You have seen that when molecules behave like hard spheres, they do not change states when the temperature changes… they always behave like they are in a gas state. There is clearly another factor involved when molecules transition from a gas state to a liquid or a solid state. It is time to learn what that second factor is.
How many drops of water do you think you can fit on top of a penny? To find out, you will need a penny, an eyedropper, a glass of water, and a sheet of wax paper. Place the penny in the center of the sheet of wax paper on a flat surface. Fill the eyedropper with water from the glass, and slowly add drops of water, one-by-one, to the penny.
After seeing how much water is in a single drop, most students predict that they will be able to fit around ten drops of water on top of a penny. In this video, a student was able to fit 36 drops of water on top of her penny (40 or 50 drops is not uncommon), and after around 25 drops, the bubble of water on the penny actually seems to defy the laws of physics by bulging over the edges of the penny but still staying on.
If water molecules behaved like hard spheres, the bubble of water on the penny would have spilled over the edges of the penny long before the student reached 36 drops. The reason that the bubble was able to grow so large and hang over the edges of the penny is because water molecules actually behave like small magnets that are attracted to each other. One end of a water molecule has a slight positive (+) charge and the other end has a slight negative (−) charge. The positive end of one water molecule will be attracted to the negative end of another water molecule, and this magnetic attraction will pull them together.
There is another, more direct, way to see that water molecules behave like small magnets. Run a nylon comb through your hair ten times (make sure your hair is dry and clean). This will generate static electricity in your hair and on the comb. Static electricity is created when two objects are rubbed together, and electrons jump from one object to the other. In this case, electrons jump from your hair to the comb, giving the hairs on your head a slightly positive charge (you may see your hairs repelling or pushing away from each other) and the comb a slightly negative charge.
At a sink, turn the faucet until you have a very thin but smooth stream of water. Move the comb close to the stream of water, but do not get the comb wet. The negatively charged comb should attract the positive ends of the water molecules in the stream, and the stream will bend towards the comb. This works best when the air is dry and the humidity is low. If you go to YouTube and search under “bend water with static electricity,” you will find a number of videos that demonstrate this magnetic behavior. You can also use a magnet instead of a comb, but the magnet would have to be extremely powerful.
Because water molecules are attracted to each other, they tend to bead up or form droplets when in a liquid state. This attraction creates a property in liquids known as surface tension. The molecules inside of a liquid are pulled equally in all directions by their attraction to the other molecules in the liquid. However, the molecules along the surface of a liquid are pulled inward by their attraction to the other molecules in the liquid.
Water molecules are not only attracted to other water molecules; they are attracted to other molecules as well. The reason why we use a sheet of wax paper in this demonstration is because the attraction between water molecules is so much stronger than the attraction between water molecules and the molecules in the wax paper. This is why water beads up so well on wax paper. If we were to do the same demonstration on a surface made up of a material that was more attractive than wax paper, then water would not bead up as well and we would not have been able to fit nearly as many water drops on the penny.
So what makes molecules attractive to each other? You will learn more about that when we study atoms, molecules, and chemical bonding later in this unit.