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Electrolysis w/ My Kids - We are stumped!

by Justinmcg67
Tags: electrolysis, kids, stumped, w or
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Justinmcg67
#1
Sep2-14, 02:58 AM
P: 14
Hello all! My son is getting ready to go into the 2nd grade here on Wednesday and although he takes breaks during the summer, I really try to keep his education rolling throughout the entire year. As such, I try to get him excited by doing experiments and other things to captivate him and get him anxious for the school year. While I had some spare time I researched some simple experiments at home. The one that sounded the most interesting as far as all the science taking place, was electrolysis. Specifically we wanted to make hydrogen gas and check out the bubbles forming. Many interesting and unexpected (but good!) things happened. Also, I will link videos (on YouTube) taken from our experiments.

First the set up is as follows:
- Copper Wire Electrodes x2
- AA Rechargable Batteries x5 (7.5V total)
- Mason Jar
- Non-Iodized Salt
- Water (Boiling when added)
- Vinegar

The first thing we got was as expected. A clear solution with bubbles forming and one electrode turning darker. I presumed the darkening to be the copper oxidizing, as in what happens to pennies over time. A video (in 1080p) is linked below to add clarity:
Link

One thing was that despite the instructions from About.com and other places (Instructables.com, etc.) we got results that were not listed. The observation we got was a red/brown almost salmon-like color in the solution. A video (in 1080p) is linked below to add clarity:
Link

As the process went on longer it got much more red/salmon-like and presented obvious flakes, or chunks that didn't seem to be stable, as in like paper when it stays in water for long period of time. A video (in 1080p) is linked to add clarity:
Link

Having looked into the experiment and its safety I noted that the solution is basic and needs to be neutralized in order to be disposed of properly (not to sure if this is accurate, but did it anyway as a caution). As such we added vinegar, and to our surprise the solution went clear with only the chunks or semi-pieces left. Most eventually settled to the bottom. A video (in 1080p) is linked to add clarity:
Link

Now what I would like to know is: Why does the copper dissolve? And, more importantly, is there chemistry that can be explored here? Can we do something with the solution?

Additionally, we did this same experiment and used iodized salt, and got the same results. We also tried using baking soda instead of table salt (read that this was a more safe form of the experiment). A video to that will be linked to as well. For the baking soda we wanted to know why the solution turned blue/teal? We noticed how similar one copper lead was turning green similar to that of the Statue of Liberty, while the other turned black. This seems obvious, but if anyone wants to break down the chemistry that would be great! (Video link below)
Link

Any and all help is much appreciated! Thanks!


TL;DR

Why does copper dissolve via electrolysis in a solution of table salt and water?
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Borek
#2
Sep2-14, 04:27 AM
Admin
Borek's Avatar
P: 23,727
I am not convinced it was copper dissolving. Have you used distilled water or tap water? Is your water iron free?

I suppose the copper was just some copper wire - as such it may contain some impurities that make the experiment result difficult to predict and explain.

Blue color is that of copper aqua complex (that is, Cu2+ complexed by water molecules). Green solid is a basic copper carbonate - hard to tell what is the exact composition, as it is variable and depends on conditions.

There are two important differences between both solutions - one is that bicarbonate has a much more stable pH, the other is that presence of carbonates makes the precipitation of the basic carbonate possible. Typical tap water contains some carbonates as well, but the amount is much lower.

Great experiment BTW


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