School Science Lessons
10. Molecular motion, diffusion, osmosis, molecular spacing
2011-12-26
Please send comments to: J.Elfick@uq.edu.au
Table of contents
10.0.0 Molecular motion
10.1.0 Diffusion, particles of matter
9.14.0 Osmosis
10.3.0 Molecular spacing
12.4.0 Siphons
10.1.0 Diffusion, particles of matter
3.55 Brownian movement
8.1.1.0 Candle, paraffin wax (diffusion flame)
3.58 Clay soil suspension
9.12.0 Colloids, diffusion, semipermeable membrane, dialysis
3.7 Congo red, microscopy stain
10.0.1 Diffusion experiments
10.1.3 Diffusion in liquids, potassium manganate (VII) or potassium dichromate
10.1.2 Diffusion of ammonia and hydrogen chloride gases
10.1.1 Diffusion of carbon dioxide
3.55.1 Diffusion of heavier than air gas, carbon dioxide
3.55.3 Diffusion of liquids
10.0.2 Graham's law of diffusion
3.55 Matter as particles
3.3.3 Measure the size of the stearic acid molecule
3.56 Particles of matter and dilution
3.57 Size of a molecule
3.3.0 Sizes of particles of matter
19.4.2.1 Stain removal
10.3.0 Molecular spacing
24.0.0 Change of state
10.3.4 Container holds more
10.3.3 Container not leaking
12.3.5.2 Inverted drinking glass
3.4.1.1 Stretched rubber bands
3.55.0 Matter as particles
10.3.5 Shrinking balloons
10.3.2 Shrinking mixture of liquids
10.3.1 Shrinking solution
3.57 Size of a molecule
12.4.0 Siphons
12.4.0 Siphons
12.4.1.0 Simple siphon
2.12 Siphon and water spray (Primary)
12.4.1.1 Siphon fountain
12.4.1.4 Siphon in a bell jar
12.4.1.2 Siphon replaces water for fish tank
12.4.1.5 Siphon mechanism apparatus
12.4.1.9 Mariotte flask and siphon
12.4.1.7 Mechanical siphon
12.4.1.6 Pressure in a siphon
12.4.1.8 Self-starting siphon
See pdf Self-siphoning gel, Poly-Ox, polyethylene oxide
12.4.1.10 Turnover siphon
12.4.1.3 U-tube siphon
10.0.1 Diffusion experiments
Diffusion is the mutual penetration of molecules of substances in contact. In diffusion, the molecules of a substance, being in permanent motion, penetrate into the spaces between the molecules of another substance, which is in contact with the first substance, and are distributed among them. Molecular motion causes the concentration of a substance in an in homogeneous material to become homogeneous. Diffusion always occurs from high concentration to low and diffusion occurs at a faster rate when the difference between concentrations is larger. The smaller the molecules, the faster the rate of diffusion. The rate of diffusion increases with temperature. Diffusion occurs in all the states of aggregation but to different extents. You can observe diffusion in gases easily. Diffusion in liquids occurs at a much slower rate than in gases. Diffusion in solids occurs at a still slower rate than in liquids.
Open bottles of perfumes or other aromatic substance at the end of the room with all windows and doors shut. Note when you can first smell the substances and estimate the rate of diffusion in air per metre at that temperature.
10.0.2 Graham's law of diffusion
The rates of diffusion of particles of gas are inversely proportional to the square root of the molar mass of the particles.
Rate of diffusion A / Rate of diffusion B = √ molar mass B / √ molar mass A
10.1.1 Diffusion of carbon dioxide
See diagram 3.34.1: Limewater test for carbon dioxide
1. Fill a jar with carbon dioxide and invert it over a similar jar full of air. After a few moments separate the jars, pour a little lime water in the lower one and shake it. The limewater will turn milky indicating that the carbon dioxide has fallen into the lower jar because it is the heavier gas. Now repeat the experiment but this time put the carbon dioxide in the lower jar and invert a jar of air over it. If the jars are left for about five minutes, some carbon dioxide will be carried into the upper jar by diffusion, in the same way some air will be carried into the lower jar. The limewater test shows the presence of carbon dioxide in the upper jar.
