The decomposition of water into hydrogen and oxygen.
Explanation
The phenomenon described is the chemical effect of electric current, specifically known as electrolysis. Pure water is a poor conductor of electricity; however, adding a small amount of an electrolyte, such as sodium chloride (salt) or an acid, introduces free ions that make the solution conductive.
Detailed Analysis
- Electrolysis Process: When electric current passes through the conducting water solution, electrical energy is converted into chemical energy, causing the decomposition of water molecules ($H_2O$).
- Reaction at Electrodes:
- Cathode (Negative Terminal): Hydrogen ions gain electrons to form Hydrogen gas bubbles.
- Anode (Positive Terminal): In a dilute solution where salt is merely facilitating conduction, oxygen is evolved. Consequently, water decomposes into its constituent gases: Hydrogen and Oxygen.
- Analysis of Incorrect Options:
- The rapid evaporation of water due to heating. Incorrect: While the passage of current generates heat (Joule heating), the primary and most significant change observed in this context is chemical decomposition, not rapid evaporation.
- The precipitation of salt at the bottom of the container. Incorrect: The salt dissolves and dissociates into sodium ($Na^+$) and chloride ($Cl^-$) ions; it does not precipitate at the bottom.
- No chemical change in the solution. Incorrect: The evolution of gas bubbles indicates that a chemical change is actively occurring.
Key Takeaway: Electrolysis is the process where the passage of electric current through an electrolyte (conducting liquid) causes chemical decomposition, such as splitting water into hydrogen and oxygen.