In the previous article, “Designing a Unit in Science: A Three-Year Journey”, I shared the story of designing a new chemistry unit for grade 10 science. The next stage in this journey is the exploration of one lesson in detail, examining how the nitty-gritty pedagogical details get woven into one learning experience. If you haven’t had a chance to read the previous article, don’t worry! The important pedagogical themes to follow are:
Authentic problems. New learning is motivated by realistic situations and questions that people outside of school might ask.
Using science to teach science. New ideas and concepts are introduced using observational evidence. Our provisional understanding is tested through experiment. This helps answer the question, “how do we know this?” or “how did people figure this out?”.
On-demand skills. New chemistry skills are introduced “on demand”, when there is an immediate need to explain, describe, or calculate something new.
Time to see where the rubber meets the road!
Part 5: Finally, a Complete Lesson! Lesson 7 of the chemistry unit, “Burning Magnesium”, uses observations and simple experiments to guide our learning. The job of the teacher is to share the conventions of chemistry work and coach students through the thinking processes that are part of doing chemistry. And just like real scientific work, when we get stuck or find something surprising, we need to learn new skills and ideas. In this lesson, we introduce core skills like naming compounds and writing formulae, but only after encountering a situation that requires these skills!
Who uses this knowledge, anyways? This lesson is framed by a story that highlights the professional use of chemistry skills. We are doing a bit of role-playing, imagining that we work for the Consumer and Hazardous Products Safety Directorate of Canada. (They actually exist!) We have received a complaint about a consumer product — sparklers used at a birthday party! It is our job to determine the chemical products coming from the burning sparkler and decide if they are hazardous.
Safety First Every time we encounter a new substance, students look up its safety characteristics in a binder of SDS sheets that is at each group’s lab station. (The SDS is the safety data sheet describing the cautions and safety requirements for chemicals.) Safety is all about establishing good habits and routines. This seems like a pretty good one.
Particle Diagrams An important part of learning science is the development of conceptual models that help us represent and explain our understanding. We begin by drawing a particle diagram for solid magnesium, showing the arrangement of its atoms. This activates a lot of good scientific thinking that benefits from regular reinforcement. We start drawing particle diagrams in grade 9, so the diagram shown below would be nothing new for our grade 10 students.
Science should generate questions When we burn magnesium, we observe evidence for a chemical change.
But what is the reaction product? There was a time when no one had the answer to a question like this, so let’s not ruin the fun for our students! The steps that help us answer this question are an important window into the world of chemistry and its history. So, we need to figure out what the magnesium might be reacting with in the air.
We come up with many possibilities: the magnesium could be reacting with substances like oxygen, nitrogen, carbon dioxide, argon, and many more. Which are the most likely? Nitrogen is the most plentiful and oxygen is the most reactive. After reflecting, students will realize that argon is non-reactive and carbon dioxide is used to extinguish fires. The two likely possibilities turn into hypotheses that we can test. This thinking process gets to the heart of doing science; this is how we come up with convincing explanations for how our world works. And notice how we are using lots of diverse chemistry knowledge as part of our reasoning, just like real chemists! The example of burning magnesium was chosen because its cool (well, actually hot) and its products can be captured and measured, showing a very interesting increase of mass! Here is how we generate one of our predictions:
The other possible product is magnesium nitride, which becomes the rival prediction. Until we do the science, we just don't know what the actual product is! And unless we have rival predictions based on all the likely hypotheses, we don’t know how reliable a positive test result actually is! (What if the mass of magnesium nitride was close to that of magnesium oxide? Experimental results pointing to magnesium oxide could actually point to both. Science is sneaky that way.)
Name that compound Even though we are in our seventh lesson of the chemistry unit, this is the first time we encounter a compound whose identity we don't know but who's composition we do (or at least that we predict). It is only now, in the example above, that we have a genuine need to create a name for a compound! Before this, we were simply told the names of compounds as we started using them — the names were on the containers. We introduce new skills only when they are needed. The skill is now relevant and immediately useful because we need to talk about this compound!
