Bio H – Basic Chem
What is pH anyway???
: Hydroxide Ion (OH-)
A
: Hydrogen Ion (H+)
B
C
D
1. Count the Hydrogen and Hydroxide ions for each beaker. Rank them in order of their approximate pH
from most acidic to the most basic. (1pts)
2. Which of the beakers contains only water? How do you know? (1pts)
3. In reality the difference between a solution with a pH of 4 and a solution with a pH of 5 is not the
addition of one hydrogen ion. Explain what the numbers do tell us about the relative number of
hydrogen ions. (1pt)
4.
a. What is the pH range of most areas of your body? (1pt) _________________
b. Name two places in your body where the pH is NOT in this range. Include what the pH is in each place
and why is it necessary for the pH to be different in order for each part to function properly. (2pts)
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Bio H – Basic Chem
5. What is a buffer: (1pt)
6. Explain why it is incorrect to say “buffers exist in the blood to make sure the pH of blood remains
neutral.” (2pts)
7. What happens to the concentration of hydrogen ions (or hydronium) AND the pH when an acid is added
to a beaker containing water? (1pts)
8. What happens to the concentration of hydroxide ions AND the pH when a base is added to a beaker
containing water? (1pts)
9. Solution A (pH 4) and Solution B (pH 9) are mixed. (1pt)
a. What type of reaction happens?
b. What are the two products that will form?
c. Approximately where on the pH scale will the resulting mixture fall? (from the information
provided you cannot provide an exact number)
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Bio H – Basic Chem
Introduction to pH:
When studying solutions, chemists are particularly interested in the concentration of two ions:
Hydrogen (H+) and Hydroxide (OH-). The concentration of these to ions in a solution will determine
the solution’s pH. One hydroxide ion will combine with one hydrogen ion to make water (H20).
Likewise, 100 hydroxides will combine with 100 hydrogens to make 100 water molecules. Therefore
anytime a solution has the same number of H+ ions and OH- ions, water is formed and the solution is
said to be neutral.
But what if they aren’t equal? If there are more hydrogens than hydroxides, the solution is acidic. If
there are more hydroxides, the solution is basic. pH is the number assigned to a solution based on
its H+ and OH- concentrations.




pH of 7 is neutral
that means that
[OH-] ions = [H+]
pH below 7 is acidic
that means that
[OH-] ions < [H+]
pH above 7 is alkaline (basic) that means that [OH-] ions > [H+]
The pH scale ranges from 0 to 14
pH is often used to compare different solutions. For example, a solution with a pH of 1 is said to be 10
times more acidic than a solution with a pH of 2, because the H+ ion concentration at pH 1 is ten
times higher than the H+ ion concentration at pH 2.
How exactly does this work?
Begin with water. Most molecules of water stay intact. Others break apart releasing an H+ from one
water molecule. Technically, H+ doesn’t float around by itself. Instead, it forms a hydronium ion
(H3O+), but for our purposes you can think of H+ and H3O+ as the same thing. (the chemistry
teachers will take care of this next year!)
 H2O  H+ + OH H+ (hydrogen ion) is acidic
 OH- (hydroxide ion) is basic
 As long as the number of acidic ions and the number of basic ions are equal, the
solution is neutral.
When an acid is added to water, the concentration of hydrogen ions increases and the pH goes
down.
 HCl + H2O  H2O + H+ + Cl REMEMBER H by itself is NOT THE SAME AS H attached to something else.
So even though we “see” the H attached to the Cl on the left, it is does not count
toward pH unless it is on its own as it is on the right.
When a base is added to water, the concentration of the hydroxide ion increases and the pH goes up.
 NaOH  Na+ + OH-
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Bio H – Basic Chem
What does any of this have to do with Biology?
Homeostasis is the body’s ability to maintain a stable internal environment. Among other things this
includes a stable temperature, chemical composition and pH. Physiological (consistent with an
organism’s normal function) pH, in the human, remains in the tight range of 6.5 – 7.5. A pH that
deviates from this range can have a negative effect on many necessary functions.
There are always exceptions:
Although most of the body maintains a pH between 6.5 and 7.5, there are a few exceptions. Can you think of
any? For starters, the stomach is significantly more acidic than the rest of the body. This low pH allows food to
be broken down more easily. Another place in the body where the pH deviates from the standard range is in the
intestines. The pH in the intestine is much more basic than the rest of the body. This allows the intestines to
counteract the acidic stomach contents as they pass through. Otherwise, that hotdog you ate would burn right
through the intestines!
Buffers are our friends:
Buffers are weak acids and bases that react with strong acids and bases to prevent sudden changes in pH. We
find buffers anywhere a stable pH is necessary. This does not mean that buffers are maintaining a neutral pH.
They are designed such that, wherever they are located, they keep the pH exactly as it should be. In the blood
there is a buffer system composed of carbonic acid and bicarbonate. Together they will contribute H+ or OHas needed to prevent any sudden pH changes that might be caused by the food we eat or other processes in the
body.
pH of Several Body fluids
 Gastric (stomach) juice 1.2 to 3.0
 saliva 6.35 to 6.85
 bile 7.6 to 8.6
 blood 7.35 to 7.45
Neutralization Reaction:
Acids and Bases have the unique ability of canceling each other out, or neutralizing one another. Whenever an
acid and a base are mixed, the hydrogen ions and the hydroxide ions come together to create water. The
remaining parts of the acid and base always form a salt. If no hydrogens or hydroxides are left over, the pH will
be a perfect 7! But if there are hydrogens or hydroxides left over, the solution will have a pH below or above 7.
Examples:
HCl (low pH) + KOH (high pH)  KCl + H20 (neutral)
HCl (low pH) + NaOH (high pH)  NaCl + H20 (neutral)
Acid
+
Base
 a Salt + Water
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