1) What is pH? (What is actually being measured?)
2) Exactly what makes a compound an acid or a base?
3) What are the numbers on a pH scale that could indicate a base? An acid? Neutral solution?
4) What is a chemical indicator?
Background info to assure you know 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.
5) What sould happen to a cell if placed in an environment too acidic or too basic?
6) Predict what the pH value of blood is.
Now check your thinking to see if you can apply pH understanding:
7) A student mixes strawberry koolaid and water. A pH indicator is used to measure pH of 5.4. What
type of solution would strawberry Koolaid be classified?
8) In the above koolaid mixture, what must there be more of, hydrogen ions or hydroxide ions?
9) A student adds an alka-seltzer to the koolaid and stirs. The pH meter now reads 8.3. What ions were
released by the alka-seltzer tablet to cause this change?
10) What does it mean if a solution is a “weak acid”? “Strong acid”? Give logical pH values for each.
11) What does it mean if a solution is a ‘weak base”? “Strong base”? Give logical pH values for each.
12) Baking soda is a weak base. Hydrochloric acid is a strong acid. What would happen if these two were
mixed?
13) Bromthymol blue (BTB) is a chemical indicator that is blue all alone. When combined with a basic
solution, it turns green as a solution becomes more acidic it turns yellow in an acidic solution. Fill in
what color you think bromthymol blue would be in each of the situations in the chart. If solution
color is given, simply provide a logical, possible pH value based on the outcomes. Fill in the missing
ions as well!
Situation
pH
Water directly out of tap
7.2
pH after exhaled air is blown through a straw into water for 5
min.
5.1
BTB color
H+
pH after a snail has lived in water for three days
Very Yellow
pH with 2mL of bleach added to the water
Very Blue
pH with instant coffee added to the water
pH after an aquatic plant is grown in water for three days in
bright sunlight
Ion present
5.0
Light Blue
OH-
14) Is BTB a chemical indicator?
15) What then does it “indicate”?
16) Based on the data above, why does drinking coffee cause stomach pain for some indivudals?
17) What effect does carbon dioxide have on tap water?
18) Is bleach an acid or a base?