Chemistry, more like cheMYSTERY to me! – Particles with internal structure

 

In this model building series, we last left off in Unit 4, a small but mighty chunk of curriculum. This unit introduced us to Dalton and his tiny particles called atoms. Here is what we learned:

  • All matter is made indestructible particles called atoms.
  • Different types of atoms are called elements.
  • All atoms of the same element are identical. Different elements have different properties.
  • Atoms combine chemically in simple, whole number ratios to make compounds.

I you remember, Dalton wasn’t sure how atoms combined together. It will be up to J.J. Thomson (and your students!) to answer that question!

NOTE: I do not follow the original modeling order where the mole comes next. I tried that my first year and it just seemed disjointed. I do not address the mole until right before stoichiometry.

Unit 5 is all about attractions. Bonds are not a stick or a hook that holds atoms together, they are electrostatic attractions. Bonds are also not something atoms “want”, because atoms are not people. I have wrestled with this unit for the last 3 years and I think I finally have something I like.

I kick off Unit 5 with an old favorite: the sticky tape lab.

Sticky Tape Lab

I know some people use this lab as a demo because it can be time consuming and sometimes the data are questionable but I think it is worth taking the time for. In the Sticky Tape lab, students observe the interactions between 2 charged pieces of tape and other materials including another set of charged tape, foil and paper.

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The tricky part of this lab is getting the tape charged correctly. I give each group a roll of tape and tell the students to give it to the best direction follower in the group. They are usually pretty self-aware. I then make the class go through the process of laying the base tape, bottom tape and top tape down on the desk, peeling up the bottom and top tapes together, stroking the 2 pieces of tape and then quickly ripping them apart as a class. Then, I go around and check to make sure every group’s tape is properly charged by discretely holding a piece of foil to each piece of tape before I allow students to collect data. Once students collect their required data, I encourage them to experiment with items around their desks. I also encourage them to rub those items on someone’s head and then see how they are attracted to the tapes, foil and paper.

The discussion of this paradigm lab really helps put the model in students’ minds. It is all about explaining microscopic phenomena using macroscopic observations. Students can quickly guess that the tape somehow becomes charged, but the important part is what that means for our model. Simply moving atoms would not make a piece of tape charged. There must be a particle within the atom that has a charge! The Thomson Plum Pudding Model is born! Charged particles were transferred from one tape to another, making one positively charged and one negatively charged. This is also a good place to talk about Benjamin Franklin and his designation of positive and negative charges.

Plum Pudding Model (1)

Students understand that the top tape and bottom tape are attracted to each other because opposites attract, but they have a hard time explaining how both tapes are attracted to the neutral paper and foil. I like the Balloons and Static Electricity PhET for this. Students can easily see that charged objects can displace the mobile negative charge in atoms to produce a partial charge. We then talk about how the electrons can move more easily in a metal because the positive core does not hold onto the electrons as tightly compared to a non-metal (soupy pudding vs sticky pudding). This explains why the tape was strongly attracted to the foil and only weakly attracted to the paper. This model also explains why metals conduct electricity and non-metals do not which can be easily demonstrated with a 9-volt battery/light bulb circuit. The discussion of electricity is a perfect lead in to conductivity testing.

Conductivity Testing

In the past, I have done conductivity testing of various atomic, molecular and ionic substances as a demonstration with a large, 110 V conductivity tester, but this year I decided to get crafty. I sacrificed a string of LED Christmas lights to make these mini conductivity testers.

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These are just a simple circuit with speaker wire as the leads, 9-volt batteries as the power source and Popsicle sticks as the base. The only tricky part was getting a good connection between the battery and the wire. Aluminum foil and a lot of tape proved very useful for this. This tutorial was also very helpful. I set up 6 stations for students to rotate through to test various solids and solutions for conductivity. The LED lights worked great at showing different levels of conductivity by lighting dimly or brightly.

Students were able to classify their data into 4 categories: elemental solids that conduct electricity, elemental solids that do not conduct electricity, solutions that conduct electricity and solutions that do not conduct electricity. After students noticed that the solutions that conducted electricity contained a metal, we named these ionic compounds and the other type of compound, molecular. Since ionic compounds conduct electricity, they must be composed of charged particles. The obvious next questions is, which is negative and which is positive?

Micro-Electrolysis of Copper (II) Chloride

This micro-electrolysis is an activity I have added to this unit to introduce anions and cations. I instruct students on how to set up a very simple micro-electrolysis with aquarium tubing, a 9-volt battery and mechanical pencil lead (.7 mm or thicker works best).

