chemical
White vinegar
Dilute acetic acid. Excellent on limescale, useless as a disinfectant, and harmful to stone.
What it actually does
Acetic acid reacts with calcium carbonate — limescale, hard-water film, kettle fur — and dissolves it. That is the whole trick.
What it does not do
It is not a disinfectant to any clinical standard. It does not "react with bicarbonate to clean"; the fizz is over in seconds and leaves sodium acetate in water.
Using it
Equal parts with water, and give it contact time. Twenty minutes on a limescaled tap does more than an hour of scrubbing.