Understanding Chemical Change by Electric Current

The question asks about the specific process where passing an electric current through a substance results in a chemical change within that substance. This phenomenon is a fundamental concept in the field of electrochemistry.

Let's examine the given options:

1. Thermionic conduction: This refers to the emission of electrons from a heated surface. While it involves electricity and can sometimes lead to chemical changes on the surface, the primary process is electron emission due to heat, not a chemical change within a bulk substance caused by current passage.
2. Electroplating: This is a process where electrolysis is used to deposit a thin layer of one metal onto another object. It is an application of the process described in the question, but not the general name for the process itself.
3. Electrodes: These are conductors, usually metal, that are used to make electrical contact with a nonmetallic part of a circuit, such as an electrolyte. They are components used in the process, but they are not the process itself.
4. Electrolysis: This is defined as the chemical decomposition of a substance by the passage of an electric current through it. The prefix 'electro-' relates to electricity, and '-lysis' means splitting or decomposition. This definition perfectly matches the description given in the question - a chemical change caused by an electric current.

Therefore, the process by which a chemical change takes place in a substance when electric current is passed through it is called electrolysis.

In electrolysis, ions in an electrolyte move towards the electrodes (anode and cathode) and undergo redox reactions (oxidation and reduction), leading to the decomposition of the substance or deposition of new substances.