A Simple Acid-Base Intro Lab
We're nearing the end of the year and this is when I do my simplified acid-base unit with first year chemistry students. We've just finished solutions and concentration, so this is a good extension unit where all of the same ideas are present (ions, concentrations, etc) in a specific context. It's also a good unit to probe students for previous learning because, at the surface, they all have a general idea of what acids are and I'm able to push into that knowledge to help them build a fuller chemical model.
For this lab, we start with a card sort). I ask them to group up the properties however they'd like as a way to get them thinking about the macroscopic observable properties for acids and bases. Many of the cards are "either/or," so I don't do much questioning unless they have contradictory cards in the same pile.
Once they have their categories, they set the cards aside and grab samples of eight common household solutions. I use 24-well microplates because it's simple cleanup and they're only doing qualitative tests. It keeps the lab tidy and I don't have beakers all over the place to clean up later. Each sample is tested with strips of Congo Red and pH paper. Then they add a couple drops of phenolphthalein as a final qualitative check. The Congo Red gives a nice deep blue in acids while phenolphthalein is a bright pink in the base. The pH paper reintroduces the pH scale and teases out their acid/base pH ranges from their memories.
Finally, they go back and re-sort their cards. Most groups land on "lemon juice is acidic" as a starting point and they're able to tease out acidic and basic properties from there.
The full Google Doc procedure is here if you want to take a look. I'm also curious if you do something similar and what suggestions you'd have for extending or improving this particular activity.
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