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Page 2 Procedure: 1. Clean a strip of magnesium and a strip of copper with steel wool. 2. Pour ~200 mL of orange juice into a 250 mL beaker. 3. Connect an electrical lead to one end of the magnesium strip and submerge the other end in the orange juice. (You could tape it to the side of the beaker). 4. Connect the second electrical lead to one end of the copper strip and submerge the other end in the orange juice on the opposite side of the beaker. 5. Connect the other ends of the electrical leads to the battery compartment of a clock that would normally run on a single AA battery. 6. If the clock does not run, reverse the electrical leads in the battery compartment. Expected Results: The second-hand of the clock will begin to move upon connection to the electrical leads. The acid in the orange (or other citrus fruits) combines with electrodes, such as copper and magnesium to generate electricity. Batteries generate electricity through a chemical reaction between two different metals, such as magnesium and copper. When placed in an acid solution, electrons flow from one of the metals to the other, creating an electric current. Connecting the electrical leads to the clock's battery compart- ment allows the current to flow through them, causing the second-hand to move. The copper and magnesium metals act as positive and negative battery terminals (cathodes and anodes)—the copper as the positive electrode (the cathode) and magnesium as the negative electrode (the anode). The most likely reactions in this cell are oxidation of magnesium to Mg2 + ions at the anode and reduction of H + ions to hydrogen gas at the cathode. (The citric acid content in the orange is a rich source of H + ions.) The copper electrode is an inert electrode in this cell. Oxidation half-reaction (anode) Mg(s) => Mg2 + (aq) + 2e – Reduction half-reaction (cathode) 2H + (aq) + 2e – => H2(g) Net reaction Mg(s) + 2H+(aq) => Mg2 + (aq) + H2(g) Teaching notes: • A voltaic (galvanic) cell is created with the magnesium the anode and the copper the cathode. • The components of the cell can be changed to explore the key components (i.e., change one of the metal electrodes) of a voltaic (galvanic) cell. • Have students create data tables to record the results of their activities. Follow up: Electrochemistry, redox reactions, voltaic (galvanic) cells. • Connect two or more cells in series to generate a higher voltage. • How would the voltage change if you substituted zinc for the magnesium? • Measure the amount of electricity being generated by attaching a Micro Ammeter to the orange battery. • Use potatoes instead of fruit. Ask students to determine how the phosphoric acid in the potato impacts the activity. Why? What happens to the voltage if you boiled the potato? Why? • What other fruits and vegetables might you try that would work as batteries? Disposal/Clean-Up: • The orange juice can be disposed of down the drain. • The metal strips (electrodes) can be cleaned and reused. Orange Clock (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.com