Cathode

The cathode is the electrode where reduction occurs. In electrolysis it is the negative electrode, and cations move to it and are reduced by gaining electrons. Malay: katod · Chinese: 阴极.

The cathode attracts the positive ions, the cations, because opposite charges attract. At its surface those cations take in electrons that arrive through the external circuit; this gain of electrons is reduction. At the cathode either a metal is deposited or, for reactive-metal ions in solution, hydrogen gas is released, according to the electrochemical series. Cathode and cation are linked, which is a useful pair to memorise together.

Example. In the electrolysis of copper(II) sulfate solution, copper(II) ions move to the cathode and are reduced: Cu²⁺ plus 2e- to Cu. A pink-brown layer of copper builds up on the negative electrode, confirming reduction there.

Confusion to avoid. Do not fix the cathode as always negative. In electrolysis the cathode is negative, but in a voltaic cell the cathode is the positive terminal, because it is now where electrons re-enter the cell to reduce ions. As with the anode, define the cathode by its process: it is always where reduction happens.

Cathode in electroplating. When an object is electroplated it is made the cathode, so that metal ions from the electrolyte are reduced onto its surface as a thin, even coating. This is the same reduction step, put to a practical use.

Following the ions. During electrolysis the cations move through the electrolyte towards the cathode while electrons flow into it from the wire, so the cathode is where positive ions arrive and collect the electrons they need. Watching the cathode also gives a clear signal about the electrolyte: a metal coating forming means a metal ion is being reduced, while bubbles of a colourless gas usually mean hydrogen from a reactive-metal solution. Reading these signs correctly turns what you see into the right half-equation.

Identifying the cathode, predicting whether a metal or hydrogen forms there, and writing the reduction half-equation are standard electrolysis skills, so this term recurs throughout the cells work in SPM Chemistry.