GED Science Practice Test: Pure Substances

Pure substances differ from the mixtures we examined in the last lesson.  Mixtures included two or more substances mixed together.  Pure substances are made up of only one type of substance.   Pure substances include elements and compounds.  Elements are pure substances that cannot be split into a simpler substance.  An example of an element is aluminum. If you keep splitting up aluminum until you have the very smallest piece of aluminum you can have, and still have it be “aluminum,” you will have an atom of aluminum.  An atom is the smallest unit of an element you can have, and it cannot be split by chemical means.

Compounds are pure substances, made up of two or more elements that are chemically combined.  Notice the difference between a compound and a mixture, which you learned about in the previous lesson.  Both compounds and mixtures are made up of two substances.  However, in a compound, the two substances are elements and are chemically combined.  In a mixture, the two substances could be elements or compounds, but they are not chemically combined.  The following flow chart and diagrams may help you make sense of the different kinds of substances.



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