pH and the pH Scale
pH and the pH Scale
• Prepare to be... shocked!!!
• Absolutely pure water contains an equal number
of free moving hydrogen ions and hydroxide ions.
• Where do these ions come from?
• If free moving ions are necessary to conduct
electricity in water, then why can’t pure water
conduct electricity?
pH and the pH Scale
• Question 1: Where do these ions come from?
• Pure water autoionizes.
• This means that water makes its own ions.
HOH 
1+
H
+
1OH
• For every water molecule that autoionizes, one
hydrogen ion and one hydroxide ion is produced!
#
1+
H
= #
1OH
pH and the pH Scale
• Question 2: If free moving ions are necessary to
conduct electricity in water, then why can’t pure
water conduct electricity?
• In pure water, only 1 in every 555,555,556 water
molecules autoionizes!
• Pure water just does not contain enough free
moving ions to conduct electricity!
pH and the pH Scale
• Pure water contains an equal number of H1+
ions and OH1- ions.
HOH HOH
HOH
HOH
1H1+HOH
HOH OH HOH HOH
HOH HOH
HOH HOH HOH HOH
HOH
HOH
H1+ HOH
HOH HOH HOH
HOH HOH
HOH HOH
HOH HOH
HOH HOH HOH
HOH HOH HOH
OH1- HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH 1+
HOH
H HOH
HOH OH1- HOH
HOH
HOH
pH and the pH Scale
• An acidic solution contains a greater number
of H1+ ions than OH1- ions.
HOH HOH
HOH
HOH
1+
HOH HOH
H1+HOH
HOH H
HOH HOH
HOH HOH HOH HOH
HOH
HOH
H1+ HOH
HOH HOH HOH
HOH HOH
HOH HOH
HOH HOH
HOH HOH HOH
HOH HOH HOH
OH1- HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH 1+
HOH
H HOH
HOH H1+ HOH
HOH
HOH
pH and the pH Scale
• An basic solution contains a greater number of
OH1- ions than H1+ ions.
HOH HOH
HOH
HOH
1HOH OH HOH HOH
OH1- HOH
HOH HOH
HOH HOH HOH HOH
HOH
HOH
H1+ HOH
HOH HOH HOH
HOH HOH
HOH HOH
HOH HOH
HOH HOH HOH
HOH HOH HOH
OH1- HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
OH1-HOH
HOH OH1- HOH
HOH
HOH
pH and the pH Scale
• All aqueous solutions contain hydrogen ion and
hydroxide ions!
NaCl(aq)
HOH HOH
HOH Cl1- HOH
HOH
HOH
1+
1Na HOH OH HOH HOH Cl1H1+HOH
HOH HOH
HOH HOH HOH HOH
HOH
HOH
H1+ HOH
HOH HOH HOH
HOH HOH
HOH HOH
HOH HOH
1+
HOH
HOH HOH HOH
HOH Na
HOH
1+
OH1- HOH HOH Na
HOH
HOH
HOH
Cl1- HOH HOH HOH
HOH HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
Cl1- HOH
HOH
HOH 1+
HOH
H HOH
HOH OH1- HOH
Na1+
pH and the pH Scale
• A neutral solution contains an equal number of
H1+ ions and OH1- ions.
NaCl(aq)
HOH HOH
HOH Cl1- HOH
HOH
HOH
1+
1Na HOH OH HOH HOH Cl1H1+HOH
HOH HOH
HOH HOH HOH HOH
HOH
HOH
H1+ HOH
HOH HOH HOH
HOH HOH
HOH HOH
HOH HOH
1+
HOH
HOH HOH HOH
HOH Na
HOH
1+
OH1- HOH HOH Na
HOH
HOH
HOH
Cl1- HOH HOH HOH
HOH HOH
HOH
HOH
HOH
HOH
HOH
HOH
HOH
Cl1- HOH
HOH
HOH 1+
HOH
H HOH
HOH OH1- HOH
Na1+
pH and the pH Scale
• The pH scale is a scale from 0 to 14 that is used
to measure the acidity or basicity (alkalinity) of a
solution.


neutral region
acid region
base region

0
1
2
strong
acid
3
4
5
weak
acid
6
7
8
9 10 11 12 13 14
weak
base
strong
base
pH and the pH Scale
• Stomach "acid" has a pH around 1.0.
pH and the pH Scale
• Sea water has a pH around 8.5.
pH and the pH Scale
• Rainwater has a pH around 6.2.
pH and the pH Scale
• Human blood has a pH of 7.4.
pH and the pH Scale
pH and the pH Scale
• A difference of 1 on the pH scale is really a
tenfold [x10] difference in the concentration
of H1+ ions!
1) A solution of pH = 4 is ______
10 times ______
more
acidic than a solution of pH = 5.
3
2) A solution of pH = ______
is 10,000 times
more acidic than a solution of pH = 7.
3) A solution of pH = 8 is ______
100 times ______
less
acidic than a solution of pH = 6.
pH and the pH Scale
4) A solution of pH = _____
11 is 1,000 times less
acidic than a solution of pH = 8.
5) Acid rain is killing many lakes and ponds in
the Adirondacks - the pH is too acidic. To
treat symptoms, Ca(OH)2 is added in order
to adjust the pH. If a quantity of Ca(OH)2
was added to a pond with a pH of 3.2 such
that the water's hydronium ion concentration
was changed by 1000 fold, then what would
be the new pH of the pond? _____
6.2