2. The molecules of gases are always in a random motion. They may not only move downwards under the gravity to form distribution according to concentrations, but also diffuse in every direction. Use a wide and thin test-tubes so that the thin test-tube just fits into the wide test-tube. 2. Fill the thin test-tube with carbon dioxide gas and fill the wide test-tube full of air. Insert the open end of the thin test-tube into the wide test-tube. Let them stand vertical. After some minutes, separate them quickly. Add some limewater into the wide test-tube and shake it gently. Note the change in colour. The solution changes from clear into muddy and finally becomes milk white. It shows some carbon dioxide gas has entered the thick test-tube because denser carbon dioxide molecules go down from the thin test-tube into the wide test-tube under the effect of gravitation. 3. Fill the wide test-tube with carbon dioxide gas and inflate the thin test-tube full of air. Insert the open end of the thin test-tube into the wide test-tube and let them stand vertical. After some minutes, separate them then quickly add some limewater into the thin test-tube and shake it gently. Observe the change in colour at the thick one. The solution changes from clear into muddy and becomes milk white liquid finally. It shows some carbon dioxide gas has entered the thin test-tube because carbon dioxide molecules in random motion diffuse in various directions including upward and downward. So some carbon dioxide molecules go up from the thick test-tube into the thin test-tube.
10.1.2 Diffusion rates of ammonia and hydrogen chloride gases
See diagram 10.1.2: Ring of ammonium chloride
Be careful! Only teachers should do this experiment because hydrochloric acid solution and ammonia water are strongly corrosive solutions.
1. Use a long horizontal glass tube with stoppers fitted at both ends. Use tongs to dip one piece of cotton wool into concentrated hydrochloric acid and another piece into concentrated ammonia solution. Drain off excess liquid. Simultaneously, put the soaked pieces of cotton wool inside the ends of the glass tube. Close the ends of the glass tube with the stoppers. Watch for a white ring forming where the ammonia gas and the hydrogen chloride gas meet after diffusing through the air towards each other. Ammonia is less dense than hydrogen chloride so the white ring of ammonium chloride should form nearer to the hydrogen chloride end of the glass tube.
2. The long glass tube should be horizontal. Corks should fit at both ends. Using a pair of tongs or tweezers, dip a piece of cotton wool into concentrated hydrochloric acid and dip another piece into concentrated ammonia solution, NH3 (aq) ("ammonium hydroxide") solution. Drain off excess liquid. Simultaneously, put the ammonia in cotton wool in one end of the tube and the acid in cotton wool in the other end. Close the ends of the tube with corks. Later, look for a white ring that will form where the ammonia gas and the hydrogen chloride gas meet after diffusing through the air towards each other. Ammonia is the less dense gas and the white ring of ammonium chloride should form nearer to the hydrogen chloride end than from the ammonia end of the tube.
3. Different gases diffuse at different rates at the same temperature. Use a glass tube 1 metre long and 2 cm diameter, open at each end, two stoppers, two identical balls of cotton wool, hydrochloric acid, ammonia, water. Place the glass tube flat on a table. Immerse one cotton wool ball in hydrochloric acid solution. Immerse the other cotton wool ball in ammonia solution. Take them out of the solutions and press them until no liquid drops. Insert the cotton wool balls into each end of the glass tube simultaneously and instantly insert the stoppers so that air cannot enter the tube. Observe the position of the white circle band of ammonium chloride formed by diffusion of two gases. Ammonia molecules diffuse at a faster rate than hydrochloric acid molecules so the two kinds of molecules do not meet at the middle of the glass tube.
10.1.3 Diffusion in liquids
See diagram 10.1.3: Diffusion in liquids
1. Place a crystal of potassium dichromate, potassium dichromate (VI), or ammonium dichromate at the bottom of a beaker of water. To do this, put a glass tube into the beaker of water so that it touches the bottom, then to drop the crystal down the tube. Close the top of the tube with your finger and remove the tube gently, leaving the crystal in the beaker. The colour of the dissolving crystal will spread throughout the water in quite a short time.
2. Fill a very small open bottle with a strong solution of potassium permanganate, potassium manganate (VII). Place this in a larger jar. Fill the larger jar very carefully by pouring water down the side until the water level is above the top of the small bottle. Leave this for a few days. The potassium permanganate solution diffuses evenly through the water.
