Puddles, Dissolving and a Cool Conductivity Tester...

Conductivity Tester

What am I doing to help kids achieve?

How do I know when they are there?

What is the evidence?

I have been teaching for more than 20 years. Everyone I meet who is not a teacher assumes that during the summer teachers kick back, sit by the pool and drink Margaritas. I try to explain that one of my favorite activities is to get to work on ideas that I can never do during the school year. One of my biggest struggles with students is to try to explain what happens when items, specific inorganic salts, dissolve in water. It might sound simple to me and you. Research shows that students have many real misconceptions when it comes to explaining inorganic salts dissolving in water. My own experience along with other teachers I know is that we are amazed and sometimes frustrated with trying to help students understand the simple process of dissolving, especially with ions. A key piece of equipment is a good conductivity tester. Just got done making a stack of them and can't wait to have students try them. But back to "dissolving"....

Here are a few ideas that might help students. First, there is the idea of trying to have students explain and understand the dissolving process on the particulate level, the macroscale and the symbolic. Second, Bob Worley introduced me to the idea of "puddle chemistry". To put it simply, students place a small drop of distilled water on an acetate sheet. They then place different crystals on opposite sides of the drop or "puddle". The really cool part is that if students were to use a crystal of sodium carbonate on one side and copper (II) sulfate on the other then they will eventually see a small line form in the middle of the puddle which is the insoluble solid copper (II) carbonate. This method has the ability to be so much more powerful than mixing the two solutions together and observing a solid form. If students do not understand the dissolving process then they certainly will not understand dissolved compounds in two liquids forming an insoluble solid. The "puddle" chemistry method forces students to develop some explanation about a solid dissolving and then forming a different solid. The only part that could make it better is using a conductivity tester throughout the different stages. Students could check the conductivity of distilled water, then distilled water with a small amount of an ionic solid dissolved. The idea of "ions" would hopefully help them develop the idea of dissociation. The only way to explain the conductivity part would be if there are ions present. I tried this and it failed miserably. I just did not have a good conductivity tester. I tried ot make one but it was giving false positives. I found plans for a new one that I believe is much better than ones I have tried in the past and has many advantages.

David Katz and Courtney Willis have written a paper that also includes a DIY conductivity tester and a lab. Here is what I love about their idea for the conductivity tester. All of the pieces are easily available. I made some minor modifications. First, I went to the hobby store and bought graphite rods that are used in building remote controlled airplanes. They conduct, are strong and unreactive. I am using these as my electrodes. Thanks Bob Worley for this idea. The other modification is the power source. Nine volt batteries are expensive. I was able to get a box of 12 volt DC power sources from a cable/cell phone store for free. The people at the store often have to swap out old modems with new modems and they trash the old stuff. The power sources work great for this type of application. I will never have to worry about buying a 9 V battery again. Here is the great part about the conductivity tester...there are two LED lights. One is green and the other is red. They are both hooked up to resistors but the green one is hooked up to a resistor with slightly higher resistance than the red one. If there is a solution that conducts really well, both lights are on. As the conductivity drops, the green light dims and only the red light will go on. It shows different AMOUNTS of conductivity. Now it is close to becoming not just a qualitative tool but also a quantitave tool. If this were not enough, there are also plans for a conductivity tester with a buzzer. So...in theory...this could be used with dissolving. Students could be asked to explain what would make the different lights go on and what is happening in the water during the stages.


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Comments 7

Bob Worley's picture
Bob Worley | Sat, 07/23/2016 - 02:57

Thanks Chad. These microscale exercises really do try to extend understanding into the area of ions. It only takes 1 grain of sodium salt in a puddle to enable the LED lights to come on.

If you come to BCCE2016, do come and have a chat with me. I am doing a workshop (W5) (including precipitation and complex puddles); there are few places left and I am working with Diane Beecroft from Arizona who is translating for me. It appears you all think I am Australian and talk in a different language. These ideas are now being included into UK practical exam exemplars.

I am doing two talks as well. The one on “What can a (Materials) Safety Data Sheet (SDS) do for you?” appears rather boring but I have some interesting observations from the UK.

Chad: I hope I am wrong with this observation but I am getting the impression that the amount of practical work is decreasing in your schools and that more emphasis is being placed on simulation, game playing (in the nicest possible way) and videos. Our new exam syllabus is now demanding that students have a lab book with at least 8 (ages to 16) and 12 (ages 16-18) practicals covering a range of skills. They are assessed by teachers but monitored at random by the Boards. This is to get away from the awfully contrived 90 minutes investigations that were done.

I hope I am wrong!                              

Chad Husting's picture
Chad Husting | Tue, 07/26/2016 - 09:31

Bob - Great to hear from you.  I will definately try to track you down at BCCE16.  Would love to hear about your thermite reaction and would  like to pick your brain about gas reactions on a microscale.  Safe travels.  See you in Denver.  If you get the chance I should be at the ChemEd X booth Tuesday morning.

