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#638. Law of Multiple Proportions

I started out by giving students a group writing assignment. They could work together, but the writing had to be their own words. The assignment was to compare and contrast the three laws: Conservation of mass, definate composition and multiple proportions. They all could get the conservation one - no problem, and the definite compostion one usually doesn't give them too many problems, but that multiple proportions one always seems to raise questions and furrow brows. I let them work to a point of frustration in groups, then I ask them to sit back down and I do the Flinn Scientific "Old Foamie" demo. Be sure to read this demo from Flinn before you try it - they give lots of safety instuctions - hydrogen peroxide is a strong oxidizer and can be dangerous to work with - especially if it is new. I got a white burn on my thumb today from it. For this demo, you put 20 mL of Hydrogen peroxide in a tall grad. add 7mL of dish soap and then add a premeasured 10 mL of NaI and Stand back. I used water the first time around instead of H202 and pretended like something was supposed to happen. I said well, let's see and wrote on the board the formulas as I said, " I was supposed to take H202 (I wrote H20 on the board) ... and add some NaI and some soap", usually by this time the students say something like that's the formula for water isn't it? and I say, "ohhhhhh, that little 2 there makes a big differnce, doesn't it? Let's see what happens when we use H20." It foams and bubble real cool and they love it. I point out that the H and the O are in two different ratios - two different proportions. The same elements can be found in compounds and the difference in ratios gives them very different properties. Remember the safety precautions to this lab. The same thing can be done with 3% H202 from the store, but just pour it on raw liver. I learned this at the Hasti conference. It smells a little, but it's gross so the kids like it a lot. Use water the first time and then the H202 . Do the same chalk board discussion. Then I let the students go back individually or in groups to that writing assignment and the difference in understanding is so neat to watch - they just start writing away with such confidence. A picture is worth a thousand words. Thanks Flinn!