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Chemistry Paper (Pure Substances)


What is a pure substance? Pure substances have two characteristics. One is that the percentage of composition is always the same from one sample to the next. Secondly, pure substances melt and/or boil at a characteristic temperature. There are two types of pure substances. Pure substances are either elements or compounds. An element is not chemically decomposable into other elements, and element properties do not vary. Compounds are elements that are combined chemically in definite proportions, and compounds properties do not vary either. No that we have the answer of what a pure substance is, we must answer the question, how is a substance identified as a pure substance?
A pure substance can be distinguished from other substances by its physical and chemical properties. A chemical property can be perceived when the substance is involved in a chemical reaction, a transformation that changes the substance into a new substance. A physical property is a property that can be observed without changing the chemical identity of a substance. Some chemical characteristics that a substance might have are its melting point, boiling point, or its solubility in a particular solvent. Pure solids will have a constant temperature during melting. This is also true with the boiling point for pure liquids, their temperatures remain constant at their boiling points as well. Solubility of a pure solid will usually decrease with lower temperatures. Some physical properties that a substance has are characteristics like the mass, volume, density, appearance, smell, etc.
You can go about separating mixtures in order to extract each substance contained in the mixture. Some of the methods used to separate matter are filtration, distillation, and chromatography, all of which were stated in the book, Chemistry: Principles & Reactions. Filtration, the simplest method stated above, is used to separate a heterogeneous solid-liquid mixture. Filtration is done by passing a mixture through a porous barrier, such as filter paper, in order to separate larger molecules from smaller molecular structures, much like another method known as dialysis. When the filtration procedure is finished the residue in the filter should crystallize to form a pure substance. Distillation is used to separate a homogeneous solid-liquid mixture. Distillation is performed by heating a mixture, allowing the liquid to vaporize and separate from the solid, thus leaving the solid residue in the heated flask. The vaporized liquid is then condensed through cooling and dispersed into a collecting flask as a pure liquid. Chromatography is a bit more complex than the other two, but equally useful. Chromatography can be used to separate all kinds of mixtures. It works by using the differences in solubility and/or the extent of absorption, characteristic of every substance, to separate a mixture.
Another method that can be used to identify a pure substance that my lab partner and I actually experimented with is the Tyndall test. The test is very simple to carry out. I simply passed a laser beam through a substance inside a test tube. By observing the substance you can tell whether or not it is pure by whether or not the laser beam is visible when shined through the substance. If the laser beam is visible, that means that there are tiny particles in the substance that the laser is being reflected off of, therefore it is not a pure substance. On the other hand, if the laser does shine through without being visible, this is a positive test for a pure substance. Although the Tyndall test is quite useful, it has much room for error. A dirty test tube can quickly make a pure substance appear to be impure.
Methods used to separate mixtures that I found to be quite interesting were gas chromatography and mass spectrometry. Both of these methods are used in tandem for Olympic drug testing, according to the SIRS Researcher on the World Wide Web,
http://researcher.sirs.com/res-article-display?003608+drug+testing. In the case of drug testing the procedure is to first take a sample of urine and inject it into a heated tube that vaporizes the liquid into its chemical components. Each individual substance takes a characteristic amount of time to sink to the bottom of the tube. Then, the compounds that emerge from the tube are bombarded with electrons, this is known as mass spectrometry.