Modeling Chemistry Workshop – Day 3 Part 2
Beginning Unit 2 – Smelly Particles
Start with a demo in which you spray some perfume/air freshener and ask students to raise their hand when they are able to smell it. (I still have not found the perfect thing to spray, and the bigger the classroom the more difficult this will likely be. If you use an air freshener, be sure to spray over a garbage can so that the floor doesn’t get slick.)
Have students draw a story board (3 panel particle picture) to explain how the smelly particles got to their noses.
What do you need to get out of the discussion:
- Particles moved (at first you didn’t smell them, then you did)
- Particles did not move only in one direction (the were sprayed forward, but they traveled to the sides and behind the spray bottle too)
- Consider the whole system (the room and ALL the particles in it – including AIR!)
Additions to “The Story So Far”: Gas particles move randomly and gas particles bump into each other.
Hot/Cold Water & Food Coloring Demo:
Do this either as a demo or have each group do it on their own – two identical beakers filled with the same amount of water, one with hot water the other cold. Drop one drop of food coloring in each beaker at the same time. (It is nice to use red for hot and blue for cold, but not essential.)
Have students create a three panel story board explaining what they saw.
What do you need to get out of this discussion:
- Particles at a higher temperature move faster
- Consider the whole system (include both water and dye particles)
- Reinforce Conservation of Mass (same # of particles in all three panels)
Additions to “The Story So Far”: Particles collide and cause mixing and particles at higher temperatures move faster.
Observations of Phases of Matter
The Modeling Chemistry curriculum has students observe the three phases of water and compare their density, fluidity and rigidity. They should create particle pictures of each to support their observations. (Note: Of course solid water is less dense than liquid water. I have emphasized with my students that this is an exception and due to ice crystals having more space between particles than there is in liquid water. If you didn’t want to bring up this exception now, you could maybe substitute another material (ideas?) or just go to the videos mentioned below…)
To support what we’ve observed about the phases of matter you can show the Eureka series videos 16-18. You can find these videos on YouTube (do a search for Eureka video 16). #16 is Molecules in Solids, #17 is Molecules in Liquids, and #18 is Evaporation & Condensation. Each video is roughly 5 minutes long.
There are some pros and cons to consider with these videos. The videos reinforce some of the ideas we’ve been discussing (particle pictures, differences between solids, liquids, and gases) but they also bring up somethings we aren’t ready to discuss yet – energy, molecules, melting/phase change… They are short and provide for a little variety in the classroom. I feel that several of my students have been helped by seeing the animation of the particles, and it is something I can refer back to in later discussions.
An addition to “The Story So Far”: Particles are always in motion. In solids, this motion is in the form of slight vibrations (show with squiggly brackets). In liquid and in gases, there is more movement.
Our particle pictures of solids, liquids and gases so far:
Thermal Expansion of Liquid Demo
For this demo you need two test tubes with stoppers and open ended glass tubes, food coloring, water, and ethanol. Use blue water in one test tube and the same amount of red ethanol in the other. Use a dry erase marker to mark the level of the liquid in each glass tube at the start. Heat the tubes in a beaker of warm water. Use a thermometer to monitor the temperature of the water bath. As they are heated, both liquids expand. Mark the level of each liquid in the tube at different temperatures (for example, every ten degrees).
What do we need to get out of this:
- Liquids expand as they are heated (they get more and more like gases, introduces the beginning of phase changes)
- Alcohol is more sensitive to temperature change – makes a good thermometer
- A thermometer works by allowing transfer of energy through collisions between the substance we are measuring and the surface of the thermometer. The liquid inside expands when it gains energy (temperature) and falls when it loses energy.
Eureka videos #19 & #20 reinforce this discussion and offer some information on the development of the Celsius temperature scale. Otherwise you will need to explain this to your students or alter Unit 2 Worksheet 1.
Unit 2 Worksheet 1: This touches on concepts covered in Eureka videos. Question 6 is a stretch and something for students to ponder. Have students attempt answers to this one, but don’t come to a final resolution on this since it will reappear again soon.
Introducing Pressure – How does a straw work?
In our workshop we discussed using Capri-Sun to demonstrate how a straw works. This method has the added benefit of keeping students quiet for a bit while they finish the drink.
Students should be able to buy-in to the idea that sucking on the straw removes particles from the straw. Once these particles are removed, the particles striking the outside of the Capri-Sun are pushing harder than the particles that remain in the straw, so these extra collisions on the outside of the bag have the effect of pushing the liquid into your mouth.
Have students whiteboard this or help you draw this out. My particle pictures are below (the circle with ears is the person, please forgive the lack of artistic talent!):
The black particles are liquid and the white particles are air. I used arrows here to show movement of the particles (being sucked into the person’s mouth or hitting against the container). We’ll talk more later about how to show movement and collisions of particles.
Other Pressure Demos
A few other ideas are included to get students thinking about pressure from gases.
You can “Blow Up a Student” by using a contractor garbage bag modified so that there are straws in each corner and it is sealed. One student lays on the bag on top of a table or teacher desk, while four students blow up the bag. The student is lifted off the desk by the bag filling. This worked well with my students. (You could also inflate an air mattress instead.)
You can show the PhET Gas Properties Simulation to show the collisions of gas particles with the sides of the container creating pressure. I use this simulation again in conjunction with the lab later in this unit.
One teacher told me about using a long piece of tubing and a trip to various parts of the stairs and the roof to show her students that there were limits to how long a straw we could use.
Unit 2 Worksheet 2
If you have access to a manometer, this would be great to show students how it works. I don’t and have usually just discussed it. But I just found this Manometer Simulation that I might use this year. It is slightly different than the manometers shown on the worksheet, but I like that it shows the liquid level changing as the pressure changes.
You will also need to give your students some basic information on atmospheric pressure and pressure units before they can do the worksheet.
Next Up: More on pressure notes & kinetic molecular theory, PVTn lab, finishing Unit 2