Formation of hydrogen bubbles at the copper plate of a primary cell is called?

Analyze the question: It asks for the name of the defect in a simple (voltaic) cell where hydrogen bubbles collect on the copper plate (the positive electrode).

Explain the concept: In a simple cell, zinc and copper plates are placed in dilute sulphuric acid.

• Zinc reacts with the acid and releases electrons.
• Hydrogen ions (H⁺) from the acid move to the copper plate, pick up electrons and form hydrogen gas.
• The hydrogen gas forms a layer of bubbles on the copper plate.

This layer causes two problems:

1. It insulates the copper plate from the electrolyte, which increases the internal resistance of the cell.
2. It sets up a reverse potential, so the current falls quickly.

This defect is called polarization. It is cured by adding a depolarizer such as manganese(IV) oxide (MnO₂), which oxidizes the hydrogen to water. This is what happens in the Leclanché cell.

Check the other options:

• B. Back e.m.f.: This is an opposing e.m.f. in a circuit, for example in motors or electrolysis cells. It is not the name for hydrogen bubbles forming on a plate.
• C. Electrolysis: This is the chemical decomposition of a substance by passing an electric current through it. It is not the defect described here.
• D. Local action: This is the defect caused by impurities in the zinc, which form tiny cells and make the zinc waste away. It is cured by amalgamating the zinc with mercury.

Answer: The formation of hydrogen bubbles at the copper plate is polarization, which is option A.

Exam tip: Remember these two pairs:

• Polarization → hydrogen bubbles on the copper → cured by a depolarizer (MnO₂).
• Local action → impurities in the zinc → cured by amalgamation (mercury).