Monthly Archives: September 2015

Chemistry, more like cheMYSTERY to me! – Democritus

Quick recap from the first post of this series: I start the year with some underpinnings (scientific process skills that are necessary to survive in a Modeling classroom) activities. It is there that we establish how to build a scientific model.

Continuing my series on model building,  let’s talk about Democritus. 

Democritus’s Atomic Theory is the foundation for all of chemistry and is incredibly relevant today. This is where the chemistry Modeling Instruction curriculum starts. Democritus made the observation that if you break a rock into tiny pieces, those pieces are still made of rock. He then inferred  that if you broke that rock into tiny particles so small we can’t see them, they would still have the same rock composition. Therefore, all matter must be made of teeny tiny indestructible particles that Democritus called atoms (I don’t use the word atom until we get to Dalton to avoid confusion about compounds). The first model of the atom was born! There are some other parts to Democritus’s model like the properties of atoms are determined by the shape of the atom but I don’t address that.

I start all of chemistry with the above story about Democritus and tell my students that this is our current model of the atom because we do not have any other evidence to tell us otherwise. Then I do the exploding can demonstration because chemistry is all about blowing things up, right?

Exploding Can Demo

The exploding can demonstration helps establish the practice of drawing particle diagrams. Students are asked to draw a particle diagram before the can is lit, while the can it lit, and when the can explodes. They come up with all sorts of explanations with their particle diagrams. Sometimes they are dead on, sometimes not. The right answer is not as important as the discussion of particles.

From the exploding can demonstration you can generate some particle diagram rules.  The 4 I always have them come up with are:

  1. Particles are represented as circles, not dots
  2. Different particles should look different
  3. Include a key so we know which particles are which
  4. You don’t need more than 20 particles in your diagram

I always get at least one group that tries to represent particle motion with whooshies or arrows. When I see this I ask, “why are there lines coming off your particles?”, to which students usually reply, “because it’s a gas and gas particles are always moving.” I then ask, “do you have any evidence that particles move?”, to which students usually reply, “yeah, my 9th grade science teacher told me they do!” I followed that up with, “but how do you know?”  It sometimes takes a few more questions to convince the students that particle motion is not currently in our model but we may add it later if we have evidence to support it. I do not tell students how the exploding can works here because they do not have the background knowledge to fully appreciate the chemistry. Instead I bring it back on the last day of class and have the students try to explain it again with their more robust model of the atom.

The discussion of particles and particle diagrams leads us straight into the “Mass and Change Lab.”

Mass and Change Lab

The “Mass and Change Lab” is a fairly standard conservation of mass lab. I have edited the lab so it is not exactly the same as what is in the Modeling Instruction materials but it includes a variety of chemical and physical changes that gain, lose and keep the same mass (depending on how you define the system). I have students use triple-beam balances  during this lab to continue to reinforce the concept of significant figures.

After every group has collected their data, I have the class compile their data on the main board in the class. Each group writes whether the experiment gained or lost mass and if so, how much? The data will not be perfect. You can usually spot which groups forgot to account for the mass of a test tube or beaker and use it as an opportunity to talk about sources of error. Once we have established the mass change for each experiment, we whiteboard a before and after particle diagram for each mini experiment.

During this whiteboard session I ask students, “how are you going to show if the mass changed or stayed the same?” This is where students make the connection that the number of particles represents the mass.  If the system gained mass, it must have gained particles. If the system lost mass, it must have lost particles. Students can then answer the questions, “where did the extra particles come from?” or “where did the particles go?” These questions can lead to a discussion on “what is a system?” and “what are open and closed systems?” After we have established particle diagrams for each mini-experiment, I ask students to come up with a definition for the Law of Conservation of Mass. The class usually comes up with something like “the total number of particles stays the same in a closed system.”

Now that we have established that the number of particles represents the mass, we can move on to density.

Mass and Volume Lab

I introduce the concept of density with a set of density balls I got from Education Innovations.

The two balls have the same mass but the smaller one feels heavier than the larger one. I ask students to account for this observation by drawing particle diagrams of both balls. I do not give this explanation the name “density” yet. We simply discuss it in terms of “the mass to volume ratio.”

Next we do the density lab. I have a few sets of aluminum cylinders and PVC cylinders of various sizes that I use for this lab. Any standard density lab kit would work. I ask students to find the relationship between mass and volume for the aluminum pieces and the PVC pieces. At this point in the year the students are well versed in finding relationships so I set the students loose to collect and graph their data. They come back with completed whiteboards and a lot to discuss.

