Jacques Hammer, age 14, of Rutland, Vt., for his question:
WHAT IS DISTILLED WATER AND DISTILLATION?
Water that has been distilled has gone through a process of vaporization and subsequent condensation for the purpose of purification.
Boiling water forms an invisible gas called water vapor. The clear space just above the spout of a boiling kettle is filled with steam. The cloud above this clear space, which is often called steam, is not really water vapor, but droplets of water formed again when the gas cooled.
The formation of steam can be collected by holding a cold vessel inverted above the kettle's spout. This condensed steam is called distilled water. It contains no impurities.
Distillation is the process of boiling a liquid, such as water, and condensing the vapor which forms. So distillation really includes two processes: vaporization and condensation.
The distilled water is purer than the original water because the process leaves behind substances, such as salt, which do not evaporate at the boiling temperature of water.
Distillation employs heat and an apparatus called a still. A still consists of a boiler, a condenser and a receiver. The mixture to be distilled is heated in the boiler. Whichever substance in the mixture boils at the lowest temperature will be the first to turn to a vapor. The vapor enters the condenser, where it cools and becomes a liquid again.
The distilled liquid, called the distillate, then collects in the receiver. Many liquids besides water are purified by distillation.
When several liquids are mixed, it is usually impossible to separate them completely by simple distillation. For example, alcohol boils at 172 degrees Fahrenheit and water boils at 212 degrees. But even the water will evaporate fairly rapidly at the boiling point of alcohol. So the distillate from an alcohol water mixture will contain some water. But the distillate collected at first will have a larger portion of alcohol than the portions that condense later.
Fractional distillation is important in petroleum refining. Petroleum is a mixture of many substances which have to be separated to be useful.
The earlier fractions give naphtha and benzine. Next comes gasoline, afterward kerosene and then the heavier lubricating oils.
Much of this separation is brought about in a single distillation by using huge fractionating towers.
The substances which make up the earliest fractions are the ones that boil off at the lowest temperatures. Their vapors rise highest in the towers and are carried off by pipes high up. Separate pipes carry off the different fractions at different levels.
Fractional distillation is also used to separate the different products obtained directly from coal tar.
Neither simple nor fractional distillation forms new substances. Each merely separates substances that have been mixed together. But when a substance such as wood is heated in a closed vessel, it decomposes and gives substances that were not there before. This destructive distillation and is a chemcai process.