Separation Of Mixtures

The separation of mixtures into their components is of great commercial importance, as well as a necessary step in the preparation of a pure compound in the laboratory. A raw ore is often treated to obtain the pure metal compound before subsequent steps are undertaken to isolate the desired metal. The juice of sugar beets ore cane is concentrated, crystallized, an centrifuged to give white table sugar. Steel objects are picked out by magnetic separators from other refuse at recycling plants. In this, we will discuss three common methods for separating mixtures: filtration, distillation, and chromatography.

Filtration is the process of seperating solid particles suspended in a liqued solution by pouring the mixture through a filter. For example, photografic developing solution contains silver compounds. If sodium chloride is added to the devoloping solution, fine crystals of silver chloride form. When this mixture is pured in to a cone made of filter paper, the solution passes throug, leaving the silver chloride behind. In this way, the silver can be recovered from the developing solution (as silver chloride)

A liquid or solid that changes readily to the gaseous state is said to be volatile, When the temperature of a volatile liquid is increased sufficiently, the liquid boils, changing rapidly to the gaseous or vapor state. Distillation uses the differences in volatility of substances to seperate a solution into its components. For example, if a solution of sodium chloride in water is heated, the water (which is fairly volatile) boils off, leaving behind sodium chloride (which is not volatile)



Figure 1: A simple distilation apparatus. Volatile components pass from the flask into the condenser. There the vapor changes back to liquid, which is collected in the receiver. Less volatile components remain in the distillation flask.

Figure 1 shows a typical laboratory distillation apparatus. The sodium chloride solution is placed in the distillation flask and heated. The water vaporizes and passes into the condenser, leaving sodium chloride crystals in the flask. In the condenser, the water vapor is cooled and changes back to a liquid. The pure (distilled) water is collected in the receiver.

Fractional distillation is useful when the solution consists of two or more volatile components. The temperature at which a liquid boils is known as its boiling point. When a solution containing liquid substances of different boiling points is distilled, the substance whith the lowest boiling point normally distills over first. A fractionating column, containing glass beads, is placed at the top of the column, being hotter at the bottom and cooler at the top. The substances with the lowest boiling point passes over into the condenser from the top of the column, whereas substances with higher boiling points condence on the beads in the column or remain in the distillation flask. As the substance of lowest boiling liquid increases and the next substance, now the one with the lowest boiling point, begins to distill. Fractional distillation is employed commercially to seperate crude oil or petroleum in to useful product. For this, the distillation is operated continuously, various fractions being takeb off at different heights up the fractionating column. Thus the lowest boiling fraction (20 -60 C), called petroleum ether, is obtained near the top of column. Light naphtha or ligroin is a fraction with a slightly higher boiling point (60 -100 C), and it is taken off the column just below the petroleum ether. Gasoline boils between 50 C and 20 C, and kerosene between 175 C and 275 C; these come off even lower on the column. Furnace and diesel fuels boil at stil higher temperatures.