Copper half cell. When a stick of copper (Cu) is inserted in a copper salt solution it loses electrons according to the reaction Cu<-->Cu2+ + 2e-. So Cu metal and Cu2+ ion form a redox couple with Cu2+ as the oxidant and Cu as the reductant. This is called a half cell and can be used in conjunction with a salt bridge and a half cell of a different metal to produce a crude battery.
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