(chem)Green powder!

I've also tried that expt. at home, so I have some idea what you're talking about.

But i'm confused by your description. You said the green solid forms on the CATHODE, right? That'd be strange, as I suppose any green solid would form on ANODE.

Make sure you're talking about Cathode, which is connected to -ve terminal of the battery; or Anode, which is connected to +ve terminal of the battery.

Theoretically, with inert electrodes, hydrogen gas would be given out at cathode, and chlorine (with some oxygen) evolves at anode.
2H2O + 2e- ------> H2 + 2OH(-)
2Cl(-) ------> Cl2 + 2e-

However, I bet you've used iron objects (paper clips, staples, etc.) as your electrodes (at least your anode), right?
Then, a possible side reaction may occur, involving the discharge of the non-inert metal:
Fe + 2OH(-) ------> Fe(OH)2 + 2e-

In such reaction, iron electrode is oxidized to give iron(II) ion, further reacts with excess hydroxide in electrolyte sol. to give iron(II) hydroxide, a green solid.


Please check your connections and identity of the electrodes.

p.s.
If the electrolysis is continued for long time, iron(II) may be oxidized by air to give yelow~brown rust-like iron(III) hydroxide solid. Did you see such observation?


2012-05-10 01:38:06 補充：
1.
Never mind about the gas. Just try to remember which terminal you've connected.

2.
Are you still keeping the solid? Well, just dump them like domestic garbage or flush into toilet is alright.
Does it turn brown / yellow?

2012-05-10 01:38:40 補充：
(Even if it turn brown, it's still harmless. Dump them as you like.)

2012-05-10 20:24:31 補充：
Ah, that's very good observation indeed; I've also forgotten this fact that anode should be reducing its mass.
My propose is, iron(II) is formed at anode; since H+ ion is discharged at cathode, solution around it becomes alkaline, forming metal hydroxide solid.