Trying to impress my 8th grade students, made some unknown stuff

[Superkase]

TL;DR Summary

Collecting hydrogen through hydrolysis of water. Put some table salt in city water, hooked up two pigtails (copper wire) to a 9V battery and let it fill up a submerged cup over night. Came in the next day to a greenish insoluble material covering all parts of my contraption.

I had previously collected small amount of hydrogen to explode as an attention getting opener for my 8th grade science students when we started chemistry. My third period kids are inquisitive and hard working and also wanted a bigger explosion, so I thought I would try to reward their effort. I filled a tub up with city water and some salt. I had two copper wires attached to a 9V battery. Above the pigtail connected to the negative anode I placed a clear plastic cup that was submerged in the water before flipping it over in the water to collect the hydrogen. I got it about half filled up before the battery ran dead over night.

There is a black material that is pretty dense sitting below the pigtail connected to the positive terminal. The water and everything else in the tub is absolutely filled up with a bluish green substance that I am assuming is some kind of copper oxide, and while it's not held in solution with the water it seems to be on the top of the water as much as at the bottom.

The question I have is that my googling of copper oxide doesn't reveal too many similarities to what I am seeing in the tub. Is there some way to know what else is going on? I am positive most of the copper is coming from the copper wire, but there is no telling what else is in the water because of the plumbing in this 50+ year old school.

 

[chemisttree]

Copper chloride and copper hydroxide probably. You may have produced a mix of O2 and Cl2 at the anode (+ side of the battery) and H2 at the cathode (- side of the battery). what you have is probably complex.

 

[.Scott]

I agree: copper mystery.
If break apart a C-cell or D-cell non-rechargeable battery you will find a carbon core element.
Clean up to of those rods and attach the copper wires to them so that there is minimal contact between the copper and the water. Use electrical tape, etc.

Then repeat the experiment and don't expect huge quantities of those gasses.

 

[BillTre]

TL;DR Summary: Collecting hydrogen through hydrolysis of water. Put some table salt in city water, hooked up two pigtails (copper wire) to a 9V battery and let it fill up a submerged cup over night. Came in the next day to a greenish insoluble material covering all parts of my contraption.

I had previously collected small amount of hydrogen to explode as an attention getting opener for my 8th grade science students when we started chemistry.

My teacher did this when I was in 8th grade.
It was a blast!

 

[Borek]

break apart a C-cell or D-cell non-rechargeable battery you will find a carbon core element.
Clean up to of those rods and attach the copper wires to them so that there is minimal contact between the copper and the water. Use electrical tape, etc.

Then repeat the experiment and don't expect huge quantities of those gasses.

Adding some sulfuric acid or NaOH to the solution helps a bit by lowering the solution resistance and increasing the current.