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High School Chemistry: Help and Review20 chapters | 191 lessons
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Nathan, a PhD chemist, has taught chemistry and physical science courses.
Imagine a cube of sugar that you might add to a cup of hot coffee or tea. If you look at the box of sugar cubes, you might see on the label the words 'pure cane sugar.' Does the word 'pure' mean the same in all cases, and what does this word mean within the context of chemistry?
The pure substance within chemistry is a very simple concept to grasp. Pure substances are defined as substances that are made of only one type of atom or only one type of molecule (a group of atoms bonded together). The measure of whether a substance is pure is known as purity. For example, pure iron would only contain iron atoms, and, as in the sugar cube mentioned above, pure sugar would only contain molecules of the substance called sucrose.
Pure substances exhibit very well-defined physical properties, or properties that are not connected with the substance's ability to combine with different substances. The temperatures where pure solids melt, known as melting points, are particularly sharp, meaning the melting occurs at a single temperature. Likewise, the temperatures where pure liquids begin to boil, or boiling points, occur at single temperatures when other factors, like air pressure, are controlled. The ability of substances to conduct electricity, or conductivity, is profoundly affected by purity. For example, copper that is used in electrical wiring must have very high purity in order to provide the most efficient conductivity. On the other hand, extremely pure liquid water is a very poor conductor of electricity because there are no dissolved electrical conductors, called electrolytes, to allow conduction to occur.
Purity can also have an important impact on the chemical properties of a substance. Pure substances have the potential to form predictable products from chemical reactions. The reason for this reliability comes from the absence of impurities that could start other reactions that might interfere with the reaction that is being studied by a chemist. Therefore, chemists often use substances of high purity when conducting chemical research.
While the concept of pure substances is very easy to understand, reality presents a more relative idea of purity. For example, the pure sugar mentioned earlier refers to sugar that is 99.9% pure. Copper wiring also falls short of the ideal definition of purity at approximately 99.99% purity. In fact, the removal of all impurities from a substance, though theoretically possible, is impossible at this time. Due to the relative nature of purity in many circumstances, the determination of purity through chemical analysis becomes vitally important. Measurements of purity can be conducted through a variety of techniques that involve chemical reactions, high-tech instrumentation, or a combination of the two.
Let's review. Pure substances are defined as substances that are made of only one type of atom or molecule. The purity of a substance is the measure of the extent to which a given substance is pure. The physical properties of a pure substance include well-defined melting and boiling points. The purity of a substance also impacts the chemical properties of a substance by reducing the chances of unexpected products being formed if impurities are present within the substance. Finally, absolutely pure substances are not highly likely in the reality of the everyday world, and the purity of a substance must be determined by chemical analysis.
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