Part One: When things don't go as planned...
This week we began a chemistry unit. The focus is on the differences between physical and chemical changes. It will be filled with what I hope is lots of fun lessons related to cooking and art materials. In addition to science content, students will practice using the steps of the scientific method and will design and conduct their own experiment as a final project.
We covered phase changes as one type of physical change yesterday, so today I introduced the concept of solutions. Using Vicki Cobb's book on kitchen science as my guide, I planned a lesson about sugar water solutions that is supposed to culminate in the creation of edible rock candy. Last night, I did a test run at home and things went smoothly, so I thought I was all set to go. Although most kids have a sense of where this project is heading, we set it up as a scientific experiment with me posing the question: What will happen if we dissolve sugar in water, heat it up, and then leave it for several days? Most students agreed with a hypothesis that the water would begin to evaporate and would leave some visible amount of sugar crystals behind.
Using Vicki Cobb's procedure, we dissolved sugar in water, heated the water and discovered that we could dissolve even more sugar in the water. Then we brought it to a boil and poured it into glasses and stuck craft sticks in the glasses, "just in case." We made observations about what the liquid looked like and left it for the afternoon. (I just realized, as I wrote this, that we didn't set up some sort of control glass of plain water, or unheated sugar water. Rats. I am a better cook than scientist.)
So mission accomplished. I left school thinking we'd check in on the glasses tomorrow morning and make more observations. Except for then I came home and discovered that the sugar solution I set up last night is a nasty slurry of sugary sludge, most of which is not clinging to the craft stick sitting in the middle of the glass.
This raises some questions:
1) Did I let the solution boil too long?
2) Would we have been better off using a piece of string instead of a craft stick?
and most importantly
3) Am I going to let my students eat this nasty sugar slime if it doesn't turn into the acceptable rocky candy form of sugar?
Here's what I have decided regarding questions numbers one and two. Who knows why this experiment didn't work? Either way, it will demonstrate that solutes can be recovered from solutions, showing that physical changes are reversible. And we got some good practice in with steps of the scientific method.
Question number three is a stumper. What do you think I should do with all the sugar sludge that may be forming in my classroom at this very moment?
To be continued...