Archive for August 24th, 2008
This is a standard demo, one I did with my 8th grade Physical Science class and it stuck with them. It uses sugar to show the difference between a physical change and a chemical change. The first step is to dissolve sugar in water and then evaporating the water over a low flame. I usually use a beaker over a burner. The sugar will crystallize out and can be dried and returned to its original form.
The second step involves heating sugar in a test tube until it carmelizes and turns to carbon. The kids smell the change and associate the smell with a property change. We try but can’t get the mess to turn back into sugar.
If you haven’t done this before, don’t go by the picture, it’s just a photo I found on the web. You want to gently heat the test tube with the sugar. You only need a small amount of sugar (1/2 at the bottom of the test tube) and if you do it slowly and carefully, you will first see the sugar melt and then start to change. Gently waft the odors to the students as it starts to change. If you go fast, you will stink up the place. I often hold the test tube in my hands as I heat the bottom. It doesn’t get hot if you go slowly.
I usually throw the test tube out, it’s just not worth cleaning it once the change takes place. If someone knows how to clean it easily, please comment. Thanks.
I just got this idea in the shower this morning. Here’s the basic lesson:
- Divide the class into groups. Give each group a complete but non-working flashlight.
- Ask them to look at the flashlight, but not take it apart. They are to predict why it isn’t working and propose a solution. They need to come up with a test to show their prediction is correct.
- Once documented, they can take the flashlight apart and attempt repairs.
- Each time they reassemble the flashlight and it doesn’t work, they need to stop and come up with a new written plan.
- The group must document each step as they try to determine what is wrong with their flashlight.
Since these are very simple devices, they can be set up as follows:
- One or more dead batteries
- Tape over battery terminal
- Batteries installed incorrectly (positives together)
- Dead bulb (see update below)
- Switch or broken circuit in flashlight
I would probably set each flashlight up with at least two failures so that the exercise is not over in a minute. For instance, if I installed the batteries incorrectly, I would also make sure that one of them is dead.
There are two ways they can problem solve. They can share parts with other groups to make the flashlights work or there can be a pile of batteries, bulbs, and bodies to use. I’m also not sure if I want to have meters available. I may have them available but only if someone thinks to ask for one.
I’m not sure how I want to conclude the lesson. I would probably go into a class discussion of how they developed their plan and made sure the test did what it was supposed to do.
This looks like it’s going to be fun to watch. I need ideas for wrapping it up.
Update… We did this lab today (9/11/08)
I wrote a lab procedure for this: Flashlight Lab. There were only a couple little changes. I used clear plastic between the bulb and the bulb holder. It made the bulb look perfectly normal, but it formed a nice invisible insulator that served to fool them for quite a while. For the switch, I put clear tape over the contact from the switch. Some of them found that though, it was hard to disguise and it stood out.
I didn’t let the students share information between groups. I told them I didn’t have spare batteries, so they had to come up with a way to test them. If they were clever enough to ask, I let them use a multimeter or spare parts from other flashlights. I didn’t show them how to use the meter, but I did set it up properly. They figured it out on their own, which impressed me. I was hoping they would think of swapping parts with their neighbor, but they didn’t.
This lesson served two purposes: it demonstrated the scientific method since they had to cycle through the hypothesis-test-results process several times until they figured out everything that was wrong. The lab also served as a lesson in following directions. One of the groups did exactly what they were to told not to do and took everything apart and got it working in a minute. They had no documentation, hypothesis, or controls. They couldn’t tell me the three problems with their flashlight, their experiment was worthless. I told them that they failed the lab and they also showed me I can’t trust them with chemicals and fire in the lab if I can’t trust them with a flashlight. Hopefully that made an impression.
This experiment was done in my 12th grade Physical Science class and the overall response was positive. The kids liked the hands-on challenge and were impressed at how devious I was at sabotaging the flashlight. I think this is definitely worth doing. Let me know what you change and improve.