van der Walls forces

Could you explain van der Waals' forces?

How does a drop of water can stay hanging from the ceiling without falling?

Question

Could you explain van der Waals' forces to me, and their role in why energy is needed to vaporize water?

Answer

It is important to remember that van der Waals' forces are forces that exist between MOLECULES of the same substance. They are quite different from the forces that make up the molecule. For example, a water molecule is made up of hydrogen and oxygen, which are bonded together by the sharing of electrons. These electrostatic forces that keep a molecule intact are existent in covalent and ionic bonding but they are NOT van der Waals' forces.

The van der Waals' forces are the forces that exist between the millions of separate water molecules, and not between the hydrogen and oxygen atoms in the case of water.

Dipole-Dipole forces are one of van der Waals' three forces. Dipole Dipole forces occur in polar molecules, that is, molecules that have an unequal sharing of electrons. For example, HCl comprised of the atom Hydrogen and Chlorine is polar. The Chlorine atom has an extra electron, which came from the hydrogen atom. Because of this, the chlorine part of the molecule is negatively charged, and the hydrogen side of the molecule is positively charged.

ie. H - Cl
+ -

So in a solution where there are thousands of these molecules around that are slightly charged on each side, the molecules naturally orient themselves the accommodate the charge. The positive part of one molecule will move until it is next to the negative part of a neighboring molecule. These forces between molecules tend to make them 'stick' together.

Dispersion forces are another of van der Waals' three forces. They exist between nonpolar molecules. For example, chlorine gas is made up of two chlorine atoms. In this bond, the electrons are equally shared and are not dominant on one side of the molecule as is the case in HCl. The atom looks like this

Cl - Cl

no overall charge on either side but, it is important to remember that within a bond, electrons are constantly MOVING. They zoom around the atoms really quickly. As a result, there may be a tiny instant in time where the electrons happen to be dominant on one side, creating a situation like this,

Cl - Cl
+ -

However, this temporary charge disappears as quickly as it appeared because the electrons are moving so fast. These temporary dipoles allow the temporarily negative side of one molecule to attract the temporarily positive side of another molecule, which is the intermolecular force.

Hydrogen bonding is the third type of van der Waals' forces. It is exactly the same as dipole-dipole interaction, it just gets a special name. A hydrogen bond is a dipole dipole interaction that occurs between any molecule with a bond between a hydrogen atom and any of oxygen/fluorine/nitrogen. So, Hydrogen Fluoride (HF), Water (H₂O), ammonia (NH₃)....any kind of substance that has a hydrogen bonded to either an oxygen, fluorine or nitrogen atom, exhibits hydrogen bonding. The hydrogen bond is just the dipole dipole force but it is extremely strong compared to either dipole dipole forces like HCl. It is extremely strong because F N and O are extremely good at attracting electrons and H is extremely good at losing them. So basically, the bond is EXTREMELY a one-sided affair, resulting in an extreme dipole situation, thus named, a hydrogen bond. The extremely positive side of the molecule will orient itself with the extremely negative side of another molecule.

The van der Waals' forces are very weak. I said the hydrogen bond is extremely strong, but that is only compared to the other van der Waals' forces. Compared to say, a covalent bond, a hydrogen bond is approximately one tenth of that strength. The dipole-dipole bond is weaker still, and the dispersion forces are the weakest of Van De Waals' forces. That is demonstrated in the fact that, take for example, Cl₂. Chlorine gas exhibits dispersion forces, the weakest of van der Waals' forces. Cl₂ is a GASEOUS compound, because the dispersion forces are not strong enough to pull the molecules together as a solid. The dispersion forces can only suffice to keep the substance as a gas, because the forces between molecules are so weak that they can float about all over the place and exist as a gas.

Now that we know what van der Waals' forces are we move on to the second part of the question.

When you vaporize water, you need to turn it from a liquid to a gas. To do this, you need to overcome the forces between the molecules, to allow them to float about freely by themselves. You supply the substance with energy in the form of heat. The heat makes the molecules vibrate. If the vibrations are strong enough, the molecules break free of the van der Waals' forces that hold them together. In the case of water, these forces are hydrogen bonding. So you supply the system with enough energy, and it is able to overcome the forces between the molecules.

This accounts for the fact that water has an unusually high boiling point. Because hydrogen bonds are stronger than the other van der Waals' forces, then water will take more energy to overcome these bonds than say, HCl as a liquid- which would need sufficiently less because it has weaker bonds between molecules to overcome.

So, in a sentence, the role of energy in vaporizing water is that is needed to overcome the van der Waals' forces at work between the molecules.

Answered by: Sam Harper-Penman, Science & Engineering Student, majoring in physics

Question

Why does oil float to the top in liquid if you try to mix it with the liquid?

Answer

I think there are 2 questions here. The first is why oil and water don't mix. The second is why oil rises, as opposed to sinking or staying in place.

