Academic journal article The American Biology Teacher

Using Beads to Teach pH

Academic journal article The American Biology Teacher

Using Beads to Teach pH

Article excerpt

The inverse relationship between pH and hydrogen ion concentration is difficult for students to visualize. More often than not, students cannot comprehend why pH increases when hydrogen ion concentration decreases. While they can understand the pH scale, interpreting its association with hydrogen ion concentration is confusing. I play" a simple game with my students to help them grasp this concept. We then do a real experiment to confirm it.

[FIGURE 1 OMITTED]

Each group of students is given five tubes with a varying number of small beads (Figure 1). They count the beads and assign a value of 1 for every, 10 beads (e.g., 100 beads equals -10:50 beads equals -5). We then tabulate and compare results. This demonstration clearly shows the students that the more beads in a tube, the lower the numerical value, and vice versa. I then explain that the same concept applies to pH. A solution with more hydrogen ions ("beads") has a lower pH value than a solution with fewer hydrogen ions.

Now we do a simple experiment to reinforce the association between pH and hydrogen ion concentration. Students obtain 10 ml of 1 M hydrochloric acid in a test tube and determine the pH using pH paper. 1 then have them assume that 10 ml of this acid contains 1000 hydrogen ions, or beads. (Note: It is very important to emphasize that the number 1000 is used for simplicity and that it is not real.) Next, students add 1 ml of the acid to 9 ml of water in a second tube. I tell them to reason that if 10 ml of the original acid contains 1000 hydrogen ions, then 1 ml contains 100 hydrogen ions. Hence. the diluted solution has 100 hydrogen ions in a total of 10 ml. …

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