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Background: What is a meniscus?

The meniscus is most easily seen in a clear narrow tube, partially filled with liquid. If you look through the side of the tube, you notice that the surface of the liquid is curved (either upwards or downwards, depending on what materials the liquid and the tube happen to be). The meniscus is the point at which that curvature is horizontal, found at the center of the surface. To make it more visible, people will sometimes place a sheet of white paper behind the tube, or even a fancy "meniscus reader" that has black and white sections, so that the contrast makes the surface easier to view. For chemists, the meniscus is very important, as this is the point at which volumetric measurements are made.

Explanation: Why does a meniscus form?

All the matter that we see and interact with is made up of molecules - individual chemical structures that add up to make everything in the world around us. A glass of water, for instance, is filled with water molecules. We do not see the individual molecules, for they are far too small, but the properties of water result from the interactions of those molecules with one another, and everything around them.

The meniscus, too, is the result of interactions between molecules. To begin, let us stick with the most familiar case of water in a glass tube, such as a test tube or burette (buret). Water (H-O-H) molecules have polar chemical bonds, and are bent, so that they have a permanent dipole. In simpler language, they have positive and negative ends. The positive ends are attracted to the negative ends of other molecules, and vice-versa. This includes other water molecules, so one property of water is that it tends to "stick" to itself. Glass, while not a specific molecule, is full of silicates - a group of chemicals which all have a common component - silicon bonded to oxygen (-Si-O). When placed in water, the silicate will pick up a hydrogen atom from the water, becoming a "silanol" (-Si-O-H) which you might notice looks similar to water on one end. The silanol group is fixed in place, as part of the glass, but it is even more polar than the water molecules, so as a result, the water molecules are attracted even more strongly to the glass than they are to one another. The water molecules move towards the glass to increase their contact with it, so much so that the level of water in the center of the tube dips noticeably. The water is, quite literally, climbing the walls of the tube. At some point, gravity

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