3. Use two beakers containing water at 90oC and at room temperature. Put the same quantity of potassium dichromate crystals into each beaker. Note in which beaker the rate of diffusion of potassium dichromate in water is faster. Use a glass tube opened at both ends. Close one end with a stopper. Then place the tube in water, the open end up. Put potassium dichromate crystals in the glass tube then gently tap it so that all the potassium dichromate falls to the bottom. Hold the glass tube in a beaker of water with your left hand to keep it upright and not touch the bottom. With the right hand, hold a glass rod, longer than the glass tube, in a vertical position so you can push down on the cork and remove it from the glass tube. Note how the colour produced by the dissolving crystals of potassium dichromate diffuse through the water. Fill a small bottle with potassium permanganate solution then put it in an empty beaker. Fill the container with water so that the water surface just reaches the top of the bottle. After some days the potassium permanganate solution diffuses completely through the water.
4. Use forceps to place a crystal of lead nitrate and a crystal of potassium iodide in a Petri dish of deionized water. Observe the crystals dissolving then forming a yellow lead iodide solid between them. The solid forms  closer to the lead nitrate crystal because the iodide ion moves the fastest.
PbNO3 + KI --> KNO3 + PbI
10.3.1 Shrinking solution
See diagram 10.3.1: Shrinking solution
1. Put about 30 mL sodium chloride crystals into a measuring cylinder. Add water until the measuring cylinder is exactly full. After a few minutes, note that as the sodium chloride dissolves the liquid level drops.
2. Repeat the experiment using sucrose crystals instead of sodium chloride crystals. The liquid level does not drop. The sodium chloride crystals dissolve to form ions that can fit between the water molecules. The sucrose crystals dissolve to form sucrose molecules that are much bigger than the sodium ions or chloride ions.
10.3.2 Shrinking mixture of liquids
1. Half fill a test-tube with water. While holding this test-tube at an angle, pour ethanol slowly from a beaker until the test-tube is full. Hold the test-tube by placing your thumb on the mouth of the tube so that no air bubble is trapped. The test-tube appears to be full. Invert the test-tube several times while keeping thumb on the opening. Do not release the pressure. The liquid level becomes lower. The alcohol or water did not evaporate and no liquid spilt because of inverting the test-tube. By inverting the test-tube, mixing of water and alcohol occurs and the alcohol molecules slip in between the water molecules in the spaces between the molecules, thus making the total volume of the mixture become less. The spaces between the molecules cannot be seen by the naked eye.
2. Repeat the experiment with methanol or propan-2-ol (rubbing alcohol, isopropyl alcohol).
10.3.3 Container not leaking
Fill a measuring cylinder by 1 / 3 with water softener pellets (Calgon, sodium hexametaphosphate) or with table salt (sodium chloride). Add water until full and mark the liquid level with a grease pencil or rubber band. Leave it to stand for 5 minutes and note the liquid level. The drop in water level is not because water is absorbed by the salt. Pour out some water before all the salt dissolves and refill it with fresh water. The water level drops again because the crystals of the dissolving salt breaks down into ions that can slip in between the water molecules and make the total volume decrease. Repeat the experiment with other salts that dissolve in water and sucrose. The sugar molecule is large and does not ionize when dissolved in water, so that the water level will not drop.
10.3.4 Container holds more
Fill a large beaker with marbles and note the top level of the marbles. Slowly add sand to the beaker while tapping to make the sand settle between the marbles. Tip out the sand water and measure its volume. Replace the sand in the beaker of marbles. Add water to the marbles and sand. Tip out the water and measure its volume.
10.3.5 Shrinking balloons
Inflate 3 balloons fully. Let about half the air out of one balloon. Let out about one quarter of the air out from another balloon. Measure the diameter of the three balloons. Leave the balloons and the next day measure the diameters again. Compare the diameters of the three balloons. The full balloon shrinks faster than the half-filled balloon than the quarter filled balloon. The rate of shrinkage depends on the pressure in the balloon.
12.4.0 Siphons
See diagram 12.4.1.0: Difference in heights of water columns | See diagram 12.4.05: Two siphon systems and as syringe
Siphons are tubes used to move liquids from a higher container to a lower container, e.g. you can siphon gasoline out of a gasoline tank in a motor car. Pumps are machines used to move liquids or gases.
