Co-op Science Week 27

Density, part 1

I think the activities in Inquiry in Action for this subject are great, but absolutely dreaded carrying them out in a carpeted second floor classroom far from any sinks.  So my brilliant idea this week was to announce to the Co-op Powers That Be that we would be holding class in the kitchen.

The kitchen has a big island counter that all the kids could group around.  I started by discussing what happened when you skipped stones on a pond (the stone eventually sinks) versus what happens when you throw a stick on a pond (the stick, no matter the size, tends to float).  Why is that?  Some of the kids answered with confidence, "Because the wood has more air in it."  Which is pretty much on target.

I stuck a giant sticky note paper on the refrigerator door and wrote out that density is a function of volume and mass/weight (explaining as I went that I was going to be using the term "weight" instead of "mass" a lot of the time -- our in-home vote was that kids this age need to be aware of the difference, but we don't need to grind it down in this situation).

I filled a plastic container with water, and threw a stone in it.  Then I floated a block of wood on it.  I took both of them out and put them in a bucket balance to show that the wood was heavier than the rock.

Next we compared the density of the wood and rock to the density of water.  To accomplish this we used displacement to come up with a volume of water equivalent to the wood -- I filled a container to the brim with water, then pushed the wood down into the container, collecting the overflow in yet another container.  I poured the overflow water into a cup, which I set in the bucket balance to compare to the piece of wood.  The water was heavier, of course.  We repeated for the rock, although it was almost impossible to get a good displacement volume of water for the rock -- the surface tension of the water made it difficult to find the exact spot at which we could get a good displacement reading PLUS trying to get the rock-plus-container out of the outer overflow container without slopping water all over was pretty much impossible.  But I think the kids got the point of how it worked.  And could see that the rock was heavier than an equivalent-ish volume of water.

Bonus discussion while setting up and doing all of this -- Archimedes.  I asked if anyone remembered that story, and one of the kids did, although he'd apparently forgotten about Archimedes running naked down the street yelling "Eureka!" Or perhaps his parents had left that little gem out of the discussion -- who knows.

I passed out copies of worksheet 7.2 and we went over it verbally (some of these kids still blanch when they see a worksheet, afraid of ridicule).  We also cleared up the misconception that ice floats on water because it has air bubbles in it (in a way it's true, but not in the way that person was thinking).

Then, the ever-popular activity of pouring corn syrup, water, and oil into a cylinder, and seeing what happened.  I used the glass jars from Frontera guacomole mix, which were perfect for this.  Plus at our house we go through that stuff like it's water, so it was easy to collect several glass jars.  The kids worked in groups, figuring out the order in which they'd like to pour their liquids.  All liquids settled the same way, regardless.  Then we tossed in pieces of dry pasta -- spaghetti and macaroni -- pieces of popsicle stick, paper clips (which tended to whack into the pasta on the way down), and broken crayon.  I hadn't seen this variation of throwing in other random bits and seeing where they hovered in the liquids -- it's pretty fun.  We discussed relative density.  And, of course, several kids asked if they could drink the mix (they always ask that -- very predictable), and then someone asked if they could stir it up with the coffee stirrers that were on the counter.  Sure, why not stir it up and see what happens.

Finally, we filled cups halfway with water, then threw in pieces of carrot, which promptly sank (during which various kids asked if they could eat the carrots, sigh ... sheesh, this gets old fast).  We passed out small containers of salt and plastic spoons, and suggested trying to make the carrots float midway in the water.  While working on this we discussed floating in salt water, the ocean, and the Dead Sea.

Clean up was So. Much. Easier. because we were in the kitchen.  Again, I think this would've been a much more tedious lesson to plan in a classroom without a sink available.  But teaching in a room set up for this type of thing made it a fun, easy lesson in density.