Molecular Attraction

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.

molecular attraction
VBending Water

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.

surface tension

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.