   

Students quickly see that their data split into two lines so they have to calculate two slopes and write two statements of relationship. On the boards pictured above, you will notice that I have my students additionally draw in the line for water so we can determine if the pieces will sink or float (steeper slope than water will sink, a shallower slope than water will float). I also have the students represent both substances with particle diagrams so they have a quantitative and qualitative representation of density. I ask many questions throughout the board meeting like, “what would be more massive, 20 mL of aluminum or 20 mL of PVC?” Or the converse, “what would take up more space, 50 g of aluminum or 50 g of PVC?” At the end of the whiteboard discussion, we establish that the slope is the mass to volume ratio which we call “density.”

I follow up this lab with some worksheets on density adapted from the Modeling Instruction materials with qualitative (particle diagram) and quantitative (graphing and proportional reasoning) density questions.

I also give students a density practicum based off of Flinn’s “Don’t Sink the Boat” activity.

Once students are comfortable with the densities of liquids and solids, we can determine the density of a gas.

Density of a Gas Lab

The “Density of a Gas Lab” is a standard collection of gas by water displacement (see Flinn’s “Scientific Laboratory Techniques Guide” for a good diagram). The gas is CO2 generated by Alka-Seltzer and water. Outlining the procedure for this lab can be a little cumbersome but my students always get great data (though there are always a few groups that need a few tries to get there).

After students have collected their mass and volume measurements of the gas they collected and calculated the density, I have them record their data on the whiteboard in the front of the room. Immediately students notice that the density of a gas is a really small number. I have students put that number in scientific notation and compare it to the densities (in scientific notation) of liquids and solids we know of. This allows us to discuss the term “order of magnitude”. I ask students “how many orders of magnitude greater is the density of water compared to the density of carbon dioxide?” Students can easily determine water is three orders of magnitude denser. What students don’t realize is that means water is 1000 times denser than carbon dioxide! That usually catches them off guard so I ask them to represent the average densities of solids, liquids and gases in 3 particle diagrams.

Students either overthink it and want their particles diagrams to be exactly quantitatively correct or they underthink it and just draw each diagram with an arbitrarily smaller number of particles. Each group presents the reasoning behind their boards and we compare each board to the actual data. After a few comparisons, students realize that to truly represent the density of a gas, they would have to draw a fraction of a particle. Since fractions of particles do not fit our model, they settle for drawing one particle in the gas particle diagram. This representation is not congruent with many textbook particle diagrams and is a big misconception among students.

We have now learned all sorts of things about how the number and arrangement of particles affects properties of matter but we still have one burning question; how tiny are these tiny particles?

Thickness of a Thin Layer Activity

I wrap up the first chemistry unit with the “Thickness of a Thin Layer” activity from the Modeling Instruction materials. In this activity, students must determine the thickness of a piece of regular foil and the thickness of a piece of heavy duty foil using what they know about the density of aluminum (calculated in density lab).

From this activity, students can determine a minimum particle size if the aluminum foil is 1 particle thick (the heavy duty foil is about 1.5 times thicker than the regular foil, so the minimum particle size is 1/3 the thickness of the heavy duty foil). I then show students a clip from “The Ring of Truth” about particle size. The examples in the clip get the minimum particle size down even smaller. You could also drop a known volume of oleic acid in a large bowl of water, calculate the area of the circle it forms and then calculate the thickness of the layer to get a smaller minimum particle size.

I wrap up the discussion by showing students the “Scale of the Universe” applet. This site does a great job of putting the size of a particle into perspective for the students (as well as the size of the universe). Make sure to show it with the sound turned up, the music is awesome!

That is the end of the first chemistry unit! To sum up the model so far…

  • All matter is made of tiny,  indestructible, hard sphere particles
  • The number and size of the particles determines the mass of the substance
  • The number of particles in a closed system does not change
  • The number of particles in a certain amount of space determines the density of the substance
  • Particles are really small; on the the order of 10^-9 m or 1 nanometer.

Chemistry, more like cheMYSTERY to me! – Underpinnings

One of my clever students wrote this on a whiteboard my first year of teaching. It was a  classic “ha ha, hmmm…” moment.

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I love the creativity, but that’s not really how you want your students to feel about chemistry! I bet a lot of students could empathize with this whiteboard. So how do we take the mystery out of chemistry?

I think the biggest “mystery” of chemistry is “how do we know?” How did scientists figure out what particles that are too small to see look and behave like? Early chemists like Dalton, Thomson and Rutherford often get passed over in chapter 1 of a textbook. They provide some interesting trivia questions, but nothing more. It seems like the best way for students to understand tiny particles is to observe them the same way Dalton, Thomson and Rutherford did. Enter Modeling Instruction.

