The toothpaste challenge

In the first week of classes I use the toothpaste challenge for my grade 12 university level classes. It is primarily a review for proper lab book protocols and reminds students to record data properly with both correct uncertainty and correct significant figures. My usual protocol is to check and help as the lab progresses. With this introductory lab I answer A LOT of questions; however, answering these questions now ensures future labs run smoothly. This is my opportunity to review lab reports, frequently encountered terms, recording observations, etc. 

Things I look for in the Observations:

1. uncertainty is listed;
2. uncertainty corresponds with the recorded value (i.e., correct significant figures); 
3. proper units are listed; and
4. no calculations are done in the observations.

Things I look for in the Calculations:

1. sample calculation is shown; 
2. the established rules of significant figures are followed (especially when adding or subtracting);
3. final results have the correct significant figures and units;
4. in my course we do not carry uncertainty into the calculations; however, if that is part of your curriculum, it would be a great addition to the lab. 

For this article, the procedure is written out; however, 
I do NOT give students any written instructions. I talk students through it step-by-step (or add notes on the blackboard as needed). This allows me to re-teach different components as needed, while preventing students from knowing what is coming in the experiment (a small element of surprise is needed for Part 2). My hope is to give you a framework you can adapt for your class. 

Prepare and review lab book protocol

I have students write down and underline the title and date. Students also list their partners, purpose, special safety and waste disposal. This serves as a reminder to my students that this is required for every lab we do throughout the semester.

Purpose

To review basic lab protocols using toothpaste.

Special Safety

Do NOT eat the toothpaste.

Waste disposal

All toothpaste ultimately ends up in the garbage. Be sure to clean up the equipment and the lab bench when done!

Materials

Watch glass, tube of toothpaste, wooden splint 

Part 1: Lab instructions 

Procedure

1. Find the volume of the toothpaste (written on the tube).
2. Determine the mass of the toothpaste (and tube).
3. Determine the mass of the watch glass.
   Observations should therefore be as follows:
     i. Volume of toothpaste (listed on tube)
    ii. Mass of toothpaste + tube
   iii. Mass of watch glass.
4. I have my class perform a little “healthy” competition — a “Ready-Set-Go” scenario where students are asked to empty their toothpaste from the tube onto the watch glass as fast as possible (indicating to the teacher when done).
5. The following observations should be recorded.
   iv.  Mass of toothpaste + watch glass
    v.  Mass of “empty” toothpaste tube. 

Look up and record the density of toothpaste. (A value of 1.33 g/mL is used in this article and taken from the Colgate website.)

Part 2: Lab instructions 

I have a lot of fun with this part. I let the class know that getting toothpaste out of the tube is no real great feat; however, getting the toothpaste back into the tube — now that’s INCREDIBLE! 

1. For the second “race”, students are given just two minutes (adjust time as necessary) to see how much toothpaste they can get back into the tube using a wooden splint only. 
2. The following observations should be recorded: 
    vi.  Mass of toothpaste + tube
   (after toothpaste is put back into tube)
   vii.  Mass of watch glass with any leftover toothpaste
   (after toothpaste is put back into tube) 

Calculations

Part 1 

1. Mass of toothpaste removed from the tube (I call this experimental value.) 
   (two different ways to calculate)
   a) [mass of toothpaste + watch glass (iv)] — [mass of watch glass iii)]
   b) [mass of toothpaste + tube (ii)] — [mass of “empty” toothpaste tube(v)]
2. Calculate ACTUAL value using the density of the toothpaste and the tube volume (I call this accepted value.)
   Mass of the toothpaste in the tube = 1.33 g/mL x volume of toothpaste (i) 
3.  Using experimental value in 1(a) determine the percentage yield     (in other words percentage amount of toothpaste removed)
    % yield (removed) = [mass of toothpaste removed from the tube/ACTUAL mass of toothpaste in tube] x 100% 
4. Determine the experimental error
   "Experimental error="  "experimental value" /("accepted value"  ) "x 100" %
5. Find percentage yield and the experimental error for 1(b). 

Part 2

1. Mass of toothpaste put back into the tube (experimental value)
   (two different ways to calculate)
   a)  [mass of toothpaste + watch glass (iv) — mass of watch glass leftover toothpaste after toothpaste back into tube (vii)]
   b)  [mass of toothpaste tube (after toothpaste put back in)(vi) — mass of “empty” toothpaste tube (v)]
2. Using the experimental value in 1(a), determine the percentage yield of the amount put back into the tube. % Yield (back in tube) = [mass of toothpaste put back into the tube/ACTUAL mass of toothpaste in tube] x 100%
   Reminder the ACTUAL mass of the toothpaste has already been calculated.
3. Determine the experimental error.
4. Find percentage yield and the experimental error for 1(b). 

Discussion 

Possible discussion questions:

1. a) Controllable Errors — what would be an error that would not show up again?
   b)  Uncontrollable Errors – what would be an error that would always show up? 
2. List any assumptions made in this experiment.
3. Expand on calculations and discuss which technique would be better in terms of accuracy for both Part 1 and 2. 

Students get comfortable with the lab, and me, while enjoying and learning about what is expected.

I finish the lab by stating to the students that next week they will perform a lab that can unscramble an egg!!