases were to mix
would be too high a price to pay for mixing.  So, water and oil remain
separate (immiscible).

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
=========================================================
OK honey, letīs speak in a way you can understand.
Take the commonest example, oil and water. They are immiscible
because they have different densities, or specific gravities.
When you put them together usually the oil  forms a  layer
at the container botton. Even if you mix them
well with a spoon , the oil will form small drops inside the
water and quite fastly the drops reunite and go down.
What is density? it is a physical property of materials,
and corresponds to the relation mass/ volume = density.
So  to make things easy, for the same volume of
2 liquids (we are speaking of liquids) , the one that
is heavier, has the higher density. There are cases
when the oil has a lower density than the water. In
that situation it will form a layer over the water layer.
OK, now letīs talk about soluble liquids, as water
and alcohol. Why they are miscible? For one thing
water is called "the universal solvent" because
it solves  everything...OK,... almost anything.
Every substance (here every liquid)  has certain
type of forces that attract their molecules
betwem themselves..
When 2 liquids (or a liquid and a solid) dissolves one
in another they are called miscible and form
what is called a solution.
The one that has more quantity is called solvent;
the one that has less is called solute.
The reason why  a liquid
(or a solid) dissolves  in another liquid  is
in a simple way due to the
following: 3 things must happen:
1)the attractive forces holding the solute together
must be overcome.
2)The attractive forces holding at least some of
the water molecules( or another solvent) together
must also be overcome.
3)The solute and the solvent molecules must
interact, that is, they  must attract one another.
The result is the solution of one  in another.
Observe that a solution formation is a
physical phenomena, so there are
no  chemical changes in the molecules, and also
it is a reversible process, it is possible to
separate solute and solvent.
I hope you did understand now, yes?
Thanks for asking NEWTON!
Tell your friends about us!
Mabel
(Dr). Mabel Rodrigues
=========================================================
Hi, Ty !!!
Two liquids become immiscible when their nature
make them very different entities. If they are similar,
they dissolve each other. If they are approximately
similar, then they are approximately miscible or
immiscible.
Everything depends on the nature of the liquids.
Lets consider water and oil. Water is called "polar"
liquid, because its molecules behave like having
two poles (one positive end and a negative end).
The oil has not this tendency, so that both liquids
do not couple.
Lets take gasoline and oil. Both liquids have the
same tendency not to be polar : as a consequence,
they dissolve each other. Lets take alcohol and water :
if we speak about ethyl alcohol (commom alcohol),
then they have similar polar tendency.
Now, lets go on the second part of your question : dielectric
constants
Lets suppose you have two parallel surfaces and with
the help of a battery you transfer some electrons
from a surface to the other. You can bring a lot
of electrons from a surface to the other using for
instance only one battery. But if you employ two
or more batteries, you can bring a lot more of
electrons from a surface to the other...till the
point where the electrons flow through the air
to the surface where they originally came from.
But this is NOT the case. Lets stop in the point
where you could get a lot of electrons with some
batteries. You can relate the electrical charge with the voltage
used, lets say :  Q/V and you call this Capacitance ( C=Q/V ).
Now, consider the distance between the surfaces  "d".
If the distance is small, the electrons will jump...but
we donīt want this !!! If  "d" is big, the tendency to
jump will be small. So, you can make a relationship
between the Voltage and distance, like this:
V/d and call this E (=electrical field). And you can
also make a relationship between the number of
electrons you have gathered in a surface and the
area of this surface, like this : Q/A and call it
electrical density "D"  ( D= Q/A )
You can also relate electrical field to the
electrical density, like this :
e = D / E = (Q/A) / (V/d) =  C (d/A)   or
C = e (A/d)
If you repeat this experience in the vacuum instead of
in air, you get similar effects, but  "e" will be different.
Lets say, you get   e1. Then, summing up:
In air :         C = e (A/d)
In vacuum:  C = e1 (A/d)
You can relate   "e" to "e1", and you get the relative "er"
er = e / e1
And THIS is what is called dielectric constant.
So, as you can see, this relative value depends on
the substance between the plates or surfaces:
If it is the air, you have er = 1,0006. If between the
plates you have rubber, than er = 3. If glass, er= 6.
As you can see, the dielectric constant is a constant,
that depends on the kind of substance you have between
the plates. It relates the capacitance to the surface area and
distance between the plates. Considering the same distance,
than the bigger the surface, the more you can put electrons
with the same Voltage.

So, last but not least, keep in mind that capacitance
is the possibility of putting more electrons in a given area,
with the same voltage per length. Depending on the
material between the plates, you can increase the
capacitance or decrease. In other words, if you have
a "better" dielectric material, like glass instead of
rubber, you get a better capacitance, what means,
more electrons pro area, considering the same potential
gradient (= voltage pro length unit).

Grohmann, Alcir
=========================================================