Classifying Matter According to Its Composition Elements, Compounds, and Mixtures

4 Classifying Matter According to Its Composition Elements, Compounds, and Mixtures  

Pure substances are composed of only one type of atom or molecule. Helium and water are both pure substances. The atoms that compose helium are all helium atoms, and the molecules that compose water are all water molecules—no other atoms or molecules are mixed in. 

Pure substances can themselves be divided into two types elements and compounds. Copper is an example of an element, a substance that cannot be broken down into simpler substances. The graphite in pencils is also an element—carbon. No chemical transformation can decompose graphite into simpler substances it is pure carbon. All known elements are listed in the periodic table in the inside front cover of this book and in alphabetical order on the inside back cover of this book. 

A pure substance can also be a compound, a substance composed of two or more elements in fixed definite proportions. Compounds are more common than pure elements because most elements are chemically reactive and combine with other elements to form compounds. Water, table salt, and sugar are examples of compounds they can all be decomposed into simpler substances. If you heat sugar on a pan over a flame, you decompose it into several substances including carbon 

A compound is composed of different atoms that are chemicaiiy united (bonded). A mixture is composed of different substanoes that are not chemioaiiy united, but simpiy mixed together. 

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