Enrichment Lesson: Sense of Scale

Overview
The pH scale is a representation of the balance between hydrogen ions (H₃O⁺) and hydroxide ions (OH^-) in a liquid. A low pH corresponds to high hydrogen ion concentration, in other words, the more hydrogen ions present, the fewer hydroxide ions present, the more acidic the solution. Conversely, a high pH corresponds to a low concentration, in other words, the more hydroxide ions present, the fewer hydrogen ions present, the more basic the solution. This concept is illustrated in the abbreviated pH scale below:

The abbreviated pH scale is a common way to represent the concept of pH, but lacks to convey an important concept about pH. The pH scale is a logarithmic scale, meaning that every step on the scale represents a multiplication of 10. If the pH of a solution decreases by one pH unit, that represents a tenfold increase in the concentration of hydrogen ions. For example, Lemon juice, with a pH of 2 (100,000 H₃O⁺ ions) is 10 times more acidic than soda with a pH of 3 (10,000 H₃O⁺ ions). This aspect of the pH scale is shown nicely in this pH scale graphic.

Even after explanation, this still can be an abstract concept for some students. This activity is designed to offer a sense of scale to a pH scale for students, showing just how far apart the numbers should be on a true pH scale. Students will quickly realize why the abbreviated version of the pH scale in found in textbooks. NOTE: Depending on the student's mathematical abilities, this lesson can be adapted to use Scientific Notation.

Objectives
Students will:

brainstorm/research common products and their respective pH levels draw a pH scale to scale and place the products under the appropriate pH level on the scale

Time
One or two 45 minute class periods.

Materials

rolls of white register/receipt tape colored pencils rulers and yardsticks calculator (optional)

Teacher Preparation

The Miami Museum of Science: the pH Factor

Procedure

Have students review a standard abbreviated pH scale.   Explain to the class that the pH scale that they are accustomed to seeing is not entirely accurate. Explain that the pH scale is actually a logarithmic scale, meaning that every step on the scale represents a multiplication of 10 and that they are going to create an accurate representation of the pH scale.   In cooperative working groups or as a class, have students research or brainstorm various products and their corresponding pH.   Distribute the rolls of register tape, colored pencils, rulers, yardsticks and calculators.   Have the students unroll and find the approximate middle point of the strip of receipt tape. NOTE: To conserve class time, complete this step for the students prior to class.   Have the students label the middle point as pH 7 - neutral.   Have the students measure 10 centimeters to the right of pH 7 and label that point pH 8. If the students found products with pH 8, have them list the products on the receipt tape.   Have the students measure 10 centimeters to the left of pH 7 and label that point pH 6. If the students found products with pH 6, have them list the products on the receipt tape.   Have the students calculate how far in centimeters pH 9 will be from pH 8. Measure to that point and label the receipt tape. If the students found products with the corresponding pH, have them list the products on the receipt tape.   Have the students calculate how far in centimeters pH 5 will be from pH 6. Measure to that point and label the receipt tape. If the students found products with the corresponding pH, have them list the products on the receipt tape.   Continue procedure until students run to the end of the paper (which will happen very quickly).   Have the students continue their calculations to determine how much more receipt tape they would need to complete the pH scale to scale.

Assessment Suggestions
You can print out the following questions and use these as a means to assess the students' learning

1. Why do you think an abbreviated pH scale is used in textbooks?
2. How much more acidic is a solution with a pH of 2 than a solution with a pH of 6?
3. How much more basic is a solution with a pH of 12 than a solution with a pH of 9?
4. How much more acidic is a solution with a pH of 3 than a solution with a pH of 8?
5. How much more basic is a solution with a pH of 11 than a solution with a pH of 5?
6. How long of a piece of paper would you need to draw a complete pH scale (using centimeters)?
7. Why do you think a change in a body of water's pH level of even one pH unit could be deadly for the organisms that live in the water?