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Name: Ty M.
Status: student
Age: 11
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
Can you tell me in simple words what makes liquids immiscible?


Replies:
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.


Replies:
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, lets 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 lets 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 dont 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



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