How Can We Check the Purity of a Substance?
Pure compounds have specific chemical properties by which they are defined. Liquid compounds have specific boiling points; solid compounds have precisely defined melting points. For example, a pure water solution boils at exactly 100 degrees Celsius, while a pure ice compound melts at exactly 0 degrees Celsius. As boiling and melting points vary greatly between substances, these temperatures are important variables for testing chemical purity. As compounds become higher in complexity, you must take additional steps to measure these points and additional properties that may aid in determining the purity of a substance.
Testing a Simple Solid
In order to test for the purity of a simple solid, heat a small sample of that solid in a thin-walled melting tube. Make sure to heat slowly, so as to record the exact melting temperature. If the temperature remains completely constant during the transition phase from solid to liquid, you know you have a pure substance. If the melting process takes place over a range of temperatures, even by one or two degrees, your substance is impure. You know that a melting phase is in process when there is no change in temperature. Therefore, the number of temperature halts corresponds to the number of substances you have in your sample.
Testing a Simple Liquid
Place a sample of the liquid solution into a test tube and slowly heat it over a flame. If the entire solution boils away at a constant point in temperature, the solution is likely a pure liquid. If the transition takes place over a range in temperatures, you are dealing with various liquids. As with melting points, the number of boiling points corresponds to the number of distinct liquid compounds in your solution. Finally, you must check for solid impurities. First, after the solution has boiled away entirely, check the test tube for any solid residue. Also, compare your measured boiling point to that of the given substance in a scientific index. If you recorded only one boiling point but your measurement is different from the index, this usually indicates the presence of dissolved solids.
Testing Complex Compounds
When dealing with highly complex compounds such as certain foods, drugs and preservatives, you need more information to test for purity and content. Chromatography can be very complex and involves a multistage process of separation that may require special laboratory tools. A simpler example is the paper chromatography test, in which you are testing the purity of a colored dye by means of polarity, another property of chemical compounds. Drop a dot of dye on the bottom of a strip of cellulose paper and then submerge that paper in a test tube filled with water. The water travels up the paper, causing the dye to run, separating into different colors (i.e., substances) along the strip. This separation occurs as the different substances bond with the cellulose paper; the more polar the substance, the quicker it will bond with the paper and become visible, and the less far it will travel up the strip. Nonpolar substances travel the farthest, and therefore become visible as they eventually bond more towards the top of the strip.
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