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The Self-ionization of Water and pH


    1.    Identify the ions present in pure water and give their concentrations.

    2.    Define pH and explain how the pH scale is used.

Notes: (19-1)

Water has many unusual properties.  One of these properties is water's ability to act as either an acid or a base. ( H2O D H3O+ + OH-)  
Amphoteric is the name given to a substance that can act as either an acid or a base. 
Water becomes hydronium and hydroxide through self ionization

The pH scale was developed to help people interpret the amount of hydronium ions compared to the amount of base in a solution.  The scale was developed by Soren Sorenson, a Danish biochemist, and is based on the -log of the hydronium ion (H3O+)

example:  A solution with 1 x 10-7 H3O+ ions has a pH of 7

                  A solution with 1 x 10-2 H3O+ ions has a pH of 2


pH Scale

The pH scale contains values between 0 and 14.  Each whole number change in the pH of a substance represents a 10 fold increase in the number of hydronium ions.

Scale is from 0 - 14
0-7 acidic (0 most acidic)
7 neutral
17-14 basic (14 most basic)
can be measured using a chemical indicator
a weak acid or base that changes color as the pH changes (gain or lose H+ ions)






Last modified: April 28, 2003