1. You can define the siphon, as a pipe system consisting of two legs as an inverted J, used to carry a liquid from one vessel to another vessel at a lower level, over an intermediate level higher than both levels. When both legs of the siphon are full, the hydrostatic force due to gravity is larger on the longer leg, thus causing the liquid to move up the shorter leg, over the bend, and down the longer leg. Start the siphoning process by 1. filling a siphon with liquid before placing it into its operating location or 2. by applying suction at the lower end after the tube is in position. Once started, the flow will continue until the liquid level in both vessels is equal, or until the level in the higher vessel falls below the inlet of the tube when air is sucked in and the siphoning action stops."
2. There is more than one explanation of how the siphon works. Explanation 1: The forces of cohesion, surface tension between water molecules, allows the water in the short side of the siphon to be pulled up the tube by the greater weight of the water in the long side. The siphon acts in much the same way as the longer end of a chain hanging out of a bucket can pull the rest of the chain from the bucket. Explanation 2: Both surface tension and atmospheric pressure are required to make the siphon work. Gravity pulls down on the water in both sides of the siphon. However because there is more water in the long side, the weight is greater in the long side than in the short side. If the columns of water are not allowed to separate, the water in the long side will pull the liquid up the short side. Atmospheric pressure keeps the water from separating. The water flowing down the long side reduces the pressure in the siphon and thus atmospheric pressure pushes water up into the short side of the siphon if it is greater than the pressure of the column of water in the short side of the siphon. If atmospheric pressure did not exist then there would be no reason for water to enter the short side of the tube. The following experiment, the siphon fountain, shows that the principle of operation for a siphon does not rely on surface tension alone.
3. After "Understanding the siphon" Kevin C. de Berg and Cedric E. Grieve, Australian Science Teachers Journal, 45 (4)
The explanatory principle based on comparing the weights of fluid in both legs of a siphon occurs in most textbooks analysis and in the work of the French scientist Pascal, 1663. The advantage of this principle is that it is relatively simple and applies to siphons as commonly employed. Its disadvantage is are that it applies only to siphons and not other fluid flow devices, e.g. syringes, and applies only where the siphon fluid is more dense than the external fluid. The following explanatory principle overcomes this disadvantages: The siphon in diagram 12.4.05 1. has a fluid disc of negligible thickness at X. You can consider the contending pressures either side, S and S1 of this disc at the moment the stopper is removed from X. The pressure on side S1 is atmospheric pressure, AP. To locate the pressure on side S of the disc, follow the pressure changes on the other side of the siphon starting at the surface of the fluid in the container where the pressure is atmospheric pressure, AP. At A the pressure is (Ap + 2). At B the pressure is (AP + 2 - 18). At X the pressure is (AP + 2- 18 + 6), i.e. (AP - 10). The pressure on side S1 exceeds the pressure on side S so the liquid will not siphon from the tube but move back into the longer leg and into the container.
In diagram 12.4.05 2. for this siphon, at X, the pressure on side S1 is again AP. On side S, the pressure (AP + 2 - 12 + 14), i.e. (AP + 4). The pressure on side S exceeds the pressure on side S1 and so fluid siphons from the beaker out of the longer leg of the siphon. In the syringe in diagram 12.4.05. The pressures on either side of a fluid disc of negligible thickness at the opening of the needle after the plunger has been moved back but before fluid has begun to flow. On side S the pressure is less than atmospheric pressure because a small sample of air at atmospheric pressure was expanded into larger volume. On side S1 the pressure is (AP + h). So the pressure on side S1 exceeds the pressure on side S and fluid moves into the syringe from high pressure to low pressure.
12.4.1.0 Simple siphon
See diagram 12.4.0: Siphons
Use two tall glass bottles and fill each about half full of water. Connect two 30 cm lengths of glass tube with a 30 cm rubber or plastic tubing. Fill the tube with coloured water and pinch it. Put a glass tube in each bottle of water. Siphon the water back and forth by varying the height of the bottles.
12.4.1.1 Siphon fountain
See diagram 12.4.1: Siphon fountain
1. Fit a flask with a two-holes stopper. Through one hole put a jet tube that extends to about half way to the top of the flask and about 2 cm outside the stopper. Through the other hole push a short length of a glass tube so that it is just flush with the bottom of the stopper. Connect a 20 cm length of a rubber tube to the jet tube. Connect a 1 m length to the other glass tube. Put some water in the flask and insert the stopper. Put the short rubber tube in a container of coloured water on a table, let the longer rubber tube go into a pail on the floor and then invert the siphon. Coloured water spurts from the jet. You can make a double siphon fountain by making another flask unit similar to the first one and connecting them. While the water is maintained in the container, the fountain cannot be stopped.
