Brownian Motion And Diffusion

GRADE 11 : PHYSICS NOTES FOR CHAPTERS 4.1 & 4.2 pages 116-120 BROWNIAN MOTION: In 1827 the biologist Robert Brown noticed that if you looked at pollen grains in water through a microscope, the pollen jiggles about. He called this jiggling 'Brownian motion', but Brown couldn't work out what was causing it. The first of the three papers that Einstein published in 1905 finally came up with an explanation. Everything around us is made up of atoms and molecules: the chair you're sitting on, the food you eat, the air you're breathing. The idea of atoms has been around since the time of the ancient Greeks, and a century before Einstein, the great chemist John Dalton had suggested that all chemicals were made of tiny invisible molecules, which in turn were made of even tinier atoms. The problem was that there was no proof of their existence, until Einstein looked into the problem of Brownian motion. Einstein realised that the jiggling of the pollen grains seen in Brownian motion was due to molecules of water hitting the tiny pollen grains, like players kicking the ball in a game of football. The pollen grains were visible but the water molecules weren't, so it looked like the grains were bouncing around on their own. Einstein also showed that it was possible to work out how many molecules were hitting a single pollen grain and how fast the water molecules were moving - all by looking at the pollen grains. Importantly, Einstein's paper also made predictions about the properties of atoms that could be tested. The French physicist Jean Perrin used Einstein's predictions to work out the size of atoms and remove any remaining doubts about the existence of atoms.

DIFFUSION AND OSMOSIS: http://hyperphysics.phyastr.gsu.edu/hbase/kinetic/diffus.html Must refer to this website as it has diagrams which help explain things better. Notes from this website have not been included in this lecture. Diffusion Processes Basic idea:

In its simplest form, diffusion is the transport of a material or chemical by molecular motion. If molecules of a chemical are present in an apparently motionless fluid, they will exhibit microscopic erratic motions due to being randomly struck by other molecules in the fluid. Individual particles or molecules will follow paths sometimes known as "random walks." In such processes, a chemical initially concentrated in one area will disperse. That is, there will be a net transport of that chemical from regions of high concentration to regions of low concentration. An analogous form of diffusion is called conduction. In this case, heat is the "chemical" that is transported by molecular motion. As in chemical diffusion, heat migrates from regions of high heat to regions of low heat. The mathematics describing both conduction and diffusion are the same.

An interesting article upon Surface Tension: Ref:

http://www.ilpi.com/genchem/demo/tension/Surface Tension [View demo] [How it works]

Introduction Water has many unusual properties as a result of its ability to hydrogen bond. For example, the density of ice is less than that of the liquid and the predicted boiling point is almost 200 degrees C higher than it would be without hydrogen bonding. The water molecules at the surface of water are surrounded partially by air and partially by water. These surface molecules would be much more stable if they could be in the interior of the liquid where all their hydrogen bonds could be fulfilled (cohesion). Therefore, water normally tends to have the smallest surface possible, i.e. it has a high surface tension, in order to achieve the lowest possible energetic state.

If a solid material more dense than water is placed on the surface of water, what happens next depends on the nature of the material. If the material is hydrophilic ("water loving") it has a surface to which water is attracted. The adhesion of water to the surface of this material coats the surface of the object with water, reduces the surface tension, and causes the object to sink. If the solid object is hydrophobic ("water fearing"),the unfavorable interactions between the water surface and the object make it difficult to wet the surface. Two forces now come into play -- the energy it would take to overcome this repulsion and the force of gravity. If the force of gravity is strong enough, it will prevail and the object will sink (assuming that the object has a density greater than water). If the gravitational force is less than the surface tension then the object will float on the surface of the water. Surface tension is what permits water striders and other insects to walk across the surface of water and what enables a needle to float. Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink. The Demo In the demo shown below, sulfur is sprinkled on the surface of water in a large beaker. The sulfur floats because the particles are very small and sulfur is a hydrophobic molecular solid. When one drop of liquid detergent is added to the beaker without stirring, the sulfur suddenly sinks to the bottom of the beaker. How it works Detergents are a class of chemicals that contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. This general class of molecules are called surfactants. Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water. Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks. PS: The contents of this article has been carefully selected to enable you to understand the process without confusing yourself.

COHESION AND ADHESION: The tension on the surface of the liquids is caused by attraction which makes its molecules cling together. This attraction between molecules of same substance is called cohesion. Adhesion-cohesion is a stabilizing mechanism by which joint surfaces wet with joint fluid are held together by the molecular attraction of the fluid to itself and to the joint surfaces. Water molecules are what are called dipoles: they have an electric 'pole' at each end of the molecule with opposite charges because the electrons in the molecule tend to congregate near the oxygen atom and away from the hydrogen atoms. Thus the negative part of one water molecule will attract the positive parts of other, nearby molecules. This is why water falls from the sky as raindrops, and not individual molecules, or why water tends to bead up on the hood of your freshly waxed car, or why you can cause water to bulge out over the rim of a glass if you fill it carefully; the molecules are all pulling together. Water molecules are not only attracted to each other, but to any molecule with positive or negative charges. When a molecule attracts to a different substance, this is termed adhesion. Think about what happens when you dip one end of a piece of paper towel into a glass of water. The water will climb up the fibers of the paper, getting it wet above the level of the water in the glass. We know gravity is pulling down on the water, so why do they move up? Because the water molecules' positive and negative charges are attracted to the positive and negative charges in the cellulose molecules in the paper. Note that both the examples above have both cohesion and adhesion occuring but one is stronger than the other. If the water molecules are more strongly attracted to each other than to the surrounding material, they bead up and try to get as close to each other as possible. If there is a stronger attraction to some other material, they spread out and try to get close to the other material.