Hydrate Labs, Microscale Chemistry and Cage Fighting....

Bottle cap set up for hydrates

What are we doing to help kids achieve?

     For as long as I can remember, my students have done some type of hydrate experiment. It is usually sandwiched in between the concepts of empirical formulas, molecular formulas, percent composition and reactions. It tends to fit well there. The lab is basically a glorified empirical formula lab and it bridges a nice gap that leads into reactions. For years my students would heat the hydrates in glassware, burn themselves, break the glassware and splatter salts and thus their data, all over the lab bench. A few years ago  came up with a great microscale technique. Essentially, it takes a used bottle cap without the plastic and this is used as the dish. Next, there is about a three inch machine screw that goes through a drilled hole in the cap. A nut is placed on the screw to hold everything in place and cheap pliers are used to hold the entire assembly over the flame. About two grams of hydrate fits in the cap.

    So here is how it played out when I tried this method with my own classes. Students were given some copper(II)sulfate pentahydrate. They recorded the mass of the empty assembly and then the assembly with the hydrate before heating. They heated the material to drive off the water. Before they measured the mass after heating they had to hand me their calculations and written prediction and then I let them measure the mass of the dish and the anhydrous salt to see how close their predictions were to the actual results. The good news is that most of the student predictions were within 1% to 5% of the actual value.  It was also fun as a teacher to watch them find the mass and see how close they got to their predictions.

     I wanted to do something different for the unknowns. Students took the mass before and after heating. I told them that they had to find the ratio of the salt to the water for the unknown. They were only allowed to ask me one question and it could not be "What is the formula of the unknown salt?". Some students asked for the molar mass of the unknown salt. They calculated moles of salt to moles of water based on their data. Others wanted the molar mass of the entire hydrate. They found the percent of water and percent of anhydrous salt based on the data. They then used that to find the percent of water in the molar mass of the hydrate and found the moles of water. They reasoned that for every one mole of hydrate they should get one mole of the anhydrous salt so they found the molar mass of the anhydrous salt from the percent composition data from the experiment. They could calculate the ratio in a round about way. This format worked pretty well.

     The fun part was watching this as a teacher. Each of the groups thought that there was only one way to solve the problem and their way must be the correct way. This is where the yelling and screaming came in (O.K.....I exagerated about the cage fighting....but things got intense...). They were convinced that if their procedure was correct, any other way must be wrong. I did not tell them there was more than one way to solve the problem. Typically, we want students to defend their ideas and usually the teacher is the one asking the questions. This time it was the students challenging each other. I wish I could say that I planned this. It was just a happy accident. Afterward we talked about doing science that makes sense and having the courage to stick with your plan.

     By the way, if you want to try this you can order the bottle caps online for a few bucks and get some screws and nuts at the hardware store. The whole set up cost me about $10 and should last a few years....my kind of lab. Maybe students are not using the same equipment as chemists, but I am more concerned that they think like chemists first.

     Do you have a lab that challenges students in a different way? If so....don't be afraid to share...

     

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Comments 2

Jennifer Clarke | Wed, 07/26/2017 - 09:03

This is a great idea- thank you for sharing.  I have had the same issues with kids spilling or breaking glassware, and this sounds like a great alternative.