distillation

distillation, process used to separate the substances composing a mixture. It involves a change of state, as of liquid to gas, and subsequent condensation. The process was probably first used in the production of intoxicating beverages. Today, refined methods of distillation are used in many industries, including the alcohol and petroleum industries.

The Basic Distillation Process

A simple distillation apparatus consists essentially of three parts: a flask equipped with a thermometer and with an outlet tube from which the vapor is emitted; a condenser that consists of two tubes of different diameters placed one within the other and so arranged that the smaller (in which the vapor is condensed) is held in a stream of coolant in the larger; and a vessel in which the condensed vapor is collected. The mixture of substances is placed in the flask and heated. Ideally, the substance with the lowest boiling point vaporizes first (see vaporization), the temperature remaining constant until that substance has completely distilled. The vapor is led into the condenser where, on being cooled, it reverts to the liquid (condenses) and runs off into a receiving vessel. The product so obtained is known as the distillate. Those substances having a higher boiling point remain in the flask and constitute the residue.

Since a perfect separation is never effected, the distillate is often redistilled to increase its purity (hence the expression “double distilled” or “triple distilled”). Many alcoholic beverages are distilled, e.g., brandy, gin, whiskey, and various liqueurs. The apparatus used, called the still, is the same in principle as other distillation apparatus.

The Fractional Distillation Process

When the substance with the lowest boiling point has been removed, the temperature can be raised and the distillation process repeated with the substance having the next lowest boiling point. The process of obtaining portions (or fractions) in this way is one type of fractional distillation. A more efficient method of fractional distillation involves placing a vertical tube called a fractionating column between the flask and the condenser. The column is filled with many objects on which the vapor can repeatedly condense and reevaporate as it moves toward the top, effectively distilling the vapor many times. The less volatile substances in the vapor tend to run back down the column after they condense, concentrating themselves near the bottom. The more volatile ones tend to reevaporate and keep moving upward, concentrating themselves near the top. Because of this the column can be tapped at various levels to draw off different fractions. Fractional distillation is commonly used in refining petroleum, some of the fractions thus obtained being gasoline, benzene, kerosene, fuel oils, lubricating oils, and paraffin.

The Destructive Distillation Process

Another form of distillation involves heating out of free contact with air such substances as wood, coal, and oil shale and collecting separately the portions driven off; this is known as destructive distillation. Wood, for example, when treated in this way yields acetic acid, methyl or wood alcohol, charcoal, and a number of hydrocarbons. Coal yields coal gas, coal tar, ammonia, and coke. Ammonia is also obtained by the destructive distillation of oil shale.

condensation

condensation, in physics, change of a substance from the gaseous (vapor) to the liquid state (see states of matter). Condensation is the reverse of vaporization, or change from liquid to gas. It can be brought about by cooling, as in distillation, or by an increase in pressure resulting in a decrease in volume. Certain natural phenomena, such as dew, fog, mist, and clouds, are the result of the condensation of water vapor in the atmosphere; the formation of dew illustrates well the fundamental principles involved in such phenomena. The explanation of condensation can be found in the kinetic-molecular theory of gases. As heat is removed from a gas, the molecules of the gas move more slowly, and as a result, the intermolecular forces are strong enough to pull the molecules together to form droplets of liquid. Similarly, reducing the volume of the gas reduces the average distance between molecules and thus favors the intermolecular forces tending to pull them together.

states of matter

states of matter, forms of matter differing in several properties because of differences in the motions and forces of the molecules (or atoms, ions, or elementary particles) of which they are composed. The states of matter are also known as phases of matter or states of aggregation. There are three commonly recognized states of matter: solid, liquid, and gas. The molecules of a solid are limited to vibration about a fixed position. This restriction gives a solid both a definite volume and a definite shape. As energy in the form of heat is added to a solid, its molecules begin to vibrate more rapidly until they break out of their fixed positions and the solid becomes a liquid. The change from solid to liquid is called melting and occurs at a definite temperature, the melting point. The molecules of a liquid are free to move throughout the liquid but are held from escaping from the liquid by intermolecular forces (see adhesion and cohesion). This gives a liquid a definite volume but no definite shape. As more heat is added to the liquid, some molecules gain enough energy to break away completely from the liquid and escape into the surrounding space (see evaporation). Finally a temperature is reached at which molecules throughout the liquid are becoming energetic enough to escape and bubbles of vapor form and rise to the surface. The change of the liquid to a vapor, or gas, in this manner is called boiling and occurs at the boiling point. The molecules of a gas are free to move in every possible way; a gas has neither a definite shape nor a definite volume but expands to fill any container in which it is placed. In addition to these three states of matter, scientists also distinguish three additional states—plasma and the Bose-Einstein and the fermionic condensates. A plasma is formed by adding still more heat to the molecules of a gas. Eventually a point is reached where the molecules are moving so rapidly that the molecules become torn apart into their component atoms and individual electrons are pulled away from the atoms. This very hot mixture of negatively charged electrons and positively charged ions has properties distinct from those of the other states of matter. Bose-Einstein condensate and fermionic condensates are formed by chilling the molecules of a gas. As temperatures approach absolute zero (-273.15°C), the motion of the individual atoms slows to the point where they combine to form a single “super atom” with properties distinct from those of other states of matter.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2006, Columbia University Press. All rights reserved.

