information:classification, structural:expansion
Acids

Lemon juice, containing citric acid which is the active bleaching agent, can effectively remove stains. Its action can be accelerated by exposing the stain to sunlight, or some other UV source, while soaking. Various acids were used in the past such as Phosphoric acid as used in Calcium Lime Rust Remover (CLR) and Hydrofluoric acid as used in the Australian product made in Queensland called "Rustiban". Both of these Acids have been removed from sale to the general public due to toxicity concerns. Both of these acids were used primarily to remove rust.Other rust removal acids are oxalic acid.