Electrolysis is a powerful chemical technique that uses direct electric current (DC) to drive a non-spontaneous chemical reaction. The term literally translates to "splitting by electricity." It is the primary method used to decompose ionic compounds into their constituent elements. The IGCSE 0654 Core syllabus requires a formal definition of electrolysis, an identification of the components of an electrolytic cell, and a descriptive analysis of key industrial and laboratory examples.

Electrolysis is defined as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

The Requirement for a Liquid State

Ionic compounds do not conduct electricity in their solid state because the ions are locked in a rigid lattice. For electrolysis to occur, the compound must be:

• Molten: Heated until it melts, allowing ions to move.
• Aqueous Solution: Dissolved in water, which breaks the lattice and frees the ions.

The liquid that conducts the electricity is known as the electrolyte.

A simple electrolytic cell consists of three primary components:

1. Electrodes: Rods that conduct electricity into and out of the electrolyte.

• Anode: The positive electrode.
• Cathode: The negative electrode. (Memory aid: PANIC - Positive Anode, Negative Is Cathode).

1. Electrolyte: The ionic liquid undergoing decomposition.
2. Power Source: A battery or DC power supply that provides the energy to move the ions.

During electrolysis, positive ions (cations) move toward the negative cathode, while negative ions (anions) move toward the positive anode. This results in the formation of new substances at the electrodes.

A. Molten Lead(II) Bromide ($PbBr_2$)

• Cathode (-): Grey beads of lead metal form and sink.
• Anode (+): Brown vapors of bromine gas are evolved.

B. Concentrated Aqueous Sodium Chloride ($NaCl$)

In aqueous solutions, water ($H_2O$) is also present, providing $H^{+}$ and $OH^{-}$ ions.

• Cathode (-): Bubbles of colorless hydrogen gas are evolved (sodium remains in solution).
• Anode (+): Bubbles of pale green chlorine gas are evolved.

C. Dilute Sulfuric Acid ($H_2SO_4$)

This is effectively the electrolysis of water.

• Cathode (-): Bubbles of colorless hydrogen gas are evolved.
• Anode (+): Bubbles of colorless oxygen gas are evolved.
• Stoichiometry: Twice as much hydrogen is produced as oxygen ($2H_2 : 1O_2$).

Electrolysis is the bridge between electrical energy and chemical change. By understanding the movement of ions within an electrolytic cell, chemists can extract reactive metals, produce essential gases like chlorine, and refine substances to extreme purity. It remains one of the most industrially significant processes in modern chemistry.