First question. There is an old saying in chemistry that like dissolves like. What this means is that a substance tends to dissolve in another substance if the molecules of the 2 substances have similar electric dipole moments. Think of an electric dipole like you think of a magnet. The magnet has a north and south pole, and south end of one magnet is attracted to the north end of another magnet, and vice versa. The electric dipole has a positively charged end and a negatively charged end. The magnitude of the positive charge can be greater than that of the negative charge, or vice versa. The difference between the magnitudes of the 2 charges and the distance between them determines the moment or strength of the dipole. In general, dipoles with similar strengths dissolve in each other more readily than dipoles with very different strengths. Oil (as in hydrocarbon-based oils) and water have very different dipole moments, so oil and water do not readily dissolve in each other.

The second part of the answer has to do with a force called the buoyancy force. This is the force that causes some objects to float in water. Suppose you want to dissolve 1 cubic cm of oil in water. For this to happen, the oil has to displace 1 cubic cm of water. The buoyancy force on the oil is equal to the weight of that 1 cubic cm of water. Oil is less dense than water, so 1 cubic cm of oil weighs less than 1 cubic cm of water. Therefore, the upward buoyancy force on the oil, which is equal to the weight of water displaced, is greater than the downward force of gravity on the oil, also known as the weight of the oil. This inequality of forces causes the oil to rise in the water. If the oil were denser than the water, the oil's weight (the downward force) would exceed the buoyancy force (the upward force), and the oil would sink in the water.

Answered by: Philip Zell, Ph.D. Physics, ACT, Inc.

Question

How come a drop of water can stay hanging from the ceiling without falling immediately on the floor? How does it stick to the ceiling surface?

Answer

Water molecules attract other water molecules. In fact, every molecule of all 'normal' liquids attract each other. In general, every neutral atom or molecule attracts one another -- more or less, when they are a little distance apart, and tend to repel each other when you push them too close together. The forces in action for this to happen are certainly electrical in nature -- they are generally known as Van der Waals' forces, and they are mainly due to forces between two dipoles (i.e. uneven charge distributions whose total charge is zero). However, a side note is that the molecules need not be polar on their own, they will induce a dipole moment when they are brought together, and thus attract each other.

Water, if left alone in a zero-gravity space (or equivalently, when in free fall), will tend to form itself into a sphere. Since every molecule pulls on one another, and the molecules on the surface have no molecule to pull on them from the outside, this causes what is known as 'surface tension'. This is the reason why one can overfill a glass with water, past the top, by a couple millimeters. Now, since there is tension in the surface, water tries to minimize the surface (it is like a balloon with extremely flexible surface), and since a sphere has the least surface area for a given volume, it forms a sphere.

Now, still assuming there is no gravity, assume we bring this sphere of water into contact with another piece of solid material, which has a planar surface. (This obviously is our ceiling.) What will happen next depends on the strength of the attraction between a water molecule and and a 'ceiling molecule'. If the attraction between water and ceiling is stronger than the attraction between water and water, the water will tend to stick to the surface. Then it is said that the water has 'wet' the surface. If water-water attraction is greater, the water will just not stick to the surface -- then we say the water did not wet the surface. This phenomenon is easily observed, water will readily wet most materials (i.e. spread out on them), but with some good quality car polish, it will just 'bead up'. When it beads up, it is NOT wetting the surface, in fact, in absence of gravity, it would form into a sphere.

Now, if the water can wet the surface, it sticks to it. It has found a way to reduce the tension by reducing the surface area further, since now the water next to the surface has very little tension, so it does not really 'count', and the rest of the area is less than the original sphere. Thus it sticks to the surface.

If gravity was absent, the story would end here. We would see all sizes of water droplets stuck on surfaces. However, the presence of gravity changes the problem. Now, the force which holds the water droplet against the surface is proportional to the area. In the case of a sphere of radius r, this is proportional to r*r. But then, the weight is proportional to the volume of the sphere, which is proportional to r*r*r for a sphere of radius r. So, for a large enough radius, no matter how strong the attraction is, the force of gravity will overcome the attraction between water and ceiling (since r^3 grows faster than r^2) and the droplet will have to drop.

As another piece of information, take mercury, which is a liquid metal. It is a metal, and atomic, so it is not polar. If you spill it on most surfaces (i.e. stone, wood, aluminum) it will bead up into almost perfect spheres. It will not wet it. The attraction between mercury-mercury is much stronger than attraction with wood, stone or aluminum. However, less is known what happens when you bring it into contact with gold -- it wets it! In fact, it will form a thin layer of mercury wherever it touches it -- the attraction between mercury and gold is so strong, one will have a hard time removing the mercury from the gold (Believe me, I know, gold is one of the few materials dense enough to sink in mercury, and I was trying to test that. The piece of jewelry had to go the jeweler's to get cleaned.) SO DON'T TRY THIS AT HOME, MERCURY IS POISONOUS. I think this example shows that the sticking has nothing to do with either having polar molecules or static charges, although the forces involved are electromagnetic in nature.

Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey