The acid that is used to remove rust is?

Analyze the question: The question asks which acid is used to remove rust. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O, so we need an acid that reacts with metal oxides to form soluble salts.

Explain the concept: Acids react with metal oxides (basic oxides) to give a salt and water. For rust removal, the acid should form a soluble iron(III) salt that washes away easily.

Solution:

1. Rust is mainly Fe₂O₃·xH₂O.
2. With hydrochloric acid: Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O Iron(III) chloride is soluble in water, so the rust dissolves and is removed. This process is called "pickling" and is used to clean steel before galvanizing or painting.

Checking the other options:

• A. Boric acid: A very weak acid, used as an antiseptic or eyewash. It is not effective on rust.
• C. Trioxonitrate(V) acid (HNO₃): A strong oxidizing acid that can attack iron, but it also dissolves the metal itself and can passivate iron surfaces. It is not the usual choice.
• D. Tetraoxosulphate(VI) acid (H₂SO₄): Dilute sulphuric acid can also remove rust in industry, but hydrochloric acid is the standard answer in textbooks and works faster at room temperature.

Answer: B.

Exam tip: Remember "Rust + HCl = pickling." In JAMB, if the question asks about removing rust, think hydrochloric acid, which is also the main ingredient in many household rust removers and toilet cleaners.