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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.
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(a) Why is it necessary to clean the rods with sand paper before dipping them into the liquids?