What am I doing to help kids achieve?
How do I know when they are there?
What is the evidence?
I am facing what many teachers are facing. It is AP week, I am trying to continue "as usual" with doing labs and learning but this time of year is anything but "as usual". There is a rates lab we do this time of year which is a good lab, rather involved with a significant amount of set up and work. I got an idea for a slightly different rates lab from Bob Worley. I found a similar large scale version from Flinn Scientific. Thanks to Bob, I decided to do a microscale version.
Here is the lab. Students take a solution of sodium thiosulfate and add a few drops of hydrochloric acid. The reaction ends with sulfur forming and making the solution cloudy. There is a suggested series of reactions that occur before the sulfur forms. There are some concerns with doing this on a large scale. First, there is a good chance of getting sulfur dioxide which smells bad and can form sulfuric acid. Second, it is hard to know exactly when the reaction ends.
Here is what the students did on a microscale. We counted drops of sodium thiosulfate. In our system, 20 drops of 0.1 M sodium thiosulfate and 2 drops of 1 M HCl had a reaction that lasted about 2 minutes. Students did other reactions with fewer drops of sodium thiosulfate and more drops of water (18 drops of sodium thiosulfate, 2 drops of water....16 drops of sodium thiosulfate and 4 drops of water etc...). The also tested substances, like copper, to see if it would act as a catalyst. Although we did not get to it, I also examined temperature on my own. Instead of using hot water baths, I placed the chemicals in a microwave for about 15 seconds. The solutions heated up just enough to show a difference in the reaction rate (caution..do not heat the sodium thiosulfate above 60 degrees). This was simple, quick, easy and gave great results. Students also counted the drops of solution that would make 1 mL so they could use this as a conversion to calculate the concentrations of the diluted solutions. Here is the part about this lab that I really liked. I had students use their cell phones and a watch to capture the reaction on video so they could go back and compare the reactions to determine the time for them to finish. Here is the video.
If they had trouble seeing how each reaction ended and the times, they could go back as many times as they needed and watch the video. Overall, the data was not bad. I would probably try this again next year with a few tweeks. I wanted it to be more inquiry but we just did not have the time to fully investigate or have students develop their own experiments, but you could do it and on a microscale it is O.K. if mistakes are made. Also, because of the small scale, the sulfur dioxide was not a problem. Give it a try...let me know what you think....