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After letting the electrolysis run for a few minutes, students see bubbles forming on the positive electrode and with some careful wafting, they can identify it as chlorine gas. When students look carefully, they see the negative electrode is turning a reddish-brown color. Students immediately call this rust. I always ask, “what is rust?” and the students reply “iron oxide?” I then ask, “is there any iron in the solution?” Students then realize that the reddish-brown substance cannot be rust and must be copper. Since the copper (metal) is attracted to the negative electrode, it must be positively charged. That means the chlorine (non-metal) must be negatively charged. This is when I introduce the terms “cation” and “anion.”

Patterns of Charge 

The Modeling materials has a worksheet called “Predicting Formulas” which I have students complete after talking about anions and cations. This worksheet gives students a variety of ionic compounds and helps them find the patterns in which the ions combine. I always intro this worksheet with, “we know from the last unit that we can find the formulas for compounds using mass ratios” so students understand where these formulas come from. After completing and discussing this worksheet, students can identify the basic patterns of charge for the main group elements.

We have been zeroing in on ionic compounds for a little while, but it is time to zoom back out and look at molecular and atomic substances as well.

Structure with MolView

In the past, I have had access to 7 laptops that I could install the Mercury Software on to look at the structures of various ionic, molecular and atomic substances. I am at a new school this year so I had to find a ChromeBook alternative. Enter MolView. MolView is an awesome Mercury Software alternative. It does not have all the compounds that Mercury does and you have to do an advanced search in the Crystallography Open Database to get unit cell structures, but it gets the job done.

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I had each student manipulating the structures on a ChromeBook and I also put the structures up on the SMARTboard so we could discuss them as a class.

I made a big deal this year about ionic compounds being bonded throughout because ions are charged spheres, meaning they attract particles of the opposite charge in every direction.

From looking at the structures, students constructed rules for classifying ionic, molecular and atomic substances. I always like to show students the structures of graphite and diamond to get the discussion started on “why structure matters?” Maybe that’s just the geologist in me!

After this activity, I have students complete the “Why Structure Matters” worksheet from the Modeling materials to relate structure to melting and boiling points.

This year, I added something new before getting to nomenclature. After talking about melting and boiling points, the next obvious place to go seemed to be intermolecular forces. In the past, I talked about how there are attractions between molecules that are not as strong as ionic and covalent bonds and in Unit 3, energy had to be put into a system to overcome these attractions to change phase, but I never gave these attractions a name.

Intermolecular Attractions (Forces)

I don’t like the term intermolecular forces so I call it intermolecular attractions (IMAs), because it is more descriptive of what is actually going on. For IMAs, I borrowed a lab from my colleague across the hall and “model”fied (that’s a thing, right?) it. Students timed the evaporation of 6 molecular substances: pentane, hexane (switching out for butane next year), ethanol, methanol, ethyl acetate and acetone.

img_0340 (1)I thought students might be bored by this lab because it is kind of like watching paint dry but they were actually very enthused about how quickly some of the substances evaporated and how they “disappeared” before their eyes. I heard some great hypotheses as the students talked about which ones would evaporate fastest: “it must have something to do with mass” and “these ones have oxygen in them and these don’t.”

Students saw that the evaporation times broke the substances into 3 groups: molecules without oxygen, molecules with oxygen but no OH group, and molecules with an OH group. I named the attractions in the first group “induced dipole-dipole attractions” and the second and third group “permanent dipole-dipole attractions” (because of the electronegative oxygen). I explained that the molecules with the OH group have a special kind of permanent dipole-dipole attraction called hydrogen bonding.  I of course had to introduce the term “dipole” and we talked about why permanent dipole-dipole attractions seemed to be stronger than induced dipole-dipole attractions. I did not use the terms dispersion forces or Van der Waals forces because they are not descriptive of what is actually happening.

Thoughts before moving on to nomenclature

I think this unit is the toughest modeling unit to teach because you have to teach bonding without the Bohr model. The great thing about that is you are not breeding the “atoms want 8 electrons” misconception. Atoms don’t “want” anything, they are atoms. Bonding is all about electrostatic attractions, not a set of rules. My advice is hit this hard!

Ions are formed by gaining or losing electrons. When an ion forms it is charged all over so it attracts particles of opposite charge in all directions. This is why ionic compounds do not exist in discrete, formula units. This is also why 1 sodium atom bonds with 6 chlorine atoms but only has a +1 charge.

By contrast, molecular compounds bond within molecules because the electrons of each atom are attracted to the positive core of the other atom. This is why molecular compounds do not form lattices but instead are held together by weaker intermolecular attractions.

