Hydrogen evolves now on the zinc and now on the copper
name: Demonstration of working of galvanic couple zinc-copper in an acid solution
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Zinc reacts with acid forming zinc ions (Zn2+) and hydrogen gas.
If thouch the zinc with a copper wire, zinc and copper constitute a galvanic cell. Zinc dissolves and gives electrones to copper. Hydrogen ions attaches electrons firming molecules of hydrogen gas. If break the contact of zinc and copper, hydrogen will evolve on a zinc surface again.