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Phases of Matter IV - Liquids

Written by Translator on June 24, 2008 – 6:33 pm -

Crossposted at Dailykos.com

Liquids share with the solids the term “condensed phase” matter, meaning that the density of solids and liquids is greater than gases and plasmas. The reason is that the molecules of matter in the condensed phase are much closer together than in noncondensed phases, due to energetics specific to the particular substance. Condensed phase materials are essentially noncompressible, because their molecules are already near as close together as they can be before repulsive forces dominate.

Liquids share with gases and plasmas the property of fluidity. Fluid materials tend to take the shape of the container in which they are confined, if any, and flow due to forces acting on them, where solids do not flow, just either stay still or fall, keeping their shape unless they are broken. Most chemical elements are solids at room temperature, some are gases, and only two, mercury and bromine, are liquid. (Gallium is close, but 20 degrees C is defined as room temperature. Unlike the well known candy, gallium will melt in your hand).

Fluids also have the property of “diffusion”, meaning that materials dissolved or suspended in them will move rapidly from one place to another. Solids show diffusion only extremely slowly, and for practical purposes the rate is zero. The more viscous the liquid, the slower diffusion occurs, and that makes sense because it takes more energy to push a particle through honey than it does through water.

Diffusion is driven by entropy, and as we remember from the Gibbs Free Energy equation, Delta E = Delta H - T Delta S, where E is the energy of the system, H is the enthalpy (heat) of the system, T is the absolute temperature, and S is the entropy. For example, if you take tin and lead powders and mix them, you can separate them out again. While a tin/lead alloy is more stable than either metal alone, the rate of reaction is so slow in the solid phase that nothing happens. Melt the mixture, and the metals instantly bond. The resulting alloy is solder, used in electronics and formerly in residential plumbing.

Liquids, like solids, have varying degrees of association. In loosely associated liquids, such as saturated hydrocarbons, the predominate force is induced dipole-induced dipole (van der Waals) attraction. This is why they are volatile. Not a whole lot of force attracts their molecules to each other, so evaporation is easy, as long as we are talking about small molecules. Within a family of compounds, volatility and viscosity are strongly related to molecular weight.

Polar liquids, like, for instance, acetone, are less volatile than the corresponding hydrocarbons because of permanent electrostatic forces. Acetone is a three carbon material, with the addition of an oxygen. This causes the whole molecule to have a partially positive charge on the two hydrocarbon ends, and a partially negative one at the oxygen. The closest molecular weight hydrocarbon analogue is butane, a gas at ordinary temperatures and pressures.

The mystery force that I keep alluding to explains the extremely anomalous behavior of water, and will be considered after we finish up gases and plasmas. It is much more than polarity, although water is highly polar, and that accounts for some of its properties.

Liquids have the unique property of boiling. Let’s look at our phase diagram again:

At boiling, a liquid is to the right of the “tee” of the graph, and energy is being input such that liquid molecules are given enough kinetic energy to break away from the bulk liquid. At a given pressure, the boiling point is a specific temperature. Water, for example, boils at 100 degrees C at one atmosphere (760 torr) of pressure. Increase the pressure, and following the graph, the temperature of boiling increases. That is why it takes longer to cook at high altitudes (lower pressure, lower boiling point) and why pressure cookers cook food fast (higher pressure, higher boiling point).

One application of boiling is distillation of liquids. This is a critically important method of purification of all kinds of things, including everything from one’s favorite Scotch whisky to making potable water from seawater to making refined petroleum products from crude oil. Distillation depends on differences in boiling points of various components of a mixture, with the ones with the lowest boiling points being driven out first. The vapors are passed into a cooled tube, usually a tube, and condense into a liquid once again, leaving the higher boilers behinds. The Arabs discovered this technique back in the day when their society was innovative, and the very term “alcohol” is derived from an Arabic word, “al-kohl”, which bizarrely refers to a compound of antimony, with nothing to do with the modern term. Such are the vagaries of language.

In later installments we will discuss things like liquid crystals, used in many computer displays (LCD displays), televisions, and hand held calculators. Coming soon, after gases and plasmas, is our discussion on water, the wonder substance, and the mysterious force that makes it what it is.

As always, comments, criticisms, and questions are always welcome. I learn much more from input from readers than I could possible hope to teach. Warmest regards, Doc.

Tags: Boiling, Distillation, Liquid, Phases of matter, Teaching
Posted in Diaries |