I have often joked that, in addition to fume hoods, chemistry laboratories come equipped with giant invisible brain vacuums mounted above the door. These giant vacuums suck the common sense right out of students’ heads as they walk into the laboratory.
How else can we account for the fact that many students lose all common sense while they’re in the lab?
Here’s an example. We’re performing a specific heat lab in my introductory chemistry lab. Students heat up a metal sample by placing it into a test tube suspended in boiling water. Since the water is boiling, the metal evnetually reaches the same temperature as the boiling water: 100oC. The metal is then put into a cup which contains a known amount of room temperature water. The students then measure how much heat goes from the metal sample to the room temperature water.
The lab manual asks students a queston: Why is the metal sample placed into a test tube and then lowered into the boiling water rather than being placed directly in the boiling water?
Putting the metal sample directly into the water – obviously – gets it wet. Transferring hot water along with the metal sample to the cup will make the temperature inside the cup go up more than expected, and the energy calculated will be too large.
One student had some trouble with the answer, and asked for a hint. But the problematic part of the answer might not be what you think …
Me: So, how is the metal sample going to be affected if you put it directly into the boiling water instead of the dry test tube?
Student: I don’t know.
Me: Okay. Hmm … Let’s say you’re riding in a boat and you fall overboard. What do you get?
Student: You get … tired? From swimming?
Me: How about this? You jump into the pool. How are you different after you jump into the pool from the way you were before you jump in?
Student: You’re … wet?
Me: Good. So now all you need to think about now is how that metal sample being wet with hot water before you put it into the cup would affect your results.