My head just exploded

I’m sitting here grading chemistry tests and cleaning bits of my brains off the walls after reading one particular student answer.

Here’s the situation: We do a freezing point depression lab involving dissolving dichlorobenzene in cyclohexane and monitoring how the freezing point changes. Pure cyclohexane freezes at about 6.6oC, and when the dichlorobenzene is dissolved into the cyclohexane, the freezing point drops a few degrees. The students use an ice bath to cool the cyclohexane enough for it to freeze while monitoring temperature with a digital thermometer.

On the test, I have a freezing point depression calculation – asking them to calculate the expected freezing point for a mixture of cyclohexane and dichlorobenzene (the same substances they have already used in the lab). To their credit, most students had no trouble with the calculation. One answer, though, just blew my mind.

The freezing point of the dichlorobenzene solution is 170 oC.

One hundred and seventy degrees Celsius. 70 degrees higher than water’s boiling point. For a solution that was frozen in the laboratory in an ice bath.

Ow, my head.

(This is what comes of treating real-world problems as math exercises without stopping to think that these numbers mean something.)

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