Solubility and Models

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         Thanks to   for a great idea that really helps students. It has been well documented, and my own experience confirms this, that students really do not seem to clearly understand dissolving an ionic solid in water. Here is the activity that I learned from Bob at Chem Ed 2015. First, I had my students examine the conductivity of a puddle of water the size of a nickel. They checked for conductivity. Then they took a very small amount of sodium carbonate and a fresh puddle of water and pushed in a few crystals from the side.  You can still see the crystals in the water but it tested positive for conductivity. They had to explain this. They did the same with a fresh puddle of water and a few crystals of copper (II) sulfate. Again, it tested positive for conductivity but they could still see the blue crystal. Finally, they started again with another fresh puddle of water, pushed a few crystals of sodium carbonate on one side and on the opposite side they pushed in a few crystals of copper (II) sulfate.  After waiting five minutes, a solid dull blue precipitate formed in the middle.  Also, the drop tested positive for conductivity.

Students had to explain how the solid formed and the clear liquid that tested positive. Bob even has some great leading questions about what makes up the solid. If it is light blue, where does the blue come from?  It was a great two day discussion about water, ions and drawing models as my students tried to explain what was happening. I would strongly recommend this activity. Bob has said, and I agree, that when we do the precipitate double displacement reactions with the reactants already dissolved, we are assuming too much about what we think students know.  This forces them to discuss and think about the nature of dissolving ionic solids. I would give this activity a thumbs up.

Concepts: 
Collection: 

NGSS

Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).

*More information about all DCI for HS-PS3 can be found at 

Summary:

Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).

Assessment Boundary:
Clarification:

Examples of phenomena at the macroscopic scale could include the conversion of kinetic energy to thermal energy, the energy stored due to position of an object above the earth, and the energy stored between two electrically-charged plates. Examples of models could include diagrams, drawings, descriptions, and computer simulations.

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Comments 3

Laurent Chassot | Thu, 11/19/2015 - 09:30

Thanks for sharing the idea, just a technical question: How do you test the conductivity of the puddle?

Laurent

Chad Husting's picture
Chad Husting | Sun, 11/22/2015 - 13:37

Laurent - Thanks for the comment.  There are several possible ways to test for conductivity.  First,  That could be an option.  The one that I like is to google DIY Conductivity Tester and tell students that it is an extra credit project over break.  An adaption that I like to the DIY conductivity tester is to use carbon electrodes.  If you go to a hobby shop that sells remote control airplaines, you can get carbon fiber rods.  These are supper durable and conduct electricity.  Here is what I did...I took a 9 v battery clip and wired it to a 56 ohm 1/4 watt resistor and 2 red LED lights.  I used some extra wire, hot glue and electricians tape with a small wood block to piece everything together.  It worked great. However, batteries are expensive.  I have a neighbor who works for a wireless phone company.  He had a bunch of old modems that his company was going to pitch.  I took the 12 V power supplies, spliced the wires and used these to power the conductivity tester.  Worked like a charm.  FYI...LED's allow current to flow only one way.  If you hook up the power and it does not work, just switch the leads.  Hope this helps.