Mixture Separation

Most materials in our world are mixtures. Very few materials are pure substances. The art of separating mixtures is important because it enables us to isolate pure substances. Mixtures are either homogeneous or heterogeneous. Homogeneous mixtures are uniform in composition. Heterogeneous mixtures are not. Salt water is a mixture of water and NaCl and is homogeneous if thoroughly mixed, with all the salt dissolved. Oil in water is a heterogeneous mixture. Both types of mixtures can be separated into their component parts by physical means. A salt water mixture can be separated by distilling or evaporating the water and collecting the salt residue. An oil and water mixture will separate into an oil layer and a water layer because the materials are not attracted to one another and gravity "pulls" the denser water beneath the less dense oil. Settling, filtration, chromatography, and manual methods are all means of separating the components of a mixture. Choice of method depends on the type of mixture and the characteristics of its components.

A heterogeneous mixture of solid and liquid or solid and gas is usually fairly easy to separate because of the 2 different physical phases. The solid may settle out, allowing you to pour off the liquid. Or, maybe the liquid can be evaporated, leaving the solid behind. Or the mixture can be poured through a filter, catching the solid on the filter and allowing the liquid or gas to pass through. We use filtration frequently--in our coffee makers, automobile fuel lines, automobile air cleaners to name only a few examples.

A mixture of two or more solids is usually separated by utilizing the different chemical or physical properties of the substances. For example, a heterogeneous mixture of red M&M's and yellow jellybeans can be separated using the different colors or the different shapes of the solids. The parts of the mixture are large enough to be separated manually. A mixture of black peppercorns and white table salt might be separated this way as well. But what could be done with a mixture of sand and sugar? True, you could get a magnifying glass and tweezers and try picking out the grains of sand, but is there an easier way? Is there some property that sugar has that sand does not (or vice versa)? Could this be used to separate sand and sugar? If you said that sugar dissolves in water and sand does not, you are on the right track.

Homogeneous mixtures of a solvent and one or more solutes (dissolved substances) are often separated by chromatography. Chromatography works to separate a mixture because the components of a mixture distribute themselves differently when they are in contact with a "two phase system". One phase is stationary and the other is moving or mobile. The stationary phase may be a solid packed in a tube or a piece of paper. The mobile phase may be liquid of gaseous. Food colorings are one example, a homogeneous mixture of a solvent and a single dye or combination of selected dyes that produce the desired color.