MadSci Network: Chemistry
Query:

Re: What is a simple exp. for measuring the diffusion of molecules in water?

Date: Wed Nov 17 10:46:47 1999
Posted By: Chris Neale, Undergraduate, Biology, University of Waterloo
Area of science: Chemistry
ID: 940382283.Ch
Message:

Hi Caitlin. I hope this doesn't come too late for your Science fair.

Your question involves a few important terms which I think it will be valuable to define. You may already know them, but other people who ask the same question might not have done any research yet. The words that I would like to define are molecule, water, food colouring, diffusion, and osmosis.

Molecule. When you look at an object you see many things including shape, colour, and brightness; and you can touch the object to find the temperature, and how hard it is. These properties of the material come from the properties of the molecules that make up the material. Mater is a collection of molecules that are generally packed together very closely (especially in solids and liquids.) Molecules move about differently depending on the state of the mater (solid, liquid, or gas.) The molecules in a gas are very spread out and they can move in any direction, so it is easy to pass your hand right through a gas. The molecules in a liquid are much closer together , but they can still move about fairly easily, so you can also pass your hand through a liquid. The molecules in a solid are about the same distance apart as the molecules in a liquid. However, the molecules in a solid cannot move about freely and this is why solids are so hard.

Water. Water is a collection of molecules. I assume that when you say water, you are refering to liquid form. Remember that water can also be a solid (ice) or a gas (steam.) The molecules in liquid water are able to move about, and if we mix other molecules into the water, they can also move about in the water.

Food colouring. Coloured molecules. Red food colouring is made up of molecules that look red. It is important to remember that these are individual molecules and not just tiny particles of a red solid.

Diffusion. If we place a red molecule (just one) into the very middle of a glass of water, what do you think will happen? Right, it will move somewhere else. The red molecule is being bumped about on either side, just like I am bumped around in a shopping mall at Christmas time. Try to stand in one place in the middle of a crowded store. It won't be long before somebody says "excuse me, could you please move out of my way." The red molecule is constantly being pushed from all sides by other water molecules that are impatient to get somewhere. This pushing is random, and it is uneven. If the pushing were even then the molecule would not move, it would just get squished. If the pushing was not random, then we could predict where the red molecule would end up. As it is, the red molecule does move, and we can not predict where it will go. Now imagine one-thousand red molecules all placed together in the center of the water. These molecules will all be pushed in different directions and so they will move. It just happens that we placed them very close together so if they move in different directions, they are bound to spread out. This spreading movement of the red molecules through the water is called diffusion. It is a very hard thing to understand. Individually, the red molecules just move about, but collectively we can see them spread apart. There is no force pulling them apart, it is just more likely that they will spread out. There are so many ways that the molecules can spread because we don't mark individual molecules. Try it yourself with a bunch of paper scraps. Rip one piece of paper into about up about thirty pieces. Now throw them up into the air and watch them hit the floor. If you do this again, you will see that the pattern is about the same. However, if you had put numbers on them and recorded the position of each individual piece of paper then you will see very different results every time. We have no way to mark the red molecules, so what we see is only the general pattern. Also, the number of red molecules in one drop of foor colouring is much more than a billion! Imagine how random your room would look if you had used more than a billion scraps of paper instead of thirty. I hope you have understood diffusion. I included so much information because it is very central to your question, and I can't find a good basic description to refer you to.

Osmosis. Osmosis is the process of water molecules passing through a very thin solid membrane. Osmosis is not involved in your experiment. You can try searching MadSci with Osmosis but most of the questions are very advanced. I outlined the fundamental idea, and that should be sufficient.

In my opinion, the project itself is up to you. That is the best way to learn. Remember, you are doing the project in order to learn about science, marks and awards are just ways to measure how much you have learned. In science, the best way to understand something is to experiment. Experiment! Find all the different shaped containers and tubes in your house and try putting water and food colouring in them in different amounts and arrangements. Try different temperature water. Try saltwater. Try water with oil in it. Try carbonated water. Try the water from all the different taps in your house and then try the water from your best friends house. Try different colours of food colouring or try two at once. Exploring is the really fun part of science. It is here that you will learn the most. After you have explored for a while, take your knowledge into the bedroom and think for a while. A good type of experiment is one where you try to assign numbers to things you already know. How long does it take .... How much do I need to add .... What size container is best to .....

Be creative! I hope this has helped. Good luck on your adventure through science.

Chris.

neale@innocent.com


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