Suzanne Irwin | Tue, 08/02/2016 - 12:38

Hi, I too have had great difficulty with both my regular students and even AP students in helping them understand the concept of dissolving.  I like your idea of puddle chemistry and will try that. In addition, I have used the Flinn version of the conductivity testers you describe.  They are expensive, and, unfortunately after widespread use in our department, they have become contaminated so that they now show conductivity without the wires being immersed in any solution.  I was wondering if you had any ideas on how to uncontaminate them?  Otherwise I am going to build some new ones for the sole use of my classes.  More interestingly, when I used them (before they were contaminated, when they were working properly), I discovered a strange phenomenon which I am wondering if you can help me explain.  I had my students compare solutions with dissolved ionic salts to a solution of sucrose, to show that covalent compounds, unlike ionic ones, do not typically conduct electricity.  Unfortunately, the sucrose solutions showed a low conductivity.  I suspected contamination, so I repeated the experiment several times myself, first using distilled water, which did not conduct, and then adding sucrose, and a low light was seen, indicating low conductivity. This contradicts the picture in our textbook, yet I could not come up with a satisfactory explanation for my students.  I am wondering if it has something to do with the carbon rings in a sucrose molecule separating when they dissolve, but my organic chemistry is not that strong to fully flesh out that explanation.  Anyway, note to self, I will not use sucrose to illustrate covalent versus ionic conductivity in the future.  If you have any explanations, please pass them on.  Thanks for the puddle chemistry suggestion and conductivity diagrams-- I will be using them!

Tracy Schloemer's picture
Tracy Schloemer | Fri, 08/05/2016 - 10:15

Hey Suzanne-

 I found this out the hard way myself after pulling my hair out after doing the same experiments you describe above. Sugars are weak acids, so that's why you're getting low conductivity (See here   http://butane.chem.uiuc.edu/pshapley/genchem2/b6/2.html). However, I myself "use sugar" in a conductivity test as an intro to bonding/properties before acids and bases... So I have a container labeled sugar water and actually put in no sugar (or if I'm feeling guilty, like 2 crystals of sugar). We don't revisit sugar as a weak acid, but we do break this model of "covalent compounds don't conduct in solution" in our acids/bases unit.


Bob Worley's picture
Bob Worley | Fri, 08/05/2016 - 22:20


The CLEAPSS UK indicator that we used at BCCE2016 is basically a 9v battery, a 20mA light emitting diode (LED) and Resistor, 390-500Ω, 0.25W to protect LED. The electrodes are the carbon fibre rod ones. I cannot give you the whole caboodle in our box because we spend hours doing this sort of thing and basically our stuff is under subscription. I think you could make one based on that info but the electrodes must not touch each other. We had a Flinn one (but not all) lighting up with distilled water at BCCE but not the UK version.

When I do this, I insert 1 grain of salt and 1 grain of sugar respectively in two puddles. The salt one is enough to cause the LED to light and that (to me ) is the “wow” moment: it only takes 1 grain to do that but Sugar does not work. However, if you use proper brown sugar the light does come on which shows that brown sugar contains ionic impurities. (It did not work in one country because their brown sugar was white sugar with a brown dye!)

Now you add 1 drop of Universal Indicator to distilled water and add the probes. Nothing lights up but after keeping the electrodes in for 30s and removing carefully colours can be seen. Does this indicate that water is in fact slightly ionised. It would be nice if it did but at this level impurities are an issue. I think I heard that Sorenson distilled water several times in Platinum equipment to ensure purity in his experiments. If you cannot afford a 9v battery , I think Pt equipment is out of the question.

​Tracy: I have not tried it with glucose only sucrose. With only grain being used the effect in the Purdue article (not seen that before) would be vey tiny. With my conductor indicator I have never sesen the light coming on but impurites at this level are important.



Chad Husting's picture
Chad Husting | Sun, 08/07/2016 - 05:58

Bob - Love the BCCE workshop.  Thanks again.  As far as the conductivity testers, I plan to try out the new ones this year and see how it goes.  I like the ones I built because they are based on the Flinn ones and it is not just "on" or "off" but it is a continuum which I hope will provide some good discussions.  As far as sugar being a weak acid, that is great to know.  I did not know that but it does make sense.  As far as the "contamination" of the testers I am not sure I have an answer but here is an educated guess.  Metal electrodes eventually might react, corrode and get a coating.  I am hoping the graphite electrodes won't.  If you go to a hobby shop that sells parts for remote controlled air planes, they are used as struts and wing parts and are not that expensive.  Hope this helps.  Will keep you posted as I continue to use them.  Thanks for the helpful comments.

Pascual Lahuerta's picture
Pascual Lahuerta | Sun, 08/07/2016 - 06:07

Chad, This is really an interesting experiment. 

I wonder if you might find interesting creating additional simulations that answer some questions related with the experiment: How do the ions leave the solid and pass to solution? What is the role of water in this process.

I beleive it won´t be difficult to create simulations that answer these and other possible questions. I prepared some that deal with this topic. 

http://www.uv.es/quimicajmol/concepttest/moduloD1/index.html and the following tests in the page. These simulations are part of a page dealing with Concept Tests. Independent simulations can be easily designed. Also : http://www.uv.es/quimicajmol/simulaciones  (entering solubilidad). I can translate to spanish the few comments in the page.

If you find worth trying similar simulations I could help creating new ones following your suggestions and related to your experiment.