Freezing Water Bottle

5th grade students had been discussing how water evaporates using a model that predicts increasing volume with increasing temperature. But they realize there’s a problem: Water expands when it freezes. The first clip begins with Jack A. describing the discrepancy.

Jack A. recalls how when you put a water bottle in the freezer, you have to leave room for the water to expand so the bottle won’t get damaged. He contrasts this experience with a student’s idea that water molecules pack together when freezing: “Wouldn’t packed together mean smaller?”

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Not all students agree there is an inconsistency. DC argues that connecting lots of molecules would make one giant molecule—it would be bigger. Jack A. and Ben argue back, using kids in the classroom as an analogy: If all the kids clumped together on the little carpet, “we would be big, but we wouldn’t have to make the classroom expand.”

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After Jonathan says he isn’t following, Jack A. reiterates the problem, using the same analogy: If everyone is spread out around the classroom, like molecules in water, and then pack together, like molecules in ice, the ice formation should be more compact than the water formation. Jack B. pushes for a mechanism, asking, “then how does it expand…?” which Jack A. agrees is the question.

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Ben has an idea. He grabs two notebooks and smacks them together to represent the two water molecules merging, asking the class to “pretend there’s metaphorical water bottle around” them. He then pops the center of the notebooks away from each other, creating a hollow space in the middle.

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Ms. Filner says Ben’s idea helps her think about the phenomenon, that there might be empty spaces between the molecules. DC cites evidence of having seen little pockets of air in ice. Ella proposes an experiment to test that idea: Fill a bottle to the very top so there is no air. If the air is responsible for expansion, and all of the air is removed from the closed system of the water bottle, the water may not expand when frozen.

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Jack B. continues to push for a mechanism to explain the expansion. Ben repeats his idea that the molecules come together and then spread apart to become bigger, which explains the pockets of air in ice cubes. DC provides more evidence: When you put ice cubes in water, they crack, which he says is their releasing the trapped pockets of air.

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Jack A. proposes an alternative theory. Instead of the molecules merging together and then pushing apart, he suggests the air gets trapped between the water molecules as they are merging together. He takes a flat sheet of paper and crumples it up, representing a puddle of water turning into an ice cube. He points out that there is air trapped between the folds of the crumpled paper, similar to what might happen as liquid water turns to ice.