I highly recommend reading Beyond Appearances: Students’ misconceptions about basic chemical ideas (Kind, 2004). The ideas in this paper really helped me get the big picture of this unit.

Ionic and Molecular Nomenclature

Moving on, the last thing to hit in Unit 5 is ionic and molecular nomenclature. I also took a new approach to this topic this year and had students work more independently than usual. I created a “Chemistry Ninja Warrior” system where students had to “level up” to different types of nomenclature. Different levels earned different cool stickers.

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Each level had a test (worksheet) that students had to demonstrate mastery on before they moved to the next level. The goal was for every student to reach level “ninja turtle” and more advanced students could move beyond that. Students worked independently or with other students at their level on POGIL activities to learn the nomenclature rules.

I liked that my students got to work at a pace that worked for them and I got to spend more time with the students who need  1:1 attention. I used the POGIL activities this year as is but next year I think I will edit them after seeing some of the snags my students ran into.

After spending some significant time on naming, the only thing left is a practicum!

Unit 5 Practicum

This practicum is more like a “demonstration of knowledge” than a lab challenge. Each group is given a set of tables containing names of elements and a pair of dice. How the students roll the dice determines the compound they will build. They must write the formula and name of the compound they roll. For molecular compounds, students roll the dice again to get the number of each type of atom in the molecule. I also have students construct a few rules for naming and differentiating ionic and molecular compounds. This is not my favorite practicum but it does a nice job of wrapping up the unit.

Whew! I think that is a hard unit to wrap your head around! Let’s sum up the model so far…

  • All atoms contain mobile, negatively charged particles called electrons whose charge is balanced by the positive (pudding) core of the atom. (Thomson’s Plum Pudding Model)
  • In metals, the positive core has a weaker attraction to the electrons so electrons can move more freely than in non-metals, allowing metals to conduct electricity.
  • Metals tend to lose electrons and become positively charged cations and non-metals tend to gain electrons and form negatively charged anions.
  • Ions are charged all over and attract ions of opposite charge from all directions. When ions of opposite charges are attracted to each other, they form ionic bonds. Ionic substances are bonded throughout and have high melting/boiling points.
  • When the electrons of two non-metal atoms are attracted to the other’s positive core, a covalent bond is formed. Molecular compounds are bonded within molecules but the molecules are only attracted to each other through intermolecular attractions. Molecular substances have lower melting/boiling points compared to ionic substances.
  • Molecules can be attracted to each other through induced dipole-dipole attractions and permanent dipole-dipole attractions.
  • Ionic compounds are named by writing the metal first and then dropping the ending of the non-metal and adding the suffix -ide.
  • Molecular compounds are named by using the prefixes -mono, -di, -tri, -tetra, etc. to denote how many atoms of each element are present in the compound. The first element only gets a prefix if there is more than 1. For the second element, you must drop the ending and add the suffix -ide.

Thanks for sticking with me through that one! Stay tuned for Unit 6: Chemical Reactions!

Chemistry, more like cheMYSTERY to me! -Dalton’s Model

We left off in this model building series with the very meaty Unit 3 on heat and temperature. While I love Unit 3, it is mentally taxing on my students and myself. Unit 4 is a welcomed break.

Here is where we left off in our model:

  • If temperature is a measure of the “hotness” of a system then heat is the quantity of “hotness”
  • Heat can go into a system (endothermic) or flow out of a system (exothermic)
  • Heat can be stored in 2 energy accounts: thermal and phase
  • A change in thermal energy means a change in particle speed and is shown by a slope on a temperature-time graph
  • A change in phase energy means a change in particle spacing and is shown by a plateau on a temperature-time graph
  • The quantity of heat transferred during a temperature change can be calculated using the mass, specific heat and change in temperature for the system
  • The quantity of heat transferred during a phase change can be calculated using the mass and heat of fusion or vaporization for the system

I start Unit 4 with a challenge…

Mixture Separation Challenge 

To kick off this unit, I give each group of students an Erlenmeyer flask with a mystery mixture in it.

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I have the students observe the mixture and try to figure out what 3 particles it is made of. Sand and salt are easy to identify but the iron filings give them trouble. When I hold a magnet to the flask, at least one student in the class is able to identify the iron. I then set the groups loose to come up with a plan and materials list to separate the mixture. When their plan is approved, the students get to work. I did not have time this year to let students boil the water off their salt so they just focused on recovering the sand and iron filings.

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I make it a competition and award a small prize to the group with cleanest separation. This year, in honor of Dinovember, the winning groups received dinosaur shaped fruit snacks.

