68 - The Cell 



Fig. 4-1 When a direct current flows through water, 

 the water molecules decompose into hydrogen (left test 

 tube) and oxygen (right test tube). Note that the quan- 

 tity (volume) of the hydrogen produced is twice that 

 of the oxygen (as in H..O). 



kinds of smaller particles that combine together 

 in fixed proportion to form a countless variety of 

 molecules. 



Substances vs. Mixtures. Every sample of matter 

 is either a substance or a mixture (Fig. 4-2). 

 Strictly speaking, a substance is a purified single 

 kind of matter in which all the molecules are of 

 the same species. Purified (distilled) water, or 

 highly refined sugar, or pure oxygen must there- 

 lore be designated as substances. Mixtures, unlike 

 substances, are made up of more than one kind of 

 molecule. Pond water, although it is composed 

 chiefly of H.,0 molecules, also contains molecules 

 of various minerals as well as the molecules ol 

 nitrogen (N.,) and oxygen (O.,), dissolved from the 

 atmosphere; air contains molecules ot oxygen, 

 nitrogen, and small amounts of other gases. Con- 



p 9° n 



Q d 



d tf v 



c> 



r? 



Q> 



d 



9> 



d 



^ v 



Fig. 4-2. Diagrams representing two substances (A and 

 B), and a mixture (C) of the two. The large open circles 

 represent oxygen atoms, the smaller dark circles hydro- 

 gen atoms. A, a free element, oxygen (O.,), in which all 

 the molecules are alike and contain only one kind of 

 atom. B, a compound, water (H.,0), in which all the 

 molecules are alike, each consisting of one oxygen and 

 two hydrogen atoms. C, a mixture of water and free 

 oxygen (i.e., oxygen dissolved in water), which may be 

 mixed in various proportions. 



sequently these latter kinds of matter are desig- 

 nated as mixtures. 



In practice one can distinguish a substance 

 from a mixture containing that substance in large 

 proportion by the fact that the purified substance 

 displays a nonvariable and standard set of physi- 

 cal and chemical properties. Each specific sub- 

 stance can be identified with absolute certainty 

 by its known properties. A purified sample of 

 water (H..O) displays the same properties regard- 

 less of whether it is obtained from Siberia or 

 Afghanistan. Its physical properties (color, taste, 

 odor, boiling point, freezing point, etc.), and its 

 chemical properties (see later), allow no question 

 as to its identity as a single substance: and the 

 same holds true for all other known substances. 

 Table sugar, or more properly sucrose, CjoH.t.O^, 



