Honors Biology
Ch. 2- Chemistry of Life
NAME: ____________________________________________
DATE: _______________
PERIOD: _____________
________
27
pH Lab
Review of Acids and Bases:
Complete the sentences with the help of your notes and textbook.
A water molecule (H2O) can split apart to form a(n) __________________________________ ion and a(n)
__________________________________ ion. The __________________________________ is a scale that measures the
concentration of hydrogen ions in a solution. The scale ranges from 0 to 14. Pure water has a pH of
__________________________________. A(n) __________________________________ is any compound that forms H+ ion
in solution. __________________________________ have a pH level below 7. A(n) __________________________________
is a compound that forms OH- ions in solution. __________________________________ have pH values above 7.
__________________________________ are weak acids or bases that react with strong acids or bases to prevent
sudden changes in pH. They are very important in maintaining homeostasis in living things.
Introduction:
Liquids all around us have either acidic or basic (alkaline) properties. For example, acids taste
sour while, bases taste bitter and feel slippery. However, both strong acids and strong bases can be
very dangerous and burn your skin, so it is important to be very careful when using such chemicals. In
order to measure how acidic or basic a liquid is, one must use the pH scale as illustrated below:
The easiest way to determine if a substance is acidic or a basic is to use an indicator. Indicators
are organic molecules that change color in an acid or a base. In this lab, you will use the juice from red
cabbage as a pH indicator to test common household liquids and determine their pH levels. You will
mix cabbage juice with different household liquids and see a color change produced by a pigment
called flavin in red cabbage. Through this color change, you will be able to successfully identify the
approximate pH of common household liquids using the table below:
Materials:
- Cabbage juice
- 7 plastic cups
Liquids to test:
- Bleach
- Lemon juice
- Vinegar
- Apple juice
- Color-coded pH scale
- Graduated cylinder
- Baking soda solution
- Baby shampoo
- Ammonia
Safety Rules:
 YOU MUST WEAR GLOVES & GOGGLES FOR THE DURATION OF THE EXPERIMENT.
 Never taste chemicals (or other substances) used for a lab experiment.
 Do not mix chemicals unless specifically directed.
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Clean up spills immediately.
If any substance gets into your eyes or in a cut on your skin, notify your teacher and follow
his/her directions.
Wash your hands before and after an experiment.
Clean up your lab area and materials after an experiment and return materials to their proper
location.
Procedure:
NB: Follow the instructions carefully. Do not skip steps.
1. Label each cup with each of the liquids.
2. Using a graduated cylinder, pour 50 mL of each test liquid into its respective cup.
3. CAREFULLY pour 25 mL of cabbage juice into the first test cup. In the data table, write down
the color change IMMEDIATELY after you pour the cabbage juice. Use the pH scale below to
find what pH this color change corresponds to.
4. Repeat Step 3 for every test cup, writing down the color change after every new test.
5. Identify whether each test liquid is an acid, a base, or neutral.
TEST LIQUID
Bleach
Lemon Juice
Vinegar
Apple juice
Baking Soda
Baby shampoo
Ammonia
COLOR OF INDICATOR
APPROXIMATE pH
ACID/BASE/NEUTRAL?
Data Analysis
Create a graph using the pH data. Label one axis “pH” and the other axis “Test Liquids.” (2 pts)
Conclusion
Answer each question related to the experiment.
1. In this experiment what was the independent variable? (1 pt)
2. In this experiment what was the dependent variable? (1 pt)
3. In general, how did the acids affect the color of the indicator? How did the bases affect the color
of the indicator? (2 pts)
4. Compare the concentrations of H+ and OH- ions in acids. Compare the concentrations of H+ and
OH- ions in bases. (2 pts)
Clean-up checklist:
Please do not use an excessive amount of paper towels.
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While the water is running, pour all liquids into the sink.
Throw away all cups and gloves.
Rinse out
Return goggles.
Wipe down any liquids on the bench or floor.
Teacher’s initials: _________________ = 3 pts