Science JunctionLesson 1

Introductory Lesson-How is pH Determined?

Students will be able to: Note: Safety goggles and aprons should be worn at all times during this lab activity.

Six unknown substances and a control substance with a known pH
pH paper, LaMotte pH test kit, or red cabbage juice indicator solution
7 small beakers or paper cups
Paper towels

1. Label the beakers or paper cups 1-7. Place approximately 2 milliliters or each substance
to be tested in the appropriate container. For example, place substance 1 in the beaker or
cup labeled 1. Continue with substances 2-7.
Note: If you are using a chemical test kit such as the LaMotte pH test kit, follow the
directions found in the kit.
2. The beaker or cup labeled 7 is the control. The pH of this substance will be provided.
3.  Based on the list provided of the uses of substances 1-7,  estimate the pH of each
unknown and record the number in the data table in the estimated pH column.
4. Test the pH of each substance, including 7, and record the actual pH number in the
appropriate column in the data table.
5. Determine if each substance has an acid, base or a neutral pH.
6. Clean up and dispose of your unknown substances according to your teacherís

Data Table:


Hypothesize about what will happen to the pH number if you mix an acid and a base together. In the hypothesis make a statement about how the pH will change.

After the observations are complete, answer the following questions.

1. How accurate were the estimated pH determinations in comparison to the actual pH determinations?

2. Which of the substances is the strongest acid? How did you make this determination?

3. Which of the substances is the strongest base? How did you make this determination?

4. If a substance has a pH of 3, which of the unknowns could be added to it to obtain a substance with a pH of approximately 7?

5. Based on your answer to #4, if an aquatic ecosystem has an acid pH, what could be done to neutralize it? Would this be a long term solution? Why or why not?

6.. Describe the method used to determine pH. Compare it to the other two methods.Which method would be the most accurate? Why?

Note: If more than one of the methods for testing pH is available, make a comparison of the methods by testing the pH of the known substance using each method. Compare the accuracy of each of the pH determinations.

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©1998 April J. Cleveland for Science Junction, NC State University. All rights reserved.
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Last Modified: 6/8/98
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