When I taught high school biology and chemistry, I noticed that when the subject of the pH scale was broached, there were generally two responses: one group of students knew that it simply existed and the other group had a slight knowledge of it that stopped at “acids at one end and bases on the other.” I also observed that many college prep biology and chemistry courses omitted it, leaving it to be covered in AP chemistry.
There are some pretty good reasons for this. First of all, the relationship between pH and concentrations of acids and bases requires the use of logarithms and knowledge of molarity. In addition, the act of making solutions of a certain pH can be a very frustrating experience. It is also very confusing to think of atomic rearrangements in solution and for those of you who have never had the joy of adding acid or base to achieve an optimal pH, it takes a very careful hand and incredible patience; in other words, skills that many students struggle with, which is all the more reason for them to try it!
The first step to effectively teaching acid and base chemistry is to temporarily put math in the backseat and discuss gross observations regarding pH. In a well protected and ventilated environment students added sodium hydroxide (NaOH) caplets to a hydrochloric acid solution (HCl). The caplets bubbled and the test tube heated up dramatically which the students all immediately responded to with a sense of wonder. Eventually, they noticed that a white solid that didn’t look like sodium hydroxide appeared on the bottom of the test tube. It was more crystallized, and some students even observed that it looked like salt. Without teaching any theory, the students learned that some pretty interesting stuff had happened so at this point, we took a break to explain the reasons for the reaction they had just seen.
For those unfamiliar with chemistry, the reaction between NaOH and HCl produces water (H2O) and table salt (NaCl). When I told the students this, it was met with a series of baffled looks and confusion, and many asked if they could drink the resulting solution. They can’t, BUT the reaction did take two chemicals that were foreign or scary sounding and produced two completely innocuous materials that they were all familiar with. From there, we discussed what happened to the actual reactants and how they came apart in water, the breaking of those bonds being the thing that produced the heat. I compared the breaking of a chemical bond to the breaking of a pencil, and that whenever anything is broken a small amount of energy in the form of heat is generated, though this didn’t answer the question of how water and salt ended up in the solution.
I next wrote on the board that if the NaOH and the HCl come apart, it forms Na, O, H, H, and Cl (we initially ignored the charges and the O and H existing as a hydroxide ion). I then asked the students what combinations they could make with those elements, and they quickly saw NaCl and H2O as two possibilities. From there, we discussed what chemically determines what an acid or base is. To do that, I brought out sulfuric, phosphoric, and acetic acid and showed them the chemical formulas. The students noticed that they all started with a hydrogen, and that the bases (lithium hydroxide, calcium hydroxide, potassium hydroxide) all ended with OH so that the H and OH form water, and whatever is left will combine to form salts. This ended our gross discussion of acid and base theory. We could now perform titrations to investigate the properties of the pH scale.
Titrations are used to determine the concentration of an unknown acid or base. It is performed with a long and expensive piece of equipment called a biuret that holds about 100 mL of an acid or base of a known concentration. An Erlenmeyer flask that contains a known volume of acid or base (if the biuret contains the base, the flask contains the acid and vice versa) is placed underneath the biuret. The flask also contains a few drops of a chemical indicator called phenopthalein which turns pink when exposed to an acid and cloudy for a base. As the students add liquid from the biuret into the flask, the indicator changes the color of the solution allowing them to see the pH changes in action.
The goal for this lab was for students to reach the equivalence point where the solution in the beaker below was neutral and could be confirmed with color as well as a pH meter or strips. At the equivalence point, it is possible to use a special formula that relates the concentration and volume of the base with that of the acid: Cbase x Vbase = Cacid x Vacid where Cbase is the concentration of the base (for us, 1 to 2 M), Vbase is the amount of base added from the biuret, Vacid is the volume of the acid (50 mL in our acid) and Cacid is the unknown. However, reaching the equivalence point is not an easy task. Given the logarithmic nature of the pH scale, the pH increases in a non linear fashion as the base is added. Initially, the biuret can be left open for seconds at a time with only a slight change in ph but must be added drop by drop as the solution reaches neutrality. I had plenty of acid and base on hand, as the students needed many attempts to reach the equivalence point. They were frustrated, but their timing and aim improved over time. Eventually, all of them ended the experiment feeling elated with their accomplishment of reaching the equivalence point. In addition to teaching them a valuable lesson about pH and titrations, turning the stopcock of the biuret in time improves fine motor skills.As an extension of our original experiment, we also titrated a weak acid solution with a strong base and discussed the nature of a strong vs. weak acid.
After a few short sessions, students from ages 8-14 now have a working knowledge of the pH scale and the chemical make -up of acids and bases. They even learned a little bit about a log scale and titrations, and they had a great time doing it!
(Will add images soon)