Last semester, when talking about the ideal gas law and the kinetic molecular theory of gases, you got an overview of the behavior of gases. This overview was particularly comprehensive because gases, by virtue of the extreme distance between their component molecules, don't tend to have properties that depend very much on the nature of the molecules themselves. Liquid's, however, have their molecules constantly in contact with one another. And so we would expect the properties of a liquid to depend in a much greater way on the nature of the molecules involved. The most basic property of a liquid that distinguishes it from a gas is that it has a defined volume. And the fact that this volume is defined by the molecules being in close contact with each other makes liquids relatively incompressible. This behavior is behind hydraulics, a technology that controls and transmits power through pressurized liquids. An important property of liquids is that they flow. Viscosity is the property that determines how much a particular liquid resists that flow. Substances like water and gasoline flow very easily, and so have a low viscosity. Honey, syrup, and motor oil flow much less easily, and so have high viscosities. The two factors that determine viscosity are the intermolecular forces, stronger forces tend to resist motion, and molecular complexity. Long molecules can entangle with each other, impeding their motion. The forces holding identical molecules together within a liquid, or solid, are known as cohesive forces. At the surface of a liquid, there are fewer cohesive forces. This means if you change the shape of a droplet to increase its surface area, you are reducing the attractive forces effectively breaking them. This takes energy to do, so the lowest energy state is one where the surface area is minimized, and there is a tension at the surface, drawing the droplet into the shape that minimizes its surface area. This is known as surface tension, and is the reason that dense objects can sometimes float on the surface of water. The intermolecular forces between two different molecules are called adhesive forces. If you have ever compared a water droplet on a clean glass to a droplet on a freshly waxed car, you've seen differences in adhesion forces. On the left, the adhesion between the water and the hydrophilic glass is strong, so the droplet spreads out. On the right, the adhesion between the water and the wax is weak, so the surface tension pulls the droplet together. There are even ways to make super hydrophobic surfaces by structuring the surfaces in novel ways. Adhesive forces between liquid and its container are particularly evident at the surface inside a small tube. On the left, we see water which has strong adhesive forces with glass. This causes the water to climb the sides of the burette. The reverse is clear on the right, where mercury has poor adhesive forces with glass, causing the interface to bulge in the other direction. If the tube is extremely small, known as a capillary, then strong adhesive forces can cause a liquid derives within the tube quite dramatically. This is the mechanism that causes liquids to climb up paper. In this case, carrying with it dies in the simplest example of chromatography, a powerful analytical tool for investigating solutions.