vaporization

vaporization, change of a liquid or solid substance to a gas or vapor. There is fundamentally no difference between the terms gas and vapor, but gas is used commonly to describe a substance that appears in the gaseous state under standard conditions of pressure and temperature, and vapor to describe the gaseous state of a substance that appears ordinarily as a liquid or solid. Although most substances undergo changes of state in the order of solid to liquid to gas as the temperature is raised, a few change directly from solid to gas in a process known as sublimation.

The Boiling Point and Latent Heat of Vaporization

When heat is added to a liquid at its boiling point, with the pressure kept constant, the molecules of the liquid acquire enough energy to overcome the intermolecular forces that bind them together in the liquid state, and they escape as individual molecules of vapor until the vaporization is complete. Vaporization at the boiling point is known simply as boiling. The temperature of a boiling liquid remains constant until all of the liquid has been converted to a gas.

For each substance a certain specific amount of heat must be supplied to vaporize a given quantity of the substance. This amount of heat is known as the latent heat of vaporization of the substance. The quantity of heat applied for each gram (or each molecule) undergoing the change in state depends on the substance itself. For example, the amount of heat necessary to change one gram of water to steam at its boiling point at one atmosphere of pressure, i.e., the heat of vaporization of water, is approximately 540 calories. Other substances require other amounts.

latent heat

latent heat, heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, or the reverse of either of these changes. It is called latent because it is not associated with a change in temperature. Each substance has a characteristic heat of fusion, associated with the solid-liquid transition, and a characteristic heat of vaporization, associated with the liquid-gas transition. The latent heat of fusion for ice is 80 calories per gram (see calorie). This amount of heat is absorbed by each gram of ice in melting or is given up by each gram of water in freezing. The latent heat of vaporization of steam is 540 calories per gram, absorbed during vaporization or given up during condensation. For a substance going directly from the solid to the gas state, or the reverse, the heat absorbed or given up is known as the latent heat of sublimation.

intermolecular forces

intermolecular forces, forces that are exerted by molecules on each other and that, in general, affect the macroscopic properties of the material of which the molecules are a part. Such forces may be either attractive or repulsive in nature. They are conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 angstroms or less, and long-range forces, which operate at greater distances. Generally, if molecules do not tend to interact chemically, the short-range forces between them are repulsive. These forces arise from interactions of the electrons associated with the molecules and are also known as exchange forces. Molecules that interact chemically have attractive exchange forces; these are also known as valence forces. Mechanical rigidity of molecules and effects such as limited compressibility of matter arise from repulsive exchange forces. Long-range forces, or van der Waals forces as they are also called, are attractive and account for a wide range of physical phenomena, such as friction, surface tension, adhesion and cohesion of liquids and solids, viscosity, and the discrepancies between the actual behavior of gases and that predicted by the ideal gas law. Van der Waals forces arise in a number of ways, one being the tendency of electrically polarized molecules to become aligned. Quantum theory indicates also that in some cases the electrostatic fields associated with electrons in neighboring molecules constrain the electrons to move more or less in phase.

Evaporation and Vapor Pressure

Liquids can also change to gases at temperatures below their boiling points. Vaporization of a liquid below its boiling point is called evaporation, which occurs at any temperature when the surface of a liquid is exposed in an unconfined space. When, however, the surface is exposed in a confined space and the liquid is in excess of that needed to saturate the space with vapor, an equilibrium is quickly reached between the number of molecules of the substance going off from the surface and those returning to it. A change in temperature upsets this equilibrium; a rise in temperature, for example, increases the activity of the molecules at the surface and consequently increases the rate at which they fly off. When the temperature is maintained at the new point for a short time, a new equilibrium is soon established.

The pressure exerted by the vapor of a liquid in a confined space is called its vapor pressure. It differs for different substances at any given temperature, but each substance has a specific vapor pressure for each given temperature. At its boiling point the vapor pressure of a liquid is equal to atmospheric pressure. For example, the vapor pressure of water, measured in terms of the height of mercury in a barometer, is 4.58 mm at 0°C and 760 mm at 100°C (its boiling point).

re•flux

Pronunciation: (rē'fluks"), [key]
—n.
a flowing back; ebb.