“What do you think will happen?”
“I think the water will go down”, stated one.
“I think the candle will go out and then the water outside the glass will rise”, added another.
“I think the candle will keep burning” hypothesized a third.
On it went. I smiled with each suggestion, anticipating their surprise when the results actually came in.
Before them sat a tea candle secured to the bottom of a glass dish. They had read their procedure and had been instructed to hypothesize as to what would happen to the level of water in the experiment.
After they had written their hypotheses, I allowed them to carry out the experiment. Deliberately they added the water, being careful not to add too much. Conscientiously they lit their candles, observing all safety rules. Then they delicately covered their lit candles with the jar and eagerly placed their faces at eye level with the candle and peered at their experiments through their goggles. The flame extinguishes and the water is pulled into the jar, rising up over the candle.
This is my favorite moment: the instant they observe something totally unexpected. Their mouths drop open, their faces lighten, they smile, and their eyes glisten with excitement and discovery. They look at me. I encourage them to explain what they have observed.
We then begin a great discussion on combustion. They easily identify that the reaction consumed all the available oxygen in the jar resulting in the extinguished flame.
I refer them to the combustion equations we’d balanced earlier.
“Hey, more oxygen is consumed than carbon dioxide is produced”, notes a student leading the group to talk about density of particles inside the jar as compared to outside the jar. How I love when they lead the way!
I ask them about the role of the flame. They then talk about heating up the air particles only to have them cool down again when the flame is gone.
It takes a little time but then they finally make the connection between the particle density and temperature change (ultimately, the air pressure) causing the water to be “drawn up” into the jar. There is something so satisfying in seeing the “lights go on” as each student grasps what has happened.
Of course they have to repeat the procedure several times. Anything with a flame is worth repeating. As long as they can explain combustion and its role in the experiment, I’m happy! They clean up and depart the classroom beaming and vibrant with newly acquired knowledge. And that is why I teach Science!