In an electrochemical cell, polarization is caused by

Analysis

The question asks for the substance responsible for polarization in an electrochemical cell (specifically a simple voltaic cell).

Concept Explanation

In a simple voltaic cell (like the Daniel or Leclanché cell), polarization is a phenomenon where the efficiency of the cell drops significantly after a short period of use.

This happens because, during the chemical reaction, bubbles of hydrogen gas are produced at the copper electrode (the cathode). These bubbles form a layer around the electrode, effectively insulating it from the electrolyte. This creates two problems:

1. It increases the internal resistance of the cell.
2. It creates a "back electromotive force" (a counter-voltage) that opposes the flow of current, thereby reducing the cell's voltage.

To prevent this, a depolarizer (like manganese(IV) oxide) is often added to the cell to oxidize the hydrogen gas into water.

Step-by-Step Solution

1. Identify the process: Polarization is the accumulation of gas bubbles on the electrode surface.
2. Identify the gas: In a standard simple cell, the reaction at the cathode involves the discharge of hydrogen ions ($2H^+ + 2e^- \rightarrow H_2$).
3. Analyze the effect: The hydrogen gas ($H_2$) collects on the copper plate, preventing further contact between the electrolyte and the electrode.
4. Conclusion: Since hydrogen is the gas that forms the insulating layer, it is the direct cause of polarization.

Comparison with Options

• A, B, C: These are not the primary products formed at the cathode that cause the insulating layer in a simple cell.
• D (Hydrogen): This matches the scientific explanation for polarization in electrochemical cells.

Exam Tip

Whenever you see the word "polarization" in the context of a simple cell, immediately think of Hydrogen gas. If the question asks how to fix it, look for a "depolarizer" like Manganese(IV) oxide ($MnO_2$), which reacts with the hydrogen to form water.