Change in Mass Lab

The Modeling Chemistry Change in Mass lab has students make six different changes and compare the mass before and after these changes.  I love the idea of this lab and it is always interesting to see students’ ideas about mass sneak out as they predict what will happen with the changes. ( Update:  Here is a copy of the lab handout I use U1 mass & change lab.  Feel free to modify it for your classroom.  Note that it includes the grids I use for grading the lab.  I may be moving away from grading this – more on that in a future post…)

Day 1 – Part 1

After a brief discussion of mass and volume, students are asked to mass a piece of steel wool before and after they stretch it out.  Students generally seem to easily buy-in to mass remaining the same when stretching the steel wool.  There are usually a few cases of groups that have a decrease in the mass and a student usually readily comes up with the explanation of steel wool pieces lost in the stretching (just look at the workspace covered in bits of steel wool).  We develop a histogram together to display our class data.

Day 2 – Parts 2-6

For parts 2-6, I release my students into the lab with a little less hand holding.  For part 1, I had to familiarize them with the lab equipment.  Now that they know how to use the tools, they should be able to do a bit more on their own.  They have general directions for each part of the lab, but…

Here’s where things can start to get a bit interesting.  For example:

Student: “Do we have to include the mass of the beaker after we mix the sugar into the water?”

Me: “Hmm…  What do you think?”

Student: Stares at me probably thinking – Uh, aren’t you supposed to tell me?  Then looks back through lab papers, discusses with lab partner…

This is also where the data tends to get a bit more interesting!  For each part below, I describe what students did and what we might expect to happen, and show our histograms.  We actually did these out-of-order to make timing work better (for Part 2 we need to wait while ice melts) and so that I can give individual attention to groups working with the burners for the first time for Part 4.

Part 2

Students melt ice in a vial.  One would expect mass to remain constant.  For a balance with enough precision, one could argue an increase in mass would be expected when the vial is warmer (see here for a detailed explanation).  An increase could also be seen with condensation forming on the outside of the vial.

Part 3

Two solutions are mixed to form a precipitate.  One would expect mass to remain the same.

Part 4

Student heat steel wool using a burner.  One would expect mass (of our system – the steel wool, a pie tin, and the crucible tongs) to increase because the steel wool undergoes a chemical reaction with the oxygen in the air forming an iron oxide.

Part 5 & Part 5 Revisited

Students dissolve sugar in water.  One would expect (and I think at least some of my students did – although I also heard other expectations) that mass would remain the same.

As students looked at the first round of results, several expressed confusion.  So I led them to agree to retry the experiment.

Part 6

Students dissolve Alka-Seltzer in water in a loosely capped vial.  One would expect the mass (of our system – the vial, 1/4 of an Alka-Seltzer tablet, water, and lid) to decrease since some of the gas from the reaction escapes.

Where have we gone wrong?

So why don’t my students’ results match what we would expect to see?  There are various reasons.

  • Our balances are not as precise as we need them to be for this (and other) experiments.  So our results that are off by 0.1 g either way, really could be nearly zero.    [Note to self:  Next request for funding should target good balances.]
  • Students were just getting the hang of massing EVERYTHING before and after.  I had more than one group whose original results for a lab would be significantly off.  Then they would realize they forgot to include a cap or vial or…  So they would mass that missing item separately (it may or may not have been their own item) and factor that into their “change in mass”.  It is some of these mistakes that make for great learning experiences!

Day 3 – Evaluating Results and Then What?

As I write this, I have led my students through Days 1 & 2.  Day 3 is coming up on Monday. (My school has several days off in September and October due to holidays, so we have a VERY slow start.)

When we left off before a week and a half break from Chemistry, there seemed to be agreement that Part 1 made sense.  Histograms for Parts 2-6 (including the both trials of Part 5) were underway (finishing them for homework).

My plan is to lead with the histograms – see if we can identify a trend and come up with a reasonable explanation for the trend or if further investigation is needed.  This seems to be a touchy area to skillfully guide students through.  They know you know what you are looking for, but how do you suggest additional data is needed without stepping into that authority role?  Maybe we’ll be okay with parts 1 & 3 and I can convince students to collect at least a few more data points for each of the others.

>>My only reason for hesitating with this approach is my class schedule.  During September I meet with my class for seven class periods over five days (6 hours and 40 minutes to be exact, but who’s counting).  Then I won’t see them until October 10th.  It’s hard to get much done under these circumstances, but I think establishing a precedent for careful lab work and student involvement, even if it means we go slowly at first.

So I think we will end up have a few groups repeat parts 2, 4, 5, and 6 (maybe four more data points for each?) – and hopefully by October we will have a solid understanding of the Law of Conservation of Mass!

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