Name (s): _______________________________________________________ Date: __________________ Bl: ____
The Chemistry of Life:
Using Acid/Base Indicators to Test Unknown Substances
Introduction: The pH scale is a numerical scale, ranging from 0-14. Any number below 7
represents an acid (stronger acids are closer to 0); any number above 7 represents a base
(stronger bases are closer to 14). A pH of 7 represents a neutral solution. The pH of a
solution can be measured with paper indicators that change to different colors at varying
pH levels. One indicator, wide-range pH paper, has the ability to measure both acids and
bases using one slip of paper. Students can use a key to match the color of the pH paper to
determine the pH of the solution.
The science laboratory is not the only place where acids and bases are found. Many item commonly
found at home are acids or bases. For example, many cleaning products owe their effectiveness to the fact
that they are alkaline (they are bases). Many foods that you eat contain acids.
Purpose: In this lab, you will use wide-range indicator to test the pH of 12 substances. For your safety, it is
important that you do not drink or taste any of the substances! Do not touch any of the chemicals; wash
your hands at the end of the lab.
Materials:
- 12 Well plate
- 12 solutions
-
Dropper/pipette (found at each station)
Paper towel
-
Wide-range pH paper
Procedure:
1. You and your lab partner(s) will rotate from station to station at your teacher’s cue. It is important
that you minimize the contact your pH paper has with air. Make sure that your containers are closed.
2. At each station, use the dropper provided to place one drop of substance into the well plate. If you
arrive at a station without a solution, complete the task assigned.
3. Dip one piece of pH paper into the solution and observe any color change. (Note: it could take up to a
minute for the color to fully develop!) Record your results in Table I (see pg 2).
4. After recording results, hypothesize as to what the solution is. Keep in mind the pH. Do NOT touch or
taste the solution!
5. Discard the used pH paper on your paper towel. Wait until the teacher’s signal to rotate to the next
station. Repeat the procedure.
6. When you have completed the lab, clean your well plate with soap and water and place it next to the
sink to dry. Throw out all paper towels and used pH papers. Return pH container to proper place.
7. Complete the analysis questions as a group.
Results:
Table I: pH of various solutions
Solution
1
2
3
4
5
6
7
8
9
10
11
12
What is it?
Color of pH
paper?
pH?
Acid or Base?
Analysis Questions: After cleaning up, complete these questions using complete sentences and detailed
responses.
1. What substances were acids?
2. What substances were bases?
3. Which substance was the strongest acid? Explain your answer.
4. Which substance was the strongest base? Explain your answer.
5. How might biologists use the techniques you utilized in the lab today in the “real world?” Give three
detailed examples where it would be vital that the pH be determined correctly.
6. The pH of your skin and hair ranges from 4.5 – 5.5 Did you test any substances today that could be
harmful to your skin or hair? If so, which substances? How do you know they might be damaging?
7. Draw your own pH scale from 0-14 utilizing color. Place the twelve substances tested in class at the
correct location on the scale based upon your pH results. Be certain to use a ruler. This can be drawn on
the reverse side of this paper or attached on a separate sheet.