The Scientist’s Essay for 3. Liquid Materials

What's important about liquid materials?

The solid and liquid states are far more like one another than either is like a gas. Physicists often lump the two together as "condensed matter".

Arguably the most important material to us humans is water. It is also just about the only common material that exists in all three phases – solid, liquid and gas — within the range of temperatures and pressures that we commonly experience. Most of the time, though, it's liquid. Of course it's not the only liquid. A lot of the liquids around us that we give names — milk, beer, soda, juice, detergent, even blood — are really just water with some stuff added. But some common liquids are really entirely different substances: gasoline, rubbing alcohol, molten wax. Moreover, under extreme conditions, most materials can become liquid — metals, rocks, natural gas, air. So there's more to liquids than just water and its relatives.

In many ways the liquid form of a material is a lot like its solid state. Its density is usually not very different (for most materials — water being the important exception — the liquid is slightly less dense than the solid, but the differences are generally small) and it often looks much the same. Molten metal looks shiny, ice (if you make it without bubbles) is clear and colorless like water. On a microscopic level the molecules in the liquid state are bound together and strongly coupled, as in the solid. As a result liquids, like solids, are nearly incompressible — if you push on them they may deform or move around, but the volume doesn't change. The solid and liquid states are far more like one another than either is like a gas. Physicists often lump the two together as “condensed matter”.

But of course solids and liquids differ in one crucial respect: liquids flow, and fill a container, while solids are rigid and retain their shape. On a microscopic level, the molecules in a liquid have enough heat energy to be able to move around and rearrange themselves, but still not enough to actually pull apart from one another.

A very useful practical consequence is that it's much easier to measure the volume of a quantity of liquid than that of a solid object — you just pour the liquid into a measuring cup or graduated cylinder and read off the volume. (Of course it's a little harder to measure the weight of the liquid, since you have to worry about the weight of the container, but that's a small price to pay.) We will exploit that property both to investigate density in some detail, and also to provide an indirect means of measuring the volumes of solids.

—Roger Tobin