Honors Chemistry

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We did the gold penny lab, where we bathed a copper penny in a hot zinc-NaOH mixture to turn it a silver color, then we heated that in a flame to turn it a gold color. There are two questions that go with this lab that I don't understand, however:

4. Did the copper and zinc undergo a physical change or a chemical change when the penny was bathed in the zinc-NaOH solution? Explain your answer.

- I would put it was a physical change because the zinc pretty much just melted onto the penny to turn it a silver color -- is this right?

5. Did the copper and zinc undergo a physical change or a chemical change when the penny was heated in the flame? Explain your answer.

- Wouldn't you also put it was a physical change because the zinc mixed with the copper of the penny to turn a gold color? Or did I get this completely backwards?

Any help is GREATLY appreciated! :D

  • Honors Chemistry -

    Both of your questions should be answered as a chemical change. In the first one, the Zn ions in solution are plated out onto the Cu penny. The silvery color you see is Zn. The reaction is
    Cu + Zn^+2 ==> Zn + Cu^+2
    Basically, you dissolved Cu and plated out Zn. Remember the activity series of metals? Zn is above Cu so Zn^+2 ion comes out of solution and Cu goes in.

    Second question.
    When you heated the bare Zn-coated copper penny, you formed brass which is a gold color. Brass is an alloy of Cu and Zn.
    Here is a site on the web where you can read about the experiment.
    http://misterguch.brinkster.net/MLX040.pdf

  • Honors Chemistry -

    Ohhhh, I get it now. Thank you so much!! :D

  • Honors Chemistry -

    1. is turning the penny silver a chemical or physical change?

    2. is turning the penny gold a chemical or physical change?

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