I will be posting a series of entries throughout the school year under this title summarizing how the model of chemistry evolves in my class throughout the year. My curriculum ia based on AMTA’s Modeling Instruction curriculum with some tweaks.

The first topic of this series is “underpinnings.” Underpinnings are all the skills necessary to study chemistry that are not necessarily related to chemistry content. This includes the nature of science, calculating slope, stating scientific relationships, sig figs, and unit conversions. You can check out my 180 blog for specific activities I do to address these topics. For now, I want to discuss just the nature of science.

The nature of science is often taught at the beginning of the school year and then ignored for the rest of the year. The beauty of Modeling Instruction is that the nature of science is a thread that is woven throughout the curriculum. For students to really appreciate that thread and understand the model of chemistry, you need to establish the nature of science well at the beginning of the year. Specifically, you need to address the idea of building a scientific model.

The model of chemistry started with an observation. That observation combined with an inference became the first model of a tiny particle. Eventually an observation was made that did not fit that model, so the model had to be amended with the new observation and a new inference. Eventually an observation was made that did not fit the new model and the model had to be amended again. And on and on it goes as better technology leads to better observations. This is the story of science. This is the idea I come back to over and over again throughout the year. Even our current model of chemistry is only the model so far.

I teach this idea of “model building” using the wax block activity (see 180 blog). I love this activity because it is both a discrepant event and a black box. Students make an observation and combine that with an inference to describe how the block words. When they are given flashlights (new technology), their observations may not fit their previous models. That is okay, models can change. When the students are given laser pointers (even newer technology), their models might change again. When the students collaborate with other students, their models might even change again! Of course (to the students’ dismay), there is no answer key for science. All you have is your evidence-based model (so far).

This theme will come back again and again. My goal for this year is to get my students to truly understand what a model is and how having a model helps us to build a problem-solving framework. I foresee the question, “what’s your model” being asked frequently this year.

Next up in the series: Democritus and his tiny particles

The SBG buy in: Getting students and parents on board

If you are making the leap to standards-based grading this year, you might be wondering how to get students and parents to buy into the idea. Sometimes as teachers, our fear of having one of these situations:

stops us from trying something new. Real talk: switching to SBG is not going to be all unicorns and rainbows. Some students and parents will have a hard time understanding why are you making this change. It is 100% worth it. To help ease your transition, I have compiled some tips that have helped me get students and parents on board over the past few years (yes, I did learn some of these the hard way).

1. Do your research 

SBG is not for the faint of heart. If you are going to commit to it, make sure you know the ins and outs of your system and the reasoning behind it. You are going to have to defend what you are doing to administrators, parents and most importantly, your students. Make sure you know what you are talking about and you can answer any and all questions. Before I dove into my first year of SBG, I read A LOT of blogs. Just Google “standards-based grading” and you will find a wealth of information.

2. Get an administrator on your side

This tip is almost as important as the first one. You need an administrator on your side who both trusts your judgement and understands what you are doing. I have been very fortunate to have supportive administration throughout my implementation of SBG. Having administrative support will give you more confidence as you present this new system to students and parents. If a student or parent is having an especially hard time adjusting, you will know that someone has your back.

3. Always be positive

This one is the real difference maker when it comes to students. The switch to SBG directly affects the students. They are the ones you need to convince. The easiest way to do this is to stay positive. Yes, SBG is going to turn everything your students know about education upside down. They are going to feel really uncomfortable about it. It will be a hard adjustment. You do not need to tell your students these things, they already know them. Instead, tell students, “we are going to use a new grading system this year that I think you are really going to like. It really works in your favor!” Do this from day 1.

4. Don’t dump all the details on day 1

SBG is already overwhelming for students and parents, don’t add to it by dumping a ton of unnecessary details on them on day 1. Give students and parents the main paradigm shifts and an overview of the logistics. Tell your students “we are going to go through this whole thing together. After your first quiz, we will look at it and talk about what it means. I will make sure you understand what your grade means.” Just be reassuring and hold their hands through the process.

5. Keep it simple

This may not be universally agreed on by SBG enthusiasts, but I believe in keeping it simple, especially if it is your first year of SBG. There are a ton of different flavors of SBG. You can have tiered targets, you can do weighted averages, you can have power standards. These are all great and I have seen them work really well in an SBG framework but I think it is harder to get students and parents to buy into a system they do not understand. Pick a calculation method that is straight forward. I opt for median for the final learning target grade and then I average all the learning targets. If you do your research from tip 1, you should be able to come up with something that works for you!

Best of luck with your SBG endeavors. Remember, if you believe in it, everyone else will too!