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I am doing an electrolysis experiment with two copper wires inside of test tubes that are both submerged underwater, with 20 V per wire. Test tube A is creating oxygen gas at a high rate, but test tube B is creating a smoke-like gas that is heavier than the water and coming out of the test tube that is submerged in water and settling on the bottom of the beaker.

Why is the wire creating the smoke-like gas and settling on the bottom of the beaker? The wires have been in the water for about a week and only turned on when we are working on the experiment. Is the smoke-like gas a reaction to the fresh tap water that has been sitting?

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    $\begingroup$ A gas that is heavier than water is quite an extraordinary claim. $\endgroup$
    – Ivan Neretin
    Dec 4, 2019 at 19:46
  • $\begingroup$ A gas heavier than water is not a gas. Not for common substances and conditions. $\endgroup$
    – Poutnik
    Dec 4, 2019 at 19:47

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It's not smoke, but Copper(II) oxide (cupric oxide, CuO), which is a black solid. It formed on the surface of your copper electrode and is falling off. Even after falling off, copper oxide could continue to form on the copper surface.

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