Saturday, May 19, 2012

Sugar High

Long ago, one of my chemistry sections surged ahead of my other chemistry section. I fought this briefly and then realized it was a futile effort, and instead of giving the fast class free time or forcing the slower class to speed up, I let them continue on at their own pace.

Typically, I try to keep all my sections together, but I just didn't see how it was going to work this time. Blame it on the sunspots if you want, but it ended up that both of my classes ended up working with food during class today.

My first hour made S'mores.

I have used this activity for years to help kids visualize limiting reactants in chemistry, and it never fails to delight. At first, it just seems like a time filler "hands on" type activity, but what I have noticed is that kids really do use this to visualize the reactants in a chemical equation.*

Smores Stoichiometry

Pretty simple, really. You want to make s'mores. You have a limited number of supplies. How many s'mores can you make?

What gets interesting here is our discussion of what constitutes the perfect s'more. It turns out that chocolate is the transition element of s'more ingredients and everyone has a different interpretation of how much you should use. Graham crackers are diatomic when found in nature and no one has ever considered adding more than one marshmallow. But once you have the formula for your s'mores it leads into a wonderful discussion of why you can't change that formula.

We also find the masses of the ingredients and that allows us to convert into "moles." Moles are extremely difficult for my kids to visualize. They have no concept of what one mole of substance really is, but they can see that a marshmallow has a mass of about 6 grams. This is real. This is concrete. So if we have 100 grams of marshmallows, how many do we have? I really need to work this in earlier because it isn't until now that it clicks for some kids.

And you will never see kids work faster than on days when they get to ingest sugar after their lab.

This activity has done more for my kids' understanding of limiting reactant than any other I have done. It's because they can SEE the limiting reactant. They know that they are running out of chocolate and someone is going to be left with the "dud" s'more. In fact, the next day, when we introduce limiting reactants with chemicals, I have kids asking "so is that the chocolate or the marshmallows?" Something visual in their mind that they can connect back to. Gotta love that.

So, now I'm thirsty.

Luckily, my next hour is hitting on molarity today. I do a lab with Kool Aid, that again, really helps visualize (or in this case taste) what it means to change the concentration of a solution. It's strange because a lot of my kids have never apparently thought much about concentration until now. I mean, they have all seem to have had Kool Aid that was improperly made, but it cracks me up to listen in on how they have reasoned it out.


This is actually the first day we discuss molarity at all, and basically, I give them the definition and let them figure out the rest. Each group has to figure out how many grams of Kool Aid makes each of three solutions of the correct molarity, then they taste each solution and relate it back to its concentration. It gives them a reference point for when I say we need 3 M NaOH instead of the 0.10 M NaOH and I think that makes a huge difference in their understanding. One of my kids decided this year that he wanted to make 3 M Kool Aid, just to see how much different it was.

So we did. We couldn't get it all to dissolve.

I almost didn't do these activities this year, simply because I wasn't sure of their actual academic value. My principal (of course) walked in towards the end of the Kool Aid lab. He often has a questioning look on his face when he enters my room, but today, it was a little more pronounced. 

Until one of my kids gave him a five minute lecture on molarity that would make any chemistry teacher proud.

*I ought to mention that I have had both of these activities for years and am fairly certain that I stole them from someone out there. If you are the original author, let me know so I can give you credit!

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