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pH Worksheet
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What is pH?
pH is an abbreviation for “Potential Hydrogen” or “Power of Hydrogen” and is a measurement of how
acidic or how basic a solution is. The pH scale starts at 0 and goes up to 14. Halfway between 0 and 14 is
7, which is neutral. Compounds are acidic if they have a pH lower than 7. Compounds with a pH higher
than 7 are said to be basic or alkaline.
On the pH scale below, label the pH ranges for a strong acid, weak acid, strong base,
weak base and a neutral solution.
Exactly what makes a compound an acid or a base?
To understand this you must understand water. Water is a molecule made up of three atoms covalently
bonded together. Think of water as HOH.
Some compounds can cause water molecules to break apart into H+ and OH- ions. The H+ ion is called a
hydrogen ion. It is actually proton with no electrons. The OH- ion is called a hydroxide ion.
If you mixed hydroxide and hydrogen ions together, they would immediately pair up and make water
molecules.
H+
+ H- ------------> HOH ----> H20
This is called a neutralization reaction. Hydroxide ions neutralize hydrogen ions.
If, after the neutralization reaction is complete, there are H+ ions left over, then the solution is acidic.
If, after the neutralization reaction is complete, their are OH- ions left over, then the solution is basic.
Why is pH important to biology?
Most cells can only survive within a certain range of pH. For example, human blood has a pH of about
7.2, which is slightly basic. Any higher or lower and the blood cells would be injured or killed. So you
could say that a healthy person’s blood has a pH range or 7.2 to 6.9.
Acids denature, or change the shape of proteins in much the same way heat does. As a matter of fact,
strong acids like vinegar and lemon juice can be used to actually cook meats like fish and eggs. Seviche is
a dish made by mixing raw fish and lime juice and letting it sit for a few hours. Acids are used by your
digestive system to break down food molecules into simpler monomers.
Bases cause oils and fats to fall apart. Your digestive system uses bile, a basic compound to help in the
digestion of fats and grease. Oven cleaners and drain cleaners contain lye, a strong base that dissolves
baked on grease and burned fats.
Conclude as to whether each of the following is an acid, base or neutral solution.
Solution
Acid, Base, or Neutral
Solution with a pH of 4
Solution has equal
amounts of H+ ions and
OH- ions
Seawater, pH 8.5
Distilled water (pure
water)
Coffee, pH 5.2
Solution with a pH of 7.0
Solution with a high
concentration of H+ ions
Solution with a pH of 11
Lime juice, pH 3.3
Solution with less H+ ions
than OH- ions
pH after a snail has lived in water for three days
pH=5.8
pH with 2mL of bleach added to the water pH=9.4
pH after exhaled air is blown through a straw into
water for 5 min. pH=5.1
pH after an aquatic plant is grown in water for three
days in bright sunlight pH=7.7
Questions.
1. What effect does carbon dioxide have on tap water?
2. Is bleach an acid or a base?
3. Is coffee an acid or a base?
4. A student mixes strawberry koolaid and water. A pH meter is used to measure pH of 5.4. What kind of
solution is strawberry Koolaid?
5. In the koolaid mixture, what must there be more of, hydrogen ions or hydroxide ions?
6. A student adds an alka-seltzer to the koolaid and stirs. The pH meter now reads 8.3. What was released
by the alka-seltzer tablet to cause this change?
7. Baking soda is a weak base. Hydrochloric acid is a strong acid. What would happen if these two were
mixed?
Enzymes and Denaturing
In general, enzymes are protein molecules which must be folded in a specific three-dimensional shape in
order to function properly. Certain environmental parameters can affect enzyme activity, including pH
and temperature. If an enzyme's shape changes significantly and it can no longer function, the enzyme is
said to have become denatured.
The enzyme pancreatic amylase is manufactured and secreted by the pancreas into the duodenum (the
large, beginning portion of the small intestine) which has a pH of 7. Pancreatic amylase breaks down
starch into maltose, a disaccharide. Pepsin is an enzyme that is released by the epithelium of the stomach,
and functions in the stomach to break down proteins into smaller polypeptide units. The pH of stomach
acid is 2. Trypsin is a protein digesting enzyme that functions in the small intestine which has a pH of 8.
Identify the enzymes on the following graph:
Amylase
Trypsin
Pepsin
1. Which of the following statements is true with respect to Figure 1?
a) Pepsin and pancreatic amylase could never function together in the same part of the body at the
same time.
b) Pancreatic amylase could function in the stomach, but its activity would be low.
c) The optimal pH for the functioning of pepsin is approximately 8.5.
d) Normally, the small intestine must be slightly acidic.
2. Figure 2 depicts the activities of three enzymes. Which curve illustrates the functioning of human
DNA polymerase, which functions in the nucleus of cells? (Hint: the average body temperature of
humans is 37 ºC)
a) Enzyme A
b) Enzyme B
c) Enzyme C
d) None of the above could represent the activity of human DNA polymerase.
3. Which curve illustrates the functioning of DNA polymerase from a great white shark? (Hint: the
average body temperature of a great white shark is 26ºC)
a) Enzyme A
b) Enzyme B
c) Enzyme C
d) None of the above, since sharks, like all fish, do not contain DNA.
4. At what temperature would enzyme B be completely denatured?
a) 37°C
c) 5°C
b) 15°C
d) 50°C
5. Based on this graph, could humans and
thermophillic bacteria that live on vocanic vents in
the ocean share some of the same enzymes?
Based on the graph below:
6. Which enzyme works best in neutral
conditions?
7. Which enzyme is not affected by pH?
8. Which enzyme works under alkaline
conditions?
9. Which enzyme might be found in acidic
gastric juices?