2. Insert 2 pieces of glass tubing through a two-holes stopper such that one extends farther than the other. Connect 30 cm of rubber tubing to each glass tube. Fill a large bottle with cold water. Pour 100 mL of water in a large jar and insert the two-holes stopper with attached tubing. Invert the jar. The ends of the tubing in the bottle should be under water at all times. Observe what happened after inverting the jar. Observe the volume of the air pocket above the water in jar as the water poured in the empty bottle. Note that water was drawn up into the jar. The bottle filled with water had to stand higher than the other bottle. See how to make the fountain flow more or less. The water in the jar was needed to "prime" the siphoning action. After inverting the jar, the water ran down and into the empty bottle causing an increase in volume of the air pocket above the water in the jar and decreasing the pressure. The lower pressure caused the sucking up of the water from the bottle filled with water. The atmospheric pressure pushes water up into the larger jar. The greater the difference in height of the water levels in the two jars, the greater the flow of water. As soon as the water levels are the same height in both bottles the water flow stops as in a siphon.
3. Fit a glass container, or a flask made from a used electric bulb with a two-holes rubber stopper. Through one hole place a jet tube which will extend to about half way to the top of the flask and about 2 cm outside the stopper. Through the other hole push a short length of glass tube so that it is just flush with the bottom of the stopper. Connect a 20 cm length of rubber tube to the jet tube. Connect a 1 m length to the other glass tube. Place some water in the flask and insert the stopper. Put the short rubber tube in a container of water on a table, let the longer rubber tube go into a bucket on the floor and then invert the siphon. Add ink to the water to see the fountain better. Make a double siphon fountain by making another flask unit similar to the first one and connect them.
12.4.1.2 Siphon replaces water for fish tank
Use a one metre length of 10 millimetre diameter rubber hose. Fill the hose with water then block both ends with fingers. Put one end into water in a tank for goldfish then move away the finger the end under water. Put another end into a bucket or basin on the floor below the end of the tube in the tank, then move away the finger from the end. Water streams out of the tank towards the bucket or basin.
12.4.1.3 U-tube siphon
Use two beakers half full of water with a connecting U-tube full of water. Lift one beaker then the other.
12.4.1.4 Siphon in a bell jar
Transfer water through an U-tube from a sealed flask to an open beaker when the assembly is placed in a bell jar and evacuated.
12.4.1.5 Siphon mechanism apparatus
This apparatus that shows atmospheric pressure, not cohesion, to be the basis for the siphon action.
12.4.1.6 Pressure in a siphon
Hook a manometer to the upper portion of a siphon.
12.4.1.7 Mechanical siphon
A mechanical model of a siphon consists of chain hung over a pulley to a lower level.
12.4.1.8 Self-starting siphon
Seal an inverted U-tube in the side of a beaker to make a self-starting siphon.
12.4.1.9 Mariotte flask and siphon
Use a gas siphon to transfer carbon dioxide from one beaker to another, intermittent siphon, Tantalus cup
Use a Mariotte flask to make a siphon with a constant flow because the height of an open tube inserted through the stopper of a jug with an outlet at the bottom regulates flow.
12.4.1.10 Turnover siphon
See diagram 12.4.10: Turnover siphon
1. Insert two pieces of glass tubing through a two-holes stopper with a longer piece drawn out to form a jet. Connect the shorter glass tube with rubber tubing to a lower jar, A. Connect the longer glass tube with rubber tubing to a higher jar, B. Add water to an elevated jar, C. Insert the two-holes stopper in C so that the jet end of the longer glass tube is above the water level. Fill jar B with water.
2. Invert jar C while keeping the ends of the rubber tubes in the jars. Water in jar C. starts to run down the rubber tube into jar A, increasing the volume of air in jar C. and decreasing the air pressure in jar C. Water in jar B. is pushed up the rubber tube by (atmospheric pressure - air pressure in jar C). The water shoots out through the jet end of the longer glass tube until the levels of water in jar B. and jar C. are the same.