General ---------Biology ------------------Online------------------   ----wilcox@sunline.net Ch.  2 :  The Chemical Context of Life

The definition of  "compound" is a substance consisting of two or more elements combined in a fixed ratio. 

Now answer some questions using this definition.
Questions:

• A water molecule is a compound H₂O.  The "two or more elements" are_______, _______.
• Sodium chloride (salt: NaCl) is a compound made up of Na and Cl. What is the "fixed ratio?" ____ : ____

The definition of compound uses the term "substance" where you might expect them to say "molecule."  The reason for not using molecule, is that some compounds are made up of several molecules.  An example we will study is the substance hemoglobin, the protein that carries oxygen in the blood stream. 

Summary: 

• molecules are sometimes called compounds or substances
• the difference between these terms in subtle 
• understanding the difference is not important for this course
• only refer to this page again if you find the usage of the these terms confusing