I also talk about distillation here and usually relate it to that person everyone knows who makes moonshine in his garage. Once students understand that physical properties remain the same when particles are physically mixed together, it is time to chemically combine particles.

Making and Breaking a Compound

I start by mixing sulfur powder and iron filings in a test tube and showing students that each substance retains its properties. Then I heat the mixture over a flame. This is best done in a hood since the sulfur gas can be quite noxious. I like to set up my iPevo doc cam so students can see what is happening in the test tube on the SMARTboard. After a few minutes of heating, it is clear that something new has been made. That something new does not have the same properties as the original sulfur and iron. This demonstration is part of the first worksheet for this unit from the Modeling materials. One of the questions requires students to draw particle models of the original mixture and the new compound.

Now that we have made a compound, it is time to see if we can break one apart. Typically, I use a Hoffman apparatus to show the electrolysis of water because you can collect enough gas to show the unique properties of hydrogen and oxygen. This year, a Hoffman apparatus was not available so I had students electrolyze water at their desks with 9-volt batteries. This was not a perfect demonstration but served the purpose of showing that water particles can actually be broken down further. Looks like we just broke apart a compound and our model! I also show the Ring of Truth video on electrolysis of water. This is where I introduce the term “element” and the periodic table. This is also the point in time where my periodic table fell off the wall and attacked me. The element of surprise is real.

Once students have distinguished elements, compounds and mixtures and pure substances I have them practice drawing and classifying a variety of particle diagrams to check their understanding.

Now that we have established that elements can combine to make compounds, we must determine the ratios in which these elements combine.

Avogadro’s Hypothesis

I use the worksheet from the Modeling materials to introduce Avogadro’s hypothesis. As a class, we explore the observations from combining volumes of gases to predict the formulas of various compounds. We also find out from this worksheet that some elements are diatomic.

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The problem is, most elements are not found as gases at room temperature. How do we figure out the formulas of other compounds?

Laws of Definite and Multiple Proportions

I also use the worksheet from the Modeling materials to explore the Laws of Definite and Multiple proportions. This worksheet has students explore mass data to conclude that different elements must have different masses. We can then use the mass ratios to determine the formulas of various compounds.

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Democritus to Dalton

I wrap up Unit 4 by having students complete the Democritus to Dalton reading on their own and taking a short reading quiz on Google Forms. I no longer have students complete the Dalton’s Playhouse activity because it seemed to confuse students more than help them. That is an activity I would like to redesign for next year though. I always like students to be able to answer the questions, “how do we know?”

Unit 4 is short and sweet but brought some big changes to our model!

Here is what we added to the model so far…

  • All matter is made indestructible particles called atoms.
  • Different types of atoms are called elements.
  • All atoms of the same element are identical. Different elements have different properties.
  • Atoms combine chemically in simple, whole number ratios to make compounds.

From Democritus to Dalton was big leap, but Dalton’s fish hook hypothesis about bonding will not be sticking around for long.

GW188H177

 

SBG and PowerSchool: They can be friends!

When I started using SBG, the gradebook my district used was not compatible with my grading system so I paid for an ActiveGrade subscription from my own pocket. I posted overall letter grades in the district system but grade details were in ActiveGrade. Students and parents were able to log in and see their grades but the biggest complain I got was, “I don’t like having to look at two gradebooks.” That was a completely valid complaint. As a teacher, I didn’t like having to keep track of two gradebooks.

I moved districts this year and my new district uses PowerSchool. I had no experience with PowerSchool so I asked my Tech Department if it could work with SBG. Luckily my Tech Department is awesome and put in a lot of time with me to get it up and running. I know a lot of districts use PowerSchool so here is a rundown of how you can make PowerSchool work with SBG.

The only catch is, you can’t do this on your own. You need to make friends with your district’s PowerSchool administrator. Once you and your PowerSchool administrator are best buds, you can jump right in!

STEP 1: Get your targets in the system

SBG is centered around your learning targets, so that is where you need to start. Get your PowerSchool administrator a list of your learning targets so he or she can put them into the system. This is where you want to plan ahead because you cannot edit these yourself. Be nice to your tech people and give them everything at once so they don’t have to keep going back in to add things. If your targets are coded, make sure to tell your tech team to include the code in the target description otherwise students will not see it.

STEP 2: Plan your rubric

Your rubric is the scale you use to grade each target. I use a “not yet”, “almost”, “got it” system which translates to “0”, “1”, “2” in my gradebook. You need to have numbers and percentages for PowerSchool to calculate your grade. I use “0%”, “50%” and “100%” respectively. PowerSchool will calculate a final grade across all of your targets using these percentages. If you are a percent mastery grader, you would want to make your mastery level worth 100% and everything else worth 0%. This is something your PowerSchool administrator needs to set up and assign to all of your learning targets.

