As I planned my acids and bases unit this spring, I was trying to incorporate an inquiry-based lab that involved the properties of acids and bases. Initially I thought I would give students unknown solutions and some equipment and ask them to identify the substances as acids, bases, or neither. Maybe I would make it more relevant by using household or common substances. I opted against that because it felt too elementary and, if I am being honest, wouldn't they just measure the pH and call it a day? I had to miss a day for inservice, so I left them a great PhET simulation to discover answers to some questions about strength and concentration. The image above is from that simulation; it shows the difference between an acid's strength and its concentration on a particle level.
When I returned from my absence, I still had not settled on exactly what we would do in the lab and my students had questions about the simulation. In my second period class, when I asked for questions from the previous day, one of my students said that he understood what they were supposed to get from the simulation, but he didn't feel he knew the right answer to the question of which property - strength or concentration - had a greater effect on pH. I started to answer, to tell him which one it was. But then I didn't. Instead, I said, "I am so glad you asked. Tomorrow we will explore that question in the lab."
The next day I provided two solutions, a solution of hydrochloric acid (a strong acid) and solution of acetic acid (a weak acid) with equal concentrations. I labeled the solutions as A and B so they wouldn't know which one was which. They had to use what they knew about pH to decide which was strong and which was weak. I also provided pH paper and asked them to design an experiment to answer the question "Which has a greater effect on pH: acid strength or acid concentration?" Their homework was to write a lab report. Here are some excerpts:
It's always fun to watch groups work on days like this. Some were completely mystified about why the pH wasn't changing as they added equal volumes of acid and water. It was a great lead in to our lessons about pH and especially the logarithmic nature of that scale. Students often smile and nod as I describe the scale as being powers of ten, but this year they had experienced firsthand how drastically different, in terms of dilution, a pH of 2 is from a pH of 3. The lab really got them thinking about the difference between strength and concentration too. Since those terms are used interchangeably outside of science - a strong drink is typically a very concentrated one - it is important to help them understand how different they are in chemistry. I think, based on the writing they did following the lab, that most did understand this difference.
When I do it again next year - and I will definitely do this one again - I will also provide a solution of a sodium hydroxide and ammonia so that they can use a strong base and a weak base in their experiments too. The trick will be, of course, getting a student to ask the question again!