Compounds and Mixtures

Often times, matter that you see around you in everyday life isn't just a pure element. Just look around in your house and you more than likely wouldn't find any Sodium sitting around, but you most likely have table salt (sodium chloride) in your kitchen. How is that there are more substances out there than there are elements? The answer is that elements can combine to form chemical compounds and mixtures.

A chemical compound is formed when elements bond to each other and combine to make a more complicated structure called a molecule. These new substances are made when atoms combine in specific ways, and they make molecules that have properties that are vastly different from any one of the elements that make them. Chemical compounds are pure substances, like elements, and have a unified composition in throughout with identical molecules.

Since these molecules are a new substance that aren't like their parent elements, and they are pure and uniform, we write them by giving their chemical formula. A chemical formula lists all elements contained within the molecule and how much of each is present. We do this by writing the element symbol, follow by the number of how many atoms of that element are present, followed by the next element symbol and it's amount, and so on.

Ex.   Table salt-  NaCl         Water- H₂O
        ethanol (drinking alcohol)- C₂H₆O
        Chrysotile (Common form of asbestos)- Mg₃(Si₂O₅)(OH)₄
        TNT (an explosive material) C₆H₂(NO₂)₃CH₃

As you may have noticed, some of these compounds have parenthesis around some elements, this means that the number following the parenthesis is applies to the everything that is in the parenthesis, much like multiplication and parenthesis in math.
For instance, the (OH)₄  in chrysotile means that there is 4 oxygen and 4 hydrogen in the parenthesis. The (NO₂)₃ in TNT means there is 2 nitrogen and 6 oxygen in the parenthesis.

You will notice that some elements are repeated in many compounds. This is to show some of the structure of the compound. This is the same reason why some parenthesis are followed by no number, and it is implied that there is a 1 in the subscript.


Mixtures are unlike chemical compounds. Mixtures are just that, mixtures of 2 or more substances without chemical bonds. Mixtures do not chemically alter the substituent substances that they contain. Anything can be a mixture as long as they don't react.

Examples: salt and sugar, water and oil, gasoline and ethanol

Mixtures can have any ratio of constituents. A mixture of salt and sugar can be mostly salt, equal parts of both, or mostly sugar.

Mixtures come in 2 different categories, homogeneous and heterogeneous.

Heterogeneous mixtures are not uniform, and have regions that vary in the ratio of composition. Our mixture of sugar and salt falls into this category. Some regions will have more salt, others more sugar.

Homogeneous mixtures are ones that have a uniform composition, with no regions varying in the composition ratio. An example of homogeneous mixture is the air that we breathe.