Question

Oxalic acid is commonly used to remove rust stains

Answer 1

Oxalic acid reacts with rust (iron(III) or ferric oxide or hydroxide) to form ferric  

oxalate, which is soluble in water. The reaction is fairly slow and heating the  

solution will speed it up somewhat. I have used it many times to remove rust  

stains from minerals like quartz. The oxalic acid reacts with the rust to form  

ferric oxalate and does not react with the quartz at all.