“Close enough” — said no chemist ever!

Ever wonder where the common "it's close enough" mentality has come from? How students can regularly grab an incorrect piece of equipment because it’s a tiny reach closer to them than the proper choice? This introductory activity is designed to help demonstrate the differences in accuracy of measurement through a fun, competitive and hands-on experience that's sure to leave your students with an understanding of the importance of appropriate equipment choice — at least for making measurements! It is also an opportunity to review density and get practice measuring out liquids in glassware and using a scale.

In this activity, you will challenge the students to measure out precisely 10 mL of water. It takes a full period and typically I use it during the first week in a grade 9 or 10 class. By reminding your students that water has a density of 1.00 g/mL at room temperature, they should agree that when they place their sample on the scale, the scale should read 10.0, 10.00 or 10.000 (depending on the precision of your scale). Tell them that anyone who gets exactly 10.00 mL will receive a prize, just to make it a bit more competitive.

To begin, make sure that the only equipment available for them to use is a beaker — any beaker will work. Tell them that they may fill and refill the beaker as many times as they need to before they are certain they have 10 mL of water, however; they are allowed to pour their sample into the tared beaker on the scale only once.

When they are certain they have 10 mL of water, have them transfer the sample into a tared beaker on the scale. The scales are in their regular spot and peers are usually good about monitoring any cheating. Then have them record the weight of the sample on a spreadsheet either online or on the board.

They will quickly see that using a beaker for measurement causes a huge range of results, and it's pretty unlikely that anyone was able to get exactly 10 mL this way!

Next, unlock the graduated cylinder cupboard and allow the students to repeat the activity using a graduated cylinder. Again, allowing them to refill as many times as needed, but weigh only once.

After this is complete, bring out the pipets. This is a great first exposure to using a pipette, as it is completely safe and fun for the students to work on measuring out the 10 mL of liquid. During this part, you can expect to see several students getting exactly 10.00 g on the scale, and several more getting within the 10.0 g accuracy. Students feel such a great level of accomplishment here when they realize that their success (or lack thereof) is very much dependent on the tools that they choose for the job.

Finally, bring out the burets. Depending on the timing, and endurance of your students you may not be able to have all students use a buret, so small groups may be needed now. Of course, everyone should be able to get pretty excellent results with the buret!

More follow-up ideas for discussion could include:

  • The difference between TC (to contain) and TD (to deliver), and talk about the accuracy of transferring samples from one container to another.
  • What types of things are reasonable "sources of error" in a lab report, and how using improper equipment isn't one of them!
  • How much your results and the success of your lab depend on your lab skills.
  • Areas in life where extreme accuracy is needed.
  • An introduction or review of significant digits.

Hopefully, this activity makes your students aware of the importance of selecting the right tool for the job!