My typical cookbook lab goes like this: Measure out a small amount of sodium bicarbonate (baking soda) and add some acid to create salt, water, and carbon dioxide. Boil away the water to leave the salt behind. After the lab, write a balanced equation and calculate how much salt should have been made. Then determine the percent error in the real product and explain how their mistakes contributed to the percent error.
My re-do went like this: Ask the students to write the balanced equation and calculate how much sodium bicarbonate they would need to use in order to make 2.5 g of salt. My hope was that this would focus them on the task and get them thinking about what they were about to do. Instead of asking them after the lab how much salt they should have done, I took a recipe approach - if you want to make this much product, how much reactant do you need? The procedure for the lab remained the same, but I hoped that there would be a different level of thinking during the lab because they completed the calculations before.
So how did it go? The lab itself took longer because students needed to complete the calculations before starting the lab. With only 48 minutes to go over homework, complete these calculations and do the lab, we were really pressed for time. The payoff, though, was in the work that came after. The average grade on this lab was 5% higher over the two previous years' versions of the lab. Does that mean the students understood it better? Maybe. It also could mean that they helped each other with the pre-lab calculations and that improved scores. Where I really noticed the main difference - though this is completely an anecdotal observation - was in what they wrote in their error statements. I ask students to explain their percent error. If the product they made weighs too much, what else is in the dish? If the product mass is too low, where did the product go? Students often struggle with this task, but most of the error statements in this madeover lab were focused and logical. This makes me wonder if they did understand the task better. I am not sure, but I know I want to try this strategy again during second semester to keep thinking about it.