School Science Lessons
Physics - Molecular motion, characteristics of matter, molecular nature
Updated: 2008-02-15 L
Please send comments to: J.Elfick@uq.edu.au
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Table of contents
10.0 Molecular motion
10.1.0 Diffusion, particles of matter, Brownian movement
10.2.0 Osmosis
10.3.0 Molecule spacing
10.1.0 Diffusion, particles of matter, Brownian movement
3.55 Brownian movement
3.55.1 Diffusion of heavier than air gas, carbon dioxide
3.55.2 Diffusion of ammonia and hydrogen chloride gases
3.55.3 Diffusion of liquids
3.56 Particles of matter and dilution
3.57 Size of a molecule
3.58 Clay soil suspension
10.0.1 Diffusion
10.1.1 Diffusion of carbon dioxide
10.1.2 Diffusion of ammonia and hydrogen chloride gases
10.1.3 Diffusion in liquids, potassium manganate (VII) or potassium dichromate
10.2.0 Osmosis
10.2.0 Osmosis, osmotic pressure
10.2.1 Osmometer using carrot or potato
10.3.0 Molecule spacing
10.3.1 Shrinking solution
10.3.2 Shrinking mixture of liquids
10.3.3 Container not leaking
10.3.4 Container keeps holding  more
10.3.5 Shrinking balloons
3.4.1.1 Stretched rubber band
24.0.0 Change of state
3.55.0 Matter as particles
3.57 Size of a molecule
12.3.5.2  Inverted drinking glass
10.0.1 Diffusion
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 inhomogeneous 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. You can easily observe diffusion in liquids also, although it occurs at a much slower rate than in gases. Diffusion in solids occurs at a still slower rate than in liquids.
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 3.55.2
Be careful! Only teachers should do this experiment because hydrochloric acid solution and ammonia water are strongly corrosive solutions.
1. 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.
2. 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
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.
10.2.0 Osmosis, osmotic pressure
Osmosis is a modification of diffusion, namely the penetration of liquids and solutions through a porous membrane. Some membranes, which are permeable to one type of liquids and solutions, are partially or completely impermeable to other liquids and solutions. Such membranes are called semipermeable membranes.
10.2.1 Osmometer, carrot or potato osmometer
See diagram 9.170
Prepare a 1-hole stopper with a long glass tube inserted through the hole, 2 cm below the stopper and 20 cm or more above the stopper. Be careful when inserting the stopper! Cut a hole in the side of a large carrot, or potato, the same diameter as the middle of the 1-hole stopper. Insert a spoon or knife into the hole to make the carrot hollow. Fill the carrot with concentrated sucrose solution, containing drops of ink, by pouring through the hole. Put some petroleum jelly on the rim of the hole. Insert the stopper with the glass tube into the hole of the carrot. It must fit tightly. By moving the glass tube through the stopper you can adjust the height of the coloured sugar solution in the tube. Fill any gap between the carrot and the stopper with hot candle wax. Hold the carrot in a tall beaker. Pour water into the beaker to submerge the carrot and a short length of the glass tube. Clamp the carrot upright without squeezing it. Record the height of the coloured sugar solution in the glass tube and note the depth of the ink colour. Also, record the height of the water in the beaker. Later, record the heights again and note depth of colour in the glass tube. Water penetrates the wall of the carrot to dilute the colour of the sugar solution that rises in the tube. The level of water in the beaker falls.
10.3.1 Shrinking solution
See diagram 10.3.1
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 keeps holding  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.