Introduction:

A substance is a form of matter that has a constant composition and constant and distinct properties such. It is possible, however, that the observed value of a particular property may be shared by more than one substance. For example, sodium chloride (table salt) is a white solid (two observed properties). But these properties alone do not uniquelyidentify the substance sodium chloride. Sodium iodide, sodium bromide, calcium phosphate, and aluminum oxide are just a few examples of different substances that are also white solids. How can sodium chloride be distinguished from these? Well, besides being a white solid, sodium chloride also dissolves easily in water (a third property), calcium phosphate and aluminum oxide don't; sodium chloride reacts with silver nitrate in water to produce a whitesolid (a fourth property), sodium bromide and sodium iodide don't. And so on. After enough simple properties are measured or observed, one will finally have a group of properties that uniquely identify a particular substance. But this process is very tedious, and chemists instead use sophisticated methods that can uniquely identify a substance from a single measurement. These methods usually involve the interaction of electromagnetic radiation (i.e. light) to produce a "fingerprint" pattern. The interaction of X-rays with crystals (X-ray diffraction), the combined interaction of radio waves and magnetic fields with nonmagnetic substances (nuclear magnetic resonance, abbreviated NMR), and the absorption of infrared light (IR spectroscopy) are common methods of recording a unique identity for a substance.

A mixture is a form of matter that contains two or more substances in which eachsubstance retains its distinct identity. Mixtures have a variable composition, since they can have different amounts of component substances, and hence their properties are variable and usually nondistinct. For example, while substances melt and boil at distinct temperatures, a mixture melts or boils through a broad range of temperatures. The identities of the component substances are often manifest in simple properties of the mixtures. For example, salt water is a mixture of salt and water that is liquid, like water, but tastes salty, like salt (Warning: Tasting anything in a chemistry laboratory is a bad idea because of potentially fatal consequences). And if one sample of salt water tastes "saltier" than anothersample (water from the Dead Sea versus water from the Atlantic Ocean, for example) we know that the first sample has a higher salt composition than the second (to show that mixtures do not have a constant composition). Distinct "fingerprint" patterns in the mixturecan also uniquely identify each of the component substances.

Heterogeneous mixtures are mixtures in which the composition is not uniform, such as chunks of one substance mixed with another or a layer of one substance over another. Asphalt pavement, which has small rocks mixed with tarry goo, is a simple example of the first case and oil-and-vinegar salad dressing, which has a layer of oil floating on a layer of vinegar, is an example of the second case. If the chunks are large enough to be seen,the mixture is easily identified as heterogeneous. What if the chunks are too small to be seen? For a mixture that is mostly liquid or gas, small chunks suspended in it will block the passage of light. These cloudy or opaque liquids and gases are also heterogeneous mixtures. Fog, for example, is a heterogeneous mixture of water drops suspended in air. The tiny water drops block the passage of light (e.g. reflect headlights back toward you) to make fog opaque. Milk is also a heterogeneous mixture of tiny fat and protein particles suspended in water that block the passage of light and make milk opaque. (Yes, "homogenized" milk is a heterogenous mixture! "Homogenized" means only that fat particles are dispersed throughout the milkrather than floating in a layer of cream on top.)

Homogeneous mixtures (also known as solutions) are mixtures in which thecomposition is uniform, there are no chunks or layers. Salt water, dust free air (mixture of nitrogen, oxygen, argon, carbon dioxide, water vapor and other gases), brass (solid mixture of copper and zinc) are all examples of homogeneous mixtures. Since homogeneous mixtures that are liquid or gas do not contain particles that block the passage of light, they are clearinstead of opaque.

Chemists like to study substances, but most matter in the universe is found in the form of mixtures. Hence much of the work in a chemistry laboratory is devoted to separating mixtures into pure substances. Since heterogeneous mixtures contain chunks or layers, they are often easier to separate than homogeneous mixtures. A mixture of solid particles in a liquid can be separated by pouring the mixture through a filter that traps the solid particles while the liquid passes through in a process called filtering. Two layers can be separated bypouring carefully the top layer off of the bottom in a process called decanting. (Or thebottom layer can be poured out using a special piece of laboratory glassware called a separatory funnel. ) Some simple methods also exist for separating homogeneous mixtures. A solid dissolved in a liquid solution can be separated by letting it dry out in the process of evaporation. A homogenous mixture of liquids with different boiling points can be separatedby heating it up until it boils and condensing the vapor. The liquid with the lowest boiling point vaporizes and is condensed first; the liquid with the next lowest boiling point vaporizes and is condensed second; and so on. This process is called distillation. To separate more complicated homogeneous mixtures chemists commonly use a technique called chromatography. There are many different types of chromatography that are used fordifferent types of mixtures. Liquid chromatography (LC), gas chromatography (GC), ion chromatography, paper chromatography, and thin layer chromatography (TLC) are common types. In general, chromatography works by carrying a mixture along a path through a solid material. Some of the substances in the mixture will stick to the solid material more easily and won't travel through as quickly as others. As the mixture travels through the solid, then, the different component substances are separated out as spots or bands. If these substances have colors, so do the spots or bands that show up--hence the name "chromatography."