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Introduction to Chemistry

GENERAL BACKGROUND

In medieval times, chemistry, as we know it, did not exist. Instead, an area of endeavor termed alchemy was popular. The three main goals/objectives of alchemy were:

• To find the panacea - the appropriate combination of ingredients that would cure all illness and diseases.
• To find the elixir of life - the secret potion that would prolong life, i.e. allow one to "live forever".
• To convert (transmute) "base metals" (notably lead - chemical symbol = Pb) into gold (chemical symbol = Au).

Medical science, in some sense, continues to look for a "panacea" and an "elixir of life" of sorts. The current focus may be on the immune system along with the role of genes and interplay of heredity and environment - but the desires to "cure all", to "retard the aging process", and to increase the "quality of life" are strong motivating forces. The areas of scientific endeavor that research these first two "alchemical" goals cross several disciplines and sub-disciplines besides chemistry, i.e. molecular biology, biochemistry, biophysics, genetics, and immunology.

The third alchemical goal - to turn lead into gold - also does not fall under the realm of "standard chemistry". In the 1930's, it was realized that the basic building blocks of chemistry, the elements - which are composed of only one (1) kind of atom - could not be transmuted by "chemical means". Hence, elemental substances - lead (Pb) and gold (Au) - were two different kinds of atoms and a fundamental "rebuilding" of the core or nucleus of the atom was needed to convert Pb into Au. These processes, which were quite violent and dangerous, were nuclear transformations and not chemical changes. The area of study came under the heading of nuclear physics - not chemistry. With the technology that existed during the time of the alchemist, the dream of turning Pb into Au was doomed to failure. Although we can perform this transmutation today, the costs and risks of carrying out these nuclear processes far outweight the value of Au that is eventually produced.

From the above discussion, it seems that the goals of the alchemists had more to do with medicine and nuclear physics than it did with modern day chemistry! So, why did we say that alchemy was the forerunner of chemistry? Although the specific goals of the alchemist may seem to involve chemistry only marginally - the actual alchemical processes and recipes were pure chemistry! In other words, although the alchemists wanted to turn Pb into Au - for example - the processes that they were actually carrying out involved chemical changes - not the required nuclear changes! Alas, although they wanted to change the elements, they were in fact just doing any or all of the following chemical changes:

• Combining different elements (different kinds of atoms) to form compounds.
• Converting one chemical compound into a different compound.
• Decomposing ("breaking down") a chemical compound into its constituent elements and/or into simpler chemical compounds.

<IMG dynsrc="http://dept.physics.upenn.edu/courses/gladney/mathphys/movies/rust.avi" START=Mouseover HEIGHT=240 WIDTH=320 ALIGN=left> <IMG dynsrc="http://dept.physics.upenn.edu/courses/gladney/mathphys/movies/sugar.avi" START=Mouseover HEIGHT=240 WIDTH=320 ALIGN=left>

Click on the picture on the left to see a fast version of rusting or on the picture at the right to see sugar dissolve in water. Which video shows a chemical as opposed to a physical change?

Quantifying - Making measurements - the Scientific Method

Our scientific understanding matures as a direct result of our increased ability to: collect and (possibly) quantify observations, analyze these measurements and recognize underlying patterns, fit these patterns to some empirical - often mathematical - "law", and develop a model or theory (i.e. formulate a hypothesis) to "explain" these laws. Our model/theory is worthwhile only if it can explain a large number of observations with a minimum number of assumptions (postulates) and if it can predict the results of future measurements. Thus, as we gain the ability to make more precise measurements of observations previously measured and/or to make measurements on new observations, our model must be able to accomodate this new information. If not, the model must be either revised or (sometimes) replaced. Such replacements are known as scientific revolutions. Because of the great importance of the measurement and analysis, it is important that measurements are repeated (by the same experimenter and by different experimenters) to test for reproducibility. Only if measurements are reproducible can they be considered reliable for model-building purposes. Hence, duplication is the cornerstone of scientific investigation.

In the previous paragraph we just summarized the "scientific method". You probably had to memorize the steps in some physical science course in junior or senior high school. However, it is important to understand the scientific method and to always apply it to any experiments you perform as well as to experiments that others have performed. When you read in a text, for example, about some set of experiments that were performed and how these observations led to a model or theory, you should follow the scientific method "checklist" to see if the procedure is "good science".

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