STEP 3: Set your standard grade calculation method

Finally, something you can do! In your PowerTeacher gradebook, go to the ‘Tools” dropdown menu and click “preferences.” Go to the “standards” tab.

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Check the boxes I have checked and set your calculation method. PowerSchool can do mean, weighted mean, median, highest, mode and most recent. Click “OK.” I don’t use higher level standards or “power standards” but you could set that up as well.

STEP 4: Add your targets to the grading quarter

This is the easiest step to forget and PowerSchool will not calculate final grades without it. In your PowerTeacher gradebook, click the “Grade Setup” tab and double-click the grading quarter you are currently in to bring up the settings.

GP Setup

Click the radio button for “Term Weights/Standards Weights” and then click “Add Standards.” This will bring up a dialog box where you can check all the targets you want to be calculated into the final grade. You can always go back and change this later.

STEP 5: Enter Grades!

You now have a gradebook. Yay! It’s time to meet your new best friend, the standards drawer. In your PowerTeacher gradebook, go to your “Assignments” tab and create a new assignment.

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Make sure your assignment is out of zero points and use the “standards” tab to tie learning targets to the assignment. Save the assignment and go to your “Scoresheet” tab.

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You will notice a box with an “S” in it appears with your assignment. Click that box to open your standards drawer. This is where you can put in your grades for each learning target for your assignments. NOTE: The box turns green when all of your students have grades for the assignment. Once you enter assignment grades, your gradebook should start calculating final grades for your students.

The “Scoresheet” tab has some nifty views for analyzing all of the data you are collecting. If you click the “Final Grades” button, you can see how your class is doing on each of your learning targets. If you click the “Student View” button, you can track the progress of individual students on their learning targets.

reassessmentFor reassessments, I make an assignment called “Unit X Reassessment 1” and tie all the targets in that unit to the assignment. As students reassess individual targets, I add the grades to this assignment. If a student reassesses a target from a unit more than once, I make a new assignment called “Unit X Reassessment 2” and record scores there. That makes it easy for me to see how many times a particular student has reassessed a single target.

STEP 6: Tell your students

Lastly, let your students know they can now see their standards-based grades in PowerSchool. You may need your PowerSchool provider to turn on the “standards tab” so students and parents can see scores for their learning targets when accessing PowerSchool from a browser. The app has a “standards tab” built in.

Good luck! Please comment with any questions or clarifications.

Chemistry, more like cheMYSTERY to me! – Heat and Temperature

I fell a little bit behind in posting after I finished Unit 2 in class so now it’s time to play a little bit of catch-up!

Here is a quick recap of where we left off in our model so far…

  • Particles are always in motion
  • Temperature is a measure of the average speed of the particles
  • Pressure is a measure of the number of particles colliding with a surface
  • Pressure and volume are inversely proportional (Boyle’s Law)
  • Pressure and temperature in Kelvin are directly proportional (Gay-Lussac’s Law)
  • Pressure and number of particles are directly proportional (Avogadro’s Law)
  • Volume and Temperature in Kelvin are directly proportional (Charle’s Law)

Unit 2 set up the idea of energy that Unit 3 builds off of. Let talk about energy!

Ice Melting Blocks

I kick off Unit 3 with one of my favorite discrepant events, the ice melting blocks. You can purchase these two black blocks from Flinn. I have students pass the blocks around the room and make some observations. Students observe that one block feels heavier and colder. They infer that the blocks are made of two different materials. I then ask the question, “which block will melt an ice cube faster?” Almost every student will say, “the warm one.” I always get a few students who say, “the cold one because the opposite of what I think will happen always happens in this class!” I put a cube of ice on each block and you can guess what happens:

That is indeed the “cold” block on the right. After students get over their amazement, I have them whiteboard an explanation. This leads to a great conversation about energy transfer and whether “coldness” is transferred or “hotness” is transferred. We also talk about thermal conductivity, refrigerators and some other material science applications.

That demonstration sets us up for the Icy-Hot Lab

Icy Hot Lab

The Icy-Hot Lab (from the Modeling materials) is an incredibly simple lab but still absolutely worth doing. Every year, my students have the same 2 misconceptions coming into this unit: the temperature changes during a phase change and the bubbles in boiling water are made of air. The Icy-Hot lab allows me to address both of those misconceptions

The set-up is easy; students heat a beaker of ice until it all melts and then eventually boils. All the while they are measuring the temperature at a set interval (Vernier Lab Pros are great for this). I used Bunsen burners for the first time this year and got much better results than when I used hot plates in the past.

