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I did a short experiment in which I connected a 1.5v battery with 2 paper clips into a bowl of water to which I added salt. After a while, I saw a yellowish-greenish cloud inside the water around one of the paper clips, and when I took it out, the part that was in the water was more black than the rest of the paper clip. After a while, some of it turned into a greenish color. What caused the paper clip to change its color?

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More information would be helpful. Were they both steel paper clips? What is the electrolyte? –  Chris Jul 27 '12 at 22:27
    
I think the paper clips are made out of steel, and the electrolyte is NaCl. –  Omer Jul 27 '12 at 22:32
    
Mono's answer and the comments are mostly on the money. If you want to try this again, you need both a different electrolyte and electrodes. Simplest would probably be magnesium sulfate (Epson salts) and carbon rods, respectively. Make certain to wrap the connection of the carbon rods and the wires to your battery with waterproof tape to avoid reactions with the wire (i.e. the comments about your paper clips). Good luck--I love this experiment! –  trb456 Jul 28 '12 at 0:57

1 Answer 1

up vote 4 down vote accepted

Without further details of your equipment, this might be a possible answer. I assume the electrolyte you added was table salt (NaCl).

What you saw as a yellowish cloud was gaseous chlorine which discharged on the clip you connected to the positive electrode (cathode) of your battery. The reason why you did see this, instead of seeing just colorless bubbles of oxygen, is because of something called "overpotential". It's a joint property of the species in your cell and the material and surface termination of the electrode; in your case, the clip (made of wire steel) has a lower overpotential for the discharge of chloride anions (from the salt you added) than oxygen.

Elemental chlorine is a very reactive oxidant, and you had a thin chlorine film around your clip electrode. The reaction between chlorine and iron in aqueous solution most probably yielded iron (II) chloride tetrahydrate which is the reason you saw the greenish colour of the previously submerged fraction of the clip.

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How can the chlorine be in gaseous state if it's in the water? –  Omer Jul 27 '12 at 22:48
    
Ferrous chloride solutions are yellowish; I thought you also had seen a "cloud" of chlorine bubbles discharging from the electrode into the air above. –  Mono Jul 27 '12 at 22:54
    
youtube.com/watch?v=iWpfHkWr5DY –  Mono Jul 27 '12 at 22:56
    
By "cloud" I meant a cloud inside the water, a yellow liquid but only around the paper clip. –  Omer Jul 27 '12 at 23:04
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Elemental chlorine is also soluble in water at a level of about 3 g/L, which could also account for the yellow color in solution. –  Ben Norris Jul 28 '12 at 0:23

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