In testing the polarity of a direct current generator, all of the following statements are true EXCEPT

• A. The bubbles consists of hydrogen gas
• B. The hydrogen ions have positive charge
• C. The pole which causes the most rapid bubbling is the anode
• D. The pole which causes the most rapid bubbling is the cathode

Answer: (C)

When current passes through an electrolyte, gas bubbles form at the electrodes: hydrogen gas evolves at the cathode, while oxygen gas forms at the anode. Hydrogen ions (H+) are positively charged, so they migrate to the cathode and gain electrons to become H2 gas. That makes the cathode the site of the more vigorous bubbling in typical aqueous solutions, because hydrogen evolution occurs readily and oxygen evolution requires more energy (higher overpotential). Therefore, saying that the pole causing the most rapid bubbling is the anode is incorrect—the cathode is the one with the fastest bubbling. Hydrogen bubbles and positively charged hydrogen ions are consistent with this, while the statement about the anode being the most vigorous site is the exception.