This lab produces a nice heating curve which gives way to a discussion about thermal energy and phase energy. Students must answer the question, “if temperature is not changing during the phase change, then what is?” When students get stuck here, I ask them to draw particle diagrams for a liquid and a gas at the same temperature. That makes it easy for students to see the difference is in the spacing of the particles. The energy is going into to separating the particles! Now, can they do it backwards?

Cooling Curve of Lauric Acid

I tried a new follow-up experiment this year, cooling lauric acid. Lauric acid has a melting point of about 110 °C so I kept a class set of test-tubes in a warm water bath all day and reused them every period.

During this lab, I uncovered a misconception I didn’t even know my students had. When the lauric acid began to solidify, I heard many students exclaim, “it’s freezing, but the test tube is still warm!”  They could not understand that the word “freezing” doesn’t necessarily mean cold. This lead to a great discussion about what “freezing” means and got us to definitions for the terms “endothermic” and “exothermic.” I also introduced energy bar charts (LOL charts as I like to call them) while whiteboarding this lab.

LOL Charts 

Energy bar charts are an awesome way to get students qualitatively representing energy transfers. I have students complete and whiteboard the energy bar chart worksheets from the Modeling materials.

For the second worksheet, I have students play the “Make a Mistake Game” from Kelly O’Shea. During this whiteboarding session, each group must purposefully make at least one intentional (and as many unintentional) mistakes as they want. It is then up to the class to find the mistakes and ask questions to correct them. As each group comes up to present, I assign a group still sitting as the main questioning group. Other people in the class can pipe in but I want to hear from as many students as possible.

Now that students have a qualitative representation of energy transfer, it is time to put a number on it.

Specific Heat of Copper Lab

I introduced specific heat this year using the balloon and flame demonstration. If you hold a flame under a balloon, it will immediately pop. If you fill a balloon with a little bit of water and do the same thing, the balloon remains intact. I asked my students to explain this phenomenon and they were able to conclude that water must have some special property that allows it to absorb more heat than air. I then ask students what else might affect the quantity of heat transferred into a system. After some probing, students come up with changes in temperature and mass which gives me the chance to introduce Q=mcΔT.

Then, students complete a pretty traditional specific heat of copper calorimetry lab. I am not in love with this lab and I think I am going to take it in another direction next year, just not sure where.

The data from this lab are usually pretty good so it does allow for us to discuss what this number with the crazy units means. I’m just not convinced that the students understand the math they are doing well enough to make it worthwhile.

Calorimetry Calculations

Now that student have been introduced to Q=mcΔT, they can use it to quantify heat transfer. I have students complete the worksheet from the Modeling materials which off simple asking for Q and then gets trickier when students have to solve for things like final temperature. Once students have temperature changes under their belts, we can get into phase changes.

Heat of Fusion Lab

I introduce the concept of “heat of fusion” with another traditional lab, but this one I like a lot. In this lab, students are challenged to determine how much energy it takes to melt ice by assuming that all the energy that goes into melting the ice comes from the warm water they are stirring it into. Students add ice to a styrofoam cup of warm water until no more ice can be melted. They then measure the change in temperature of the water and the volume of ice melted. Using those 2 numbers, students can calculate the quantity of heat that went into the ice and then use the mass of ice melted to figure out the heat of fusion. Again, students get pretty good data from this lab.

Heat of Fusion/Vaporization Calculations

I use the worksheet from the Modeling materials for heat of fusion/vaporization calculations and it is pretty short. To make whiteboarding more interesting, I came up with a new technique I dubbed the “gallery lot.” It is a hybrid between a gallery walk and a parking lot. Each group creates a whiteboard for a problem and props it up around the room. The rest of the class must tour the room and leave post-it notes on boards they have questions about. We then look at each board and give each group a chance to answer the questions posed to them.

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Combined Calorimetry Calculations

Once students have mastered calculating energy transferred during a temperature change and phase change individually, it is time to combine them! I approach this hurdle by having students break down the process for a simple situation: ice at -10°C is melted, heated to boiling and boiled away. First, students construct the heating curve. Then they figure out which equation they need to use for each region of the curve. Finally, they plug in the measurements and add up all the Qs.

