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strikes a balance in the system, so tubes of water don't spontaneously overflow, but the result is quite observable. In very narrow tubes (capillary tubes) the effect is so great that water can climb inches within the tube before it is stopped by gravity. In a larger tube, like a glass of water, the effect is much smaller, though still visible if you look carefully.

Glass and water is only one of many possible combinations of tubes and liquids. Not all solids are more attractive to water than water itself is. Consider a tube made of wax. Waxes are less polar than water, and so water is not very attracted to it. Water would prefer to stick to itself. As a result, water bunches together, and pulls away from the wax. If you have waxed your car lately, you have probably seen the same result. The water forms a dome - it "beads up" as it tries to maximize contact with other water molecules. Again, gravity puts a limit on this, pulling the drop flat against the car (or down into the tube). If, however, you were out in zero-gravity space, a drop of water would be able to pull itself completely into a sphere, maximizing its contact with itself, and minimizing contact with less polar surfaces.

The liquid mercury is also a popular illustration of the "upside-down" meniscus. Mercury is non-polar, and is more attracted to other non-polar chemicals, like itself, than it is to polar substances, like glass. As a result, when mercury is placed in a glass tube, it bunches up to form a dome.

All in all, the meniscus is the result of intermolecular forces. (There's a Helium article on intermolecular forces just waiting for you to read more.) In general, if the liquid and solid have a stronger interaction than the liquid molecules do with themselves, then the meniscus will be the low point of the dip formed in the surface of the liquid. If the liquid molecules are more strongly attracted to one another than to the solid of the container, the meniscus will be the high point of the dome formed by the liquid's surface. In either case, it should be easily recognizable, and found towards the center of the tube.

Learn more about this author, Ernest Capraro.
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