When Chemists Attack

As a chemistry instructor, one of the difficulties I face when getting incoming students proficient in the laboratory is that of using units for measured quantites. Often, students just don’t have a good grasp of how various units relate to each other.

To some extent, I can’t blame my students for being confused by units. In the USA, we use a system – and I use the word “system” here loosely – of units based almost entirely of a bunch of things that just seemed like a good idea at the time.

To see what I mean, stop and take a look at the set of units we Americans use for length:

• Starting with the small, we have the inch, which is just about the width of my thumb.
• Going a little bigger, we have the foot, which is the same thing as twelve inches.
• Going bigger than that, we can use the yard, which is 3 feet or 36 inches.
• Finally, for the big distances we have the mile, which is 1760 yards, or 5280 feet, or 63360 inches.

So, 1 mile = 1760 yards = 5280 feet = 63360 inches. Or, 1 inch = 1/12 feet = 1/36 yards = 1/63360 miles Simple, right? Easy to use and remember, right?

Of course not, since these units of length don’t relate to each other in any obvious and easily-remembered way. The situation gets even worse when you move to units for other quantities. Volume units, for instance, use completely different relationships between their units than length units do. A teaspoon is 1/3 of tablespoon, 1/6 of a fluid ounce, 1/48 of a cup, etc. – very different from the relationships between the length units. No wonder students are confused about units!

The solution to this problem is pretty obvious …. use metric. The most obvious benefit of metric is that students only need to learn one set of relationships (the metric prefixes) that work for all units, rather than a set of conversions for each kind of unit. Plus, the prefixes are based on powers of ten – which means that we can do metric unit conversions very easily.

The metric prefix milli- means 1/1000 – no matter whether you are applying it to the unit of length (as in the millimeter – 1/1000^th of a meter) or a unit of volume (as in the milliliter, 1/1000^th of a liter). Now that’s an easy system to deal with.

So, how well are we doing with metric here in the USA? Each semester I give a pre-test to my incoming chemistry students, and a few of the questions are about the metric system. In a recent semester, I found that:

• Just under twenty five percent of the students I surveyed in a college-level course didn’t know what the metric prefix “kilo-” meant. A little over seventy percent of them didn’t know what the metric prefix “centi-” meant.
• Of students who were surveyed in a non-college-transfer course (usually meaning that their goal was an associate’s degree), the percentage that didn’t know what the prefix “kilo-” meant was about seventeen percent, while the percentage that didn’t know what “kilo-” meant was about sixty four percent

What does that mean? It says to me that we’re not doing a good job getting the metric system across to students before they hit college. Either that, or all the complexity of the units in the US system is making our students confused with how all unit systems work. Food for thought …

I really like this story. It’s food for thought for all those people who think that education “should be run more like a business”.

“That’s right!” she barked, “and we can never send back our blueberries. We take them big, small, rich, poor, gifted, exceptional, abused, frightened, confident, homeless, rude, and brilliant. We take them with ADHD, junior rheumatoid arthritis, and English as their second language. We take them all! Every one! And that, Mr. Vollmer, is why it’s not a business. It’s school!”

(Thanks, PZ)