Proportions of elements Historically, a lot of painstaking work went into the determination of the proportion of each element within a compound. Now we introduce the well-known “criss-cross” technique to produce a formula, which is an amazing labour-saving strategy. But there is always a danger when we introduce shortcuts: that the science behind them becomes lost. The differing ionic charges of magnesium and chlorine lead to the 1:2 proportion of elements in this compound. This arises naturally from the electrostatic interactions of these particles and their random motions! They just bump around until they settle into these proportions (thermodynamics is awesome). To illustrate this, I created two simulations:
As an awesome bonus, we can see that ionic solids don’t have distinct molecular units, just repeating ratios of elements. I set up this simulation in my old-and-trusty Interactive Physics program, plugged in the charges, and the physics did the rest! Visually exploring ionic solids is a natural consequence of the choice to draw particle diagrams. These visual tools encourage all sorts of rich explorations. As a teacher, we can choose how to emphasize deeper understanding like this. We can mention it in an off-the-cuff kind of way to plant the seeds of later learning, or we can dive right in and explore why these solids are not molecular substances.
Testable Predictions Back to our two possible reaction products! With a little work, we create a prediction for each hypothesis. We can compare masses before and after the reaction to determine a mass ratio.
Did you notice the scaffolding we still have for our equation balancing skills that were introduced in lesson 6? After a discussion, we determine that we need to trap the smoke of the burning magnesium to properly compare the masses. I did this in a crucible to slow the reaction and limit the smoke production. Here are the observations:
We figured out a chemical reaction! From the experiment video, we see the mass of the solid change from 0.16 g to 0.25 g, giving a mass ratio of 1.56. The result exceeds the mass ratio of Mg2N3 (1.38) and is close to the mass ratio for MgO (1.66). This supports the hypothesis that magnesium oxide was the product and rules out magnesium nitride. (There is even some evidence from the video supporting why the ratio is a bit lower than the prediction — smoke!) Armed with evidence, we can finally put together the chemical reaction! We represent our reactions using multiple strategies in a Reaction Chart: words, equations, and shapes. In physics education research, using multiple representations has been shown to deepen student conceptual understanding. Each different representation highlights or illustrates different aspects of the chemical reaction. The representations reinforce one another by encouraging accuracy and consistency. Cognitively, this helps students bind differing ideas into one robust understanding of a reaction. This doesn't happen when skills are used in isolation, in long lists of practice questions. There is a lot of understanding and skill required to complete one chart: separately, each element of this chart could be rote work, but together they form a rich task.
So, is it safe? At long last, we can return to our day job as safety scientists. We look up magnesium oxide in our SDS book:
Did the magnesium oxide smoke cause the illness at the party? It is not a hazardous substance, so this is not likely. It was more likely something in the food! Or just some stomach bug running through the children.
Energy in the chemical system We finish the lesson with a look at energy changes in this chemical system, represented in an energy flow diagram. Our chemical system consists of the magnesium and the oxygen of the air. Inside the system, chemical energy (energy stored in the configuration of the particles) transfers to thermal energy. The system is now warmer than the environment, so energy begins to flow out of the system resulting in an exothermic reaction.
One challenging aspect of this is the need for a starting “kick” of energy from the flame of the Bunsen burner. This causes students to guess that this reaction is endothermic. I show a video of a wood model I created to illustrate energy transfers and the need for some activation energy to break the starting bonds.
Time to practice Rather than doing pages of exhaustive skill practice naming ionic compounds and determining compound formulae, we introduce skills in small pieces and use them regularly throughout the unit. The skills will become more complex as the lessons progress and are regularly reinforced through repeated use in interesting contexts. New skills soon combine with other skills; they are seldom used in isolation. The homework exercises are connected to a meaningful context, so students are never just moving around symbols.
Why this works and why it’s important Here we reach the end of the unit design journey. Congratulations on making it all this way! I could go on and on about each individual lesson, but it’s time to wrap things up. After teaching this for two years, the final point I would like to make is why this approach is successful. Learning is more interesting and enjoyable when:
there is a meaningful context
there is an emphasis on making sense of what is happening
we can make and test surprising predictions
we are not overwhelmed with content that is not used in a meaningful way
These four factors help our teaching reach a wider range of students, encouraging them to become engaged and successful. While the Burning Magnesium lesson is more challenging than a traditional lesson, it is also more rewarding, as it combines scientific inquiry skills with a broad set of chemistry skills not usually found in grade 10. By working in cooperative groups and carefully building skills, average students and traditionally less successful students can do some very impressive and engaging thinking. Don't underestimate the motivational power of being able to make and test predictions - it has an amazing effect on students. If you have never seen this, you must try it to believe it!
Try it out and learn more The full set of chemistry lessons and PowerPoint can be found here.
If you have any questions or would like to visit my class, please feel free to email me at the address at the beginning of this article.
