Water!

"Study the most common things,
for from such are seen the universal laws."
-- Cornelius Vermeulen (c 1961)

      The most plentiful substance on the face of this "Blue Planet," water is also one of the most unorthodox of chemical compounds. So let us make some studies of this most common stuff.

1. How do we know what water is made of? ELECTROLYSIS! We shall electrically breakdown water. Usually it is not thought smart to play with water and electricity at the same time. But what the heck, let's live dangerously today! Each group will be given a small homemade device that will be used to break asunder the water molecule into two gases. Happily, you are already somewhat skilled at collecting gases, and you will thus understand the apparatus. We will then set about determining what those gases are. I suspect you already know, but pretend that you aren't sure. Click to get a webpage dealing with this simple shocking device.

2. We now know which gases combine to make water - alas, it's only a qualitative finding. But how much of each? You've seen how chemists like to see constant compositions. (Don't try this in your home - or the lab, for that matter!) Some obviously crazy Italian guy by the name of Volta decided to mix our two gases in various amounts inside of several glass spheres and then, using a sparkplug to provide the necessary activation energy, he ignited the gases and looked to see how much water he obtained once the globes cooled down. (It can be supposed that he had a few rather traumatic experiences along the way!) Only in the globe that contained exactly two parts of one gas and one part of the other did water not only condense into a puddle, but there was almost a pure vacuum in the container - indicating that all the gases had been consumed. (This experiment is today done using a gentler way of doing it: no spark plug, put instead a few granules of platinum catalyst into the globe. The gases combine on the surface of the catalyst:

   2 H₂ + O₂ 2 H₂O

3. So now we know the molecular formula of water, H₂O. But what can we say about the shape of the water molecule? Is it H-O-H? Or is it some other shape? You shall inspect some models of water molecules. Pretend you looked at the jiggling molecule using a strobe light in a dark room. What would you see? Can you draw the statistically average shape?

   Click to obtain a one-page data sheet you can fill in for the following exercises.

4. What are the implications of this non-linear shape? Does the molecule have any portions that are more positively charged than others? Draw a diagram showing the somewhat positive and somewhat negative areas. Can you draw a line through the molecule in such a way that one side of the line is "+" and the other is "-"? If you can, the molecule is an electrical dipole (quite like magnets are magnetic dipoles). The further apart the center of "+" and the center of the "-", the greater the dipole moment. (At this point you have become a direct academic descendant of the Dutchman Prof. Debye, the discoverer of dipole moments (see Dr. V's academic tree).

         (It might be useful hereafter to consider a bucket of water more like a collection of magnets all tossed together, rather than a bunch of non-magnetic pieces of metal. This is called having a "mental model," which is a tool that you can do mind experiments on.)

         Suppose you were to put a zillion of these warped molecular magnets into a beaker and started heating it. They'd start jiggling around. Heat it hotter and they'd be bouncing around. But would it be easy to get any to bounce out of the beaker and escape into the air? No, their intermolecular attractions would tend to keep them bunched together. Only with extreme heat, would some bounce away into oblivion. (Hopefully, you see the analogy to boiling here!)

5. What if we did something to weaken the dipole moment? Would it take more or less heat to get them to bounce away?

         Let's try this with water. Let's lay our hands on some ANALOGs of water. Chemists might write water as R-O-H, where R is H. But they can also pretend that R is something else - such as a methyl group, or an ethyl group. So let's compare the boiling points of H-O-H, Me-O-H (methanol; C-O-H), Et-O-H (ethanol; C-C-O-H), and Pr-O-H (propanol; C-C-C-O-H). For safety's sake, we'll let the H-O-H people use fire, and the other three groups use electrical hotplates - we don't want too much excitement (read "explosion") - oh, and do it under the hoods so the boiling fumes won't wander over to the water group's station and go BOOM!

6. Another implication we can see might exist is that more strongly dipolar compounds might be better solvents of ionic things. Let's get each of our groups measuring how much of each of four solids they can dissolve in 25 ml of their liquid at room temperature. The solids are NaCl (table salt sucrose (table sugar), and napthalene (moth balls). Click for a refractometric method. Or click for an even faster method.

   naphthalene:

   =

   =

   

7. You've seen the floating the needle on water trick. Let's do this a bit more quantitatively - using capillary tubes. The higher the liquid rises in the tube, the greater the liquid's surface tension. Why? Anyway, each group should test the liquids. You will use a little trick that Galileo thought of - doing things on a tilt - to study quantitatively the law of gravity. But you will use the constant tilt of the wooden block to make quantitative measurements of capillaries. So-, be sure that when youu tilt over your capillary tube its base is submerged in the liquid you are testing, AND the tube is lying flush with the 30-degree slope of the block.