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Students then complete the worksheet from the Modeling materials that goes with this topic and we whiteboard it the next day. I would like to say that these calculations are a breeze after that, but that would be a lie. I tell my students this is the top of Unit 3 mountain and they will get a little bit of a brain break in Unit 4. To get to the very top of the mountain, I give my students a Lord of the Rings inspired challenge problem where they must calculate the amount of energy needed to melt the gold statue in Desolation of Smaug.

With all of that covered, there is only one thing left…

Unit 3 Practicum

The main reason I keep the traditional specific heat lab I mentioned earlier around is for the practicum I am currently using. Using the same lab procedure from the specific heat lab, I challenge students to use copper shot heated to 100°C to raise the temperature of room temperature water to 25.0°C. Students need to rearrange their equation from the lab to solve for mass of copper.

I like the challenge of this practicum but I think it is too narrowly focused. Next year I think I will give students an energy transfer to measure and they will have to construct the LOL chart and temperature-time graph for the scenario and calculate the quantity of heat transferred.

Unit 3 is hefty! Let’s sum it all up with the model so far…

  • If temperature is a measure of the “hotness” of a system then heat is the quantity of “hotness”
  • Heat can go into a system (endothermic) or flow out of a system (exothermic)
  • Heat can be stored in 2 energy accounts: thermal and phase
  • A change in thermal energy means a change in particle speed and is shown by a slope on a temperature-time graph
  • A change in phase energy means a change in particle spacing and is shown by a plateau on a temperature-time graph
  • The quantity of heat transferred during a temperature change can be calculated using the mass, specific heat and change in temperature for the system
  • The quantity of heat transferred during a phase change can be calculated using the mass and heat of fusion or vaporization for the system

Don’t worry, we will cool it down a bit in Unit 4 (see what I did there :0).

 

Chemistry, more like cheMYSTERY to me! – Particles in Motion

Here is a quick recap of the model so far from the previous post in this model building series:

  • 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.

Unit 1 was all about building a functional model of a particle. Unit 2 is the first time students will make an observation that doesn’t fit their model, and they will have to amend the model. Let’s jump right in!

Diffusion Demonstrations

I start Unit 2 with the diffusion demonstrations outlined the Modeling Instruction materials. First, I place something with a strong scent on a watchglass, and place that on a triple beam balance. I have used different extracts, fresh-squeezed orange juice and essential oils for this. They have all worked about equally well for me. I ask students to raise their hands when they smell the scent. Students observe that the smell travels around the room randomly. I then ask students to whiteboard what they think is happening. They come up with many different explanations but the conclusion we come to is that the particles must be moving! We have added something to our model!

We refine this addition to the model by observing what happens when you drop food coloring in warm and hot water.

From this observation, students infer that particles move faster in hot water and slower in cold water. Therefore, temperature is a measure of the average speed (kinetic energy) of the particles.

To end this demo day, I show students a thermal expansion demonstration.

Students observe two liquids (water and isopropyl alcohol) rise up capillary tubes as they are submerged in a hot water bath. The yellow liquid (alcohol), appears to rise more quickly than the red liquid (water). Students immediately respond that materials expand when they are heated but it takes a little more probing to get them to explain what is happening at the particle level.  This allows us to talk about how a thermometer works. I also talk about Celsius and Fahrenheit here and where those scales come from.

Now that we know that particles move, we can ask some interesting questions, like, “how does a straw work?”

“How Does a Straw Work?” Discussion

For this discussion, I tell all my students to bring a drink to class with them.  I then give every student 2 straws. The first questions is simple, take a drink with your straw and tell me how it works.

I get all sorts of answers to this question and hear lots of sciencey words like “pressure, vacuum” and my favorite “sucks.” It always becomes very clear that students actually have no idea how a straw works. The first step is to get them to see that nothing is being “pulled.” That means, the liquid is being pushed up the straw. But by what? Enter pressure. I also have students try to drink out of their straws with one on the inside of their drink and one on the outside. This helps cement the idea that you remove particles from your mouth to create a partial vacuum when you use a straw. Then the air on the outside of your drink is able to push your drink up the draw. I end class that day by asking the question, “is there a maximum straw length?” and showing the Veritasium video, “World’s Longest Vertical Straw.”

Now that we have defined pressure, we can figure out what affects it.

Gas Laws

I have moved to completely separating each gas law and then combining them in the end as opposed to deriving all the gas laws during one lab. I start the discussion by blowing up a balloon and asking, “what could I do to change the pressure in this balloon?” Students are able to come up with, “change the volume, change the number of particles, and change the temperature.” From there we explore each of the gas laws, starting with Boyle’s Law.

