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.

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.

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.

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.

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.