Published on January 10, 2008
Trials and Tribulations of “A Homemade pH Lab”(Or, How My Students Turned Lemons Into Lemonade): Trials and Tribulations of “A Homemade pH Lab” (Or, How My Students Turned Lemons Into Lemonade) A Middle School Prior Knowledge Investigation/Lesson by Laura Delo Fall 2004 Interview Questions: Interview Questions Based on discussion with the future earth science teacher of these physical science students. 1. What do you know about pH, have you heard of it before? 2. What is an acid? A base? What does neutral mean? 3. On the pH scale of 0-14 (this is the measure of pH), what part is acidic, what part is basic, where is neutral found? 4. What happens when a strong acid and strong base are combined? Selection of Students: Selection of Students Chose 6 students who would represent the diverse population of the class. Students were interviewed individually. Interviews were oral and recorded, lasting 3-5 minutes each. Student Responses: Student Responses Question 1: pH None of the respondents had heard of pH before. I had to explain that pH was not the local high school, ‘PH’. Student Responses: Student Responses Question 2: acids, bases, and neutral Only two responded with citrus fruits. One other replied,“Acid is something that can burn, [it’s a] powerful liquid.” One other student replied that neutral, “Deals with neutrons.” Sources say that students often have the misconception that “only acids are harmful” (Brooks, 1992) or that any pH other than neutral is bad in some way (Grady, 2004). Student Responses: Student Responses Question 3: the pH scale Only one student correctly labeled scale, even though he said he had never heard of, nor seen a pH scale. Brooks (2004) also says that students see the pH scale as “additive” (I.e. neutral being 0, bases @ 7, acids @ 14.) Student Responses: Student Responses Question 4: reaction of acids/bases Three replied “neutral” (correct) One responded, “powerful gas” One responded, “foam” One responded, “explode” Lesson to Address Prior Knowledge: Lesson to Address Prior Knowledge Began by using boiled red cabbage juice as an indicator of pH on various household items. Grouped items based on indicator color change. Discussed characteristics and similarities of like-colored samples. From here had students try to create the scale. Based on this knowledge, had them elaborate on a pH related topic (action of antacids in the stomach.) Day 1 of Lesson: Day 1 of Lesson Gave students time to free write on what they knew about pH. Had them discuss definitions while creating notes. Began testing samples like lemon juice, cleansers, water, etc. with the cabbage juice. Created a table to record findings. Day 2 of Lesson: Day 2 of Lesson Students were excited to rotate through all of the sample stations. We were able to regroup to discuss results and group samples by color. We then began talking more about acids and bases and what their actual pH measure is. We were able to get to the scale and label it (including the range of acidity and basicity, and neutral.) Changes in Conceptions?: Changes in Conceptions? Whole class evaluation: 71% Focus group evaluation: 79% 5 of 6 in focus group could answer “What is pH?” The sixth student was actually absent the first day when we went over this. Students still had a hard time characterizing bases, but they improved on characterizing acids. Student Responses:Lingering Misconceptions?: Student Responses: Lingering Misconceptions? Before “I know what acid is…it’s something that can burn, like a powerful liquid.” “[Neutral] Deals with neutrons.” After “[Acids] burn a hole in things.” Did not answer, but did not repeat old response either. Student Responses:Some Good Progress: Student Responses: Some Good Progress Before 5 out of 6 students originally incorrectly labeled pH scale Students responded “powerful gas” and “foam” Student responds“explode” After 5 out of 6 correctly labeled scale. Respective students both respond “neutral” “change” (not quite there, but close!) Analysis: Analysis Improvement was seen mainly because they were receiving their first real lesson on pH. Some misconceptions could be coming from scientific jargon being used in everyday language/marketing (Nakhleh, 1992). This could explain why they were able to identify with acids more easily than bases. Most student ‘misconceptions’ were resolved after the lesson. Conclusions: Conclusions Keep it simple to get through the lesson, don’t leave any information hanging! Students did a great job of making this lesson work, even with all the unexpected changes! ALWAYS HAVE A BACKUP PLAN!!! Slide16: References Brooks, D. (November 29, 2004) pH of Familiar Products – Conducting the Laboratory Activity. Retrieved from http://22.214.171.124/chemistry/LABS/LABS11.html Nakhleh, M. (1992). Why Some Students Don’t Learn Chemistry – Chemical Misconceptions. Journal of Chemical Education, 69(3), 191-196.