To explore Boyle’s Law, students use Vernier Lab Pros connected to TI-83 plus calculators to graph the relationship between pressure and volume.  Students push in and pull out the plunger on a syringe connected to a gas pressure sensor to collect 8 data points.

Students whiteboard their data and we are able to determine that pressure and volume are inversely proportional by the shape of the graph. I no longer have students linearize this graph because I do not think it adds enough to the discussion to be worth the time it takes to teach them how to linearize.

For each gas law, I have students draw 3 particle models to show what is changing and how that change affects the pressure. After discussing the relationship between pressure and volume, students work on just pressure-volume problems with initial-final-effect tables.

For Gay-Lussac’s Law, students again derive the relationship by collecting data with Vernier LabPros. This is an interesting whiteboard meeting because students collect their temperature data in Celsius and so their y-intercepts are far from zero.  I ask students, “what would a y-intercept of zero mean?” Students are able to reply that it would mean the particles are no longer moving which would be absolute zero. Usually one student in the class remembers something about Kelvin at this point and what temperature absolute zero is. We then have a quick discussion about why scientists use Kelvin and the need for a temperature scale without negatives. With all of this in mind, students then work on just pressure-temperature problems.

I cut the Avogadro’s Law lab this year to avoid confusion with terms like “puffs” because students do not know how to count particles yet. Instead we talked about Avogadro’s Law and Charle’s Law and how they make sense at the particle level. Avogadro’s Law will come back at the end of the year when we get into PV=nRT.

This year, I added a day of gas laws review day before jumping into combined gas law problems. For the review, students whiteboarded a graph with particle models and a statement of relationship for each gas law. Students also put this information on a summary sheet for themselves. From there, we were able to cleanly transition into combined gas law problems with initial-final-effect tables.

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That is the last major topic in Unit 2! All that’s left is a challenge problem and a practicum!

Challenge Problem

To wrap up the gas laws, I give students a challenging ranking task that a colleague of mine created. It challenges students to use their conceptual understanding of the gas laws instead of relying on their math skills. The ranking task gives students various scenarios like, “the volume is doubled and the temperature is halved”, and “the volume is tripled and the temperature is tripled.” The students must determine what effect the scenarios would have on the pressure and rank them from least to greatest final pressure. 

Practicum

For my Unit 2 practicum,  I use the term “practicum” loosely. For the practicum, students are not challenged to solve a problem using their model but rather explain various demonstrations using their model.

I have students explain a lung model,

a marshmallow expanding in a syringe,

an water balloon being pushed into the mouth of a flask,

and the always fantastic “Crush the Can” demo using the gas laws. The students always find these demonstrations are not as simple to explain at the particle level as they first appear.

To sum up what we added to our model so far…

  • Particles are always in motion
  • Temperature is a measure of the average speed of the particles
  • Pressure is a measure of the number of particles colliding with a surface
  • Pressure and volume are inversely proportional (Boyle’s Law)
  • Pressure and temperature in Kelvin are directly proportional (Gay-Lussac’s Law)
  • Pressure and number of particles are directly proportional (Avogadro’s Law)
  • Volume and Temperature in Kelvin are directly proportional (Charle’s Law)

Whew! That is a hefty unit! Look out for some more additions to the model concerning energy in the next post of this series!

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).

EDIT: I have gotten a lot of questions about the wax block activity over the last year! I wanted to offer some answers to everyone in case you have the same questions. I make these myself buy you can find instructions from Flinn here. You can buy parafin blocks at your grocery store with the pickling supplies. I also put together a script I use that loosely outlines how I structure the activity here. Here are some pro tips as well:

  1. Make sure the foil is smaller than the blocks. Inspect the sides of the blocks carefully to make sure you cannot see any foil. Students will get curious if they see something and scratch away the wax on the side of the block.
  2. If you need to break the block apart to redo it, just smack the side of it on the countertop a few times, it should come right apart. With that said, at least one of your blocks will get dropped and it will break open during class. Be on high alert so you can swoop in before the magic is ruined!
  3. Don’t let your students get away with an easy answer. They will likely draw you a picture of “something” inside the block but not elaborate. This is a good opportunity for students to make more observations and then infer what that “something” could be. Given them a flashlight, put the block on top of objects of different colors, etc.
  4. Don’t tell them the answer. This is the hardest part! Students get so excited about this activity and at the end all they want to know is “was I right?” This is a perfect opportunity to start building your classroom culture. It’s not just about being right! These wax blocks will come back to haunt you for the remainder of the school year but stay strong!

This is one of my favorite activities of the year. Students ask me about it until the last day of school. Let me know how it goes for you and feel free to message me with any questions!

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