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In an experiment, rods of metals P, Q and R were cleaned with sand paper and placed in a beaker containing water. Another set of rods was also cleaned
and placed in a beaker containing dilute acid. After placing the rods in two liquids bubbles of gas were seen around some of the rod
as shown in the diagrams below.
(a) Why is it necessary to clean the rods with sand paper before dipping them into the liquids?