Understanding pH: How Acidic Is It?
Outcomes:
1. Students will use indicators to determine the pH of solutions.
2. Students will classify solutions as acids, bases, or neutral solutions.
3. Students will observe the effects of acid rain on building materials.
4. Students will compare the effects of acid rain on lakes with and without buffers.
Background Information:
Pour yourself a glass of pure water. What are you drinking? A glass of pure water contains a lot
of water molecules and relatively few ions. Any charged atom or group of atoms is an ion. A
water molecule (H2O) can break apart to form a hydrogen ion (H+) and a hydroxyl ion (OH-).
There can never be a large number of these ions in pure water because when these ions meet, they
join to form a water molecule (H2O).
If the liquid in the glass is pure water, the number of hydrogen ions and the number of
hydroxyl ions is equal. But water is seldom pure. It usually contains dissolved gases – including
carbon dioxide and oxygen. Water may also contain dissolved minerals such as calcium and iron.
Whether it is in a bottle or a lake, water is almost always a solution.
If the solution contains extra hydrogen ions (H+), it is acidic. If there are extra hydroxyl ions
(OH-) present, the solution is basic or alkaline. Any substance that can donate hydrogen ions
(H+) is an acid. Any substance that donates hydroxyl ions (OH-) or accepts hydrogen ions (H+) is
a base.
A scale was developed to measure a solution’s acidity or alkalinity. The pH scale is based on
the number of hydrogen (H+) ions in the solution. The middle of the scale is 7; a solution with a
pH of 7 is neutral. A pH value below 7 describes an acid because the number of hydrogen ions
(H+) is greater than the number of hydroxyl ions (OH-).
The pH number always reflects the concentration of hydrogen ions (H+) in the solution. As the
number of hydrogen ions increases, the pH number decreases. On the pH scale, a 5 means that the
number of hydrogen ions is ten times greater than a pH of 6. This makes a solution with a pH of
5 ten times more acidic than a pH of 6. A solution with a pH of 4 is one hundred times more
acidic than a solution with a pH of 6. And a solution with a pH of 3 is 1000 times more acidic
than a solution with a pH of 6. Each integer change represents a factor of 10. (See the pH Scale
chart.)
The Importance of pH:
An understanding of pH is very important to the study of environmental science because living
things are very sensitive to changes in pH. Life can exist only between a pH of 3 and pH of 8.5.
Many important chemical reactions that take place in living cells are affected by acids and bases.
The growth of plants, bacteria, and fungi is affected by the pH of the environment in which they
live.
There is a very narrow range of pH in which a specific Species can exist. If the pH of the
environment becomes too high or too low, the organisms will die. If the pH of soil is too low,
certain kinds of plants will not grow. If the pH of the water is too low, certain species of fish will
not survive.
Blood plasma is the environment for some very important cells (red and white blood cells) in
many organisms. It is also the delivery system for many chemicals. The pH of human blood must
remain between 7.35 and 7.45. We cannot survive if our blood becomes too alkaline (greater than
7.8) or even slightly acidic (less than 7.0). Our kidneys and lungs help eliminate extra acids and
bases from our blood.
The blood also contains chemicals that control acidic conditions. These chemicals that help to
maintain the correct pH are called buffers. Buffers are important to living organisms because
they prevent large changes in pH that would be harmful to the organism.
Finding the pH:
To study the effect of acids, bases, and buffers we must be able to determine the pH. The pH may
be measured by using complex electrical instruments called pH meters. It can also be measured
by using indicators. Indicators are chemicals that change color at a specific pH. An example of a
natural indicator is the chemical that gives red cabbage its color.
The pH Scale
Concentration of H+ ions
(parts per liter)
1.0
100
0.1
10-1
0.01
10-2
0.001
10-3
0.0001
10-4
0.00001
10-5
0.000001
10-6
0.0000001
10-7
0.00000001
10-8
0.000000001
10-9
0.0000000001
10-10
0.00000000001
10-11
0.000000000001
10-12
0.0000000000001
10-13
0.00000000000001
10-14
1
2
3
4
<---more acidic
5
6
Concentration of OH- ions
(parts per liter)
10-14
0.00000000000001
-13
10
0.0000000000001
10-12
0.000000000001
10-11
0.00000000001
-10
10
0.0000000001
-9
10
0.000000001
-8
10
0.00000001
-7
10
0.0000001
-6
10
0.000001
-5
10
0.00001
-4
10
0.0001
-3
10
0.001
-2
10
0.01
10-1
0.1
100
1.0
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
7
neutral
8
9
10
11
12
more basic----->
13
14
Review Questions:
1. Define the following words.
Acid (acidic) ____________________________________________________________
________________________________________________________________________
Alkaline (basic) __________________________________________________________
________________________________________________________________________
Base ___________________________________________________________________
________________________________________________________________________
Buffer__________________________________________________________________
________________________________________________________________________
H+ ____________________________________________________________________
Indicator________________________________________________________________
________________________________________________________________________
Ion ____________________________________________________________________
Neutral
_______________________________________________________________________
OH- ___________________________________________________________________
pH scale _______________________________________________________________
_______________________________________________________________________
2. The two ions that are formed from a water molecule are _____________________ and
_________________________.
3. Why aren’t large numbers of these ions found in pure water?_____________________
________________________________________________________________________
4. Match the following terms to descriptions below.
____ gives up extra hydrogen ions
____ pH of 9
____ accepts extra hydrogen ions
____ pH of 3
____ gives up extra hydroxyl ions
____ pH of 7
____ the number of hydrogen ions is greater that the number
of hydroxyl ions
____ the number of hydrogen and hydroxyl ions are equal
A. acid
B. base
C. neutral
5.
Match the following terms to the description below.
___ any charged atom or group of atoms
A. acidic
___ a scale that indicates the number of hydrogen ions in a
B. alkaline
solution
C. buffer
___ a chemical that can be used to determine the pH
D. indicator
___ a chemical that prevents large changes in pH
E. ion
___ a pH greater than 7
F. pH
___ a pH less than 7
6. Why is it important we understand pH?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
7. Nearly all organisms require a pH greater than ___ but less than ___. The pH of human
blood must be between ___ and___.
8. A pH of 4 is (two times/ten times/100 times) more acidic than a pH of 5.
A pH of 4 is (two times/three times/ten times/100 times/1000 times) more acidic than a
pH of 6.
9. What pH number is 10,000 times more alkaline than a pH of 8? _______
Seven Important Facts to Remember when Using the pH Scale
 At pH 7, the number of hydrogen ions (H+) is equal to the number of (OH-) ions.
 A solution with a pH less than 7 is an acidic solution.
 The lower the pH, the greater the number of hydrogen ions (H+) and the more
acidic the solution.
 A solution with a pH number greater than 7 is an alkaline (basic) solution.
 The higher the pH number, the greater the number of (OH-) ions and the more
basic the solution.
 Solutions with the higher pH numbers are less acidic and have fewer hydrogen
ions (H+).
 A change of one whole number on the pH scale changes the concentration of H+
ions by a factor of ten.