You might do it too. The teacher before you and the teacher before them had a unit at the beginning of the year to teach dimensional analysis and metric units. So, you teach dimensional analysis. I think we need to look past the millimeters and miles and look to the moles.
The students can feel it too. It's hard to convince them that should learn the metric system just in case America pulls it's head out and starts using the superior measuring system. They are pretty sure that won't be happening (and aren't you?).
I believe that we need to teach dimensional analysis with our primary goal being to prepare students for stoichiometry. We need to pound the logic of unit cancellations and the telling of a story within the process so that when kids are faced with the novelty of stoich, they have the framework developed. I will break down some of the ways that I instill these processes in the students.
1. Tell them that you are starting to teach stoichiometry skills
Right off the bat, I tell the students that converting units is only one of my goals. And it's actually a lesser goal. The real reason that I am spending time to teach unit conversion is that we will use this skill during a much more challenging unit. I don't even call it dimensional analysis with them. I call it, "Doing the Move." That is capital "M" move. It's that important. You will be doing the move a lot later on in the year. Let's learn to do it correctly.
2. Give them tools to make metric conversions efficient.
When students are efficient, they can use their brainpower on new concepts. Since converting to milliliters to liters is actually still new, if you can make it simpler, that will be better. (I know, I know, converting milliliters to liters should not be new to them. They have been taught it for years. Go ahead. Test it. See how many sophomores can accurately do more than a couple of these questions correctly. It's sad.) I use the Stoich Steps. It's a visual step tool to show them which way to move the decimal. It's not hard, but it gives them a visual to use. After using it, they become more efficient and can wean off of the tool. You can download it in the resource page.
3. Make them categorize the answer.
I came up with four categories of numbers. Before they get lost in a problem, I always want them to categorize the answer. I wrote a blog about it here. I will summarize.
Small Numbers: Less than one. Decimals. Fractions.
Number Numbers: Numbers that I might count. Tens. Hundreds. Small thousands.
Big Numbers: Too annoying to count, but I still know what they are. Hundreds of thousands. Millions.
Freaking Huge Numbers: Ones that I have to use exponents to understand. 6.022 x 10^23.
If they categorize a number first, and then plug it into their calculator and get a crazy answer. they at least know it's crazy. After 23 years of teaching, I am still blown away at the ability of students to trust their calculators when the answer is obviously completely idiotic. Dare I say that the calculator just does what the user tell it. I know what that makes the user...if they don't have a guess first.
4. Create story within dimensional analysis.
I had an AP student walk by me yesterday at lunch, and say, "I saw you teaching from the hall yelling in your classroom. Must be teaching the metric unit."
Yep. That is exactly what I was doing. You see, there is a story about converting between the metric system and the Imperial system. In my classroom we only speak metric. It is our language. But in the hall, they only speak English units. On the door to my classroom, I have four units taped up. On the classroom side of the door, it shows 454 g and 2.54 cm. On the hallway side it shows 1 inch and 1 pound. I create a physical story that the students can connect to. When I am way out in the hall, I might be at the unit of miles. In order to enter the classroom, end start speaking metric, I have to make it to the door. The door says inch. I have to convert my unit to inch before I can walk through the door. So, I physically do it. I stand off at a distance in the hall. Ask the kids how to convert miles to feet. Then I step closer. They tell me how to convert feet to inches. Then I step all the way to the threshold of the door. Now I have to walk through the doorway. That ratio is 1 inch is 2.54 cm. Now I can step into the metric world. Then, if I need to go to kilometers, I step further into the classroom. It gives the students a real physical way to interpret the purpose of changing units. It also gives a clear picture that there are particular units that we must reach to interconvert.
When it's time for stoichiometry, they know about doorways. And the doorway can simply become the molar ratio. See, we started in grams, are we at the doorway? No. Better get to the door. The only way to walk through the door is to leave from the reactant side and step to the product side. That happens in moles at the door. Want to go to liters of a gas at STP after that? Go for it. Just step away from the door. It's simple but it honestly helps kdis have a visual plan for what they are trying to accomplish.
5. Show how the story doesn't change the value, just the unit.
Unit cancellation is REALLY easy for some kids. Usually, it is the kids that think like you and me. The rest of the world has very little idea why they would want to put a unit on the bottom of a fraction versus the top. Reinforce the reasoning. Have them demonstrate the cancellation of units through every problem. They begin to feel that the story that was taught also has mathematical and physical implications.
6. Practice.
The more times that a student can go through the process of creating the story in their mind, putting it on paper, and getting a realistic answer, the better. I have two extra homework pages with keys posted digitally in our class. There is also a practice test with a key posted. They can see my handwriting doing the Move over and over and over and over... The more times they interact with it, the more skilled they will become. Then when stoich rolls around, you can focus on teaching the mole rather than the Move.
7. Remind them of bigger things coming.
"Stoichiometry is coming" can start to sound like the end of the world. Don't let it. Remind the students how excited you are to teach them a new skill that they will be able to use in a new way. They are going to be completely new students after this class. Let them enjoy the ride, while looking forward, but not fear it. You will be there with them. They will make it. Can't wait!
Hopefully this gave you a few ideas that might make the potentially dry subject of dimensional analysis have a better place in your curriculum as you move toward stoichiometry. After all, stoich rules.
Want a way to make the introduction to stoichiometry more fun? Want to teach the mole with no notes? Try using Up & Atom. It is the chemistry classroom game that helps teach the relatedness of the mole to atoms and grams. A classroom set can be used multiple times through the unit to build and reinforce the skills we all wat them to have.
Better yet, get a classroom set of Up & Atom with CHeMgO for a reduced price!
Comments