What happens when dilute hydrochloric acid is added to iron filings?

Concept

Any metal placed above hydrogen in the reactivity series can hand its electrons to the H+ ions supplied by a dilute mineral acid. The governing pattern is: metal + dilute acid → salt + hydrogen gas, where the anion of the acid becomes the anion of the salt. Nothing in this pattern can oxidise the acid’s anion or supply hydroxide ions.

Application to iron and dilute hydrochloric acid

1. Iron stands above hydrogen in the reactivity series, so the electron transfer is possible.

2. Each iron atom loses two electrons: Fe → Fe2+ + 2e-.

3. Each pair of H+ ions from the dilute acid takes those electrons and leaves the solution as a diatomic gas: 2H+ + 2e- → H2↑.

4. The chloride ions already present in the acid pair with Fe2+ to give the salt iron(II) chloride, FeCl2.

5. Overall: Fe + 2HCl (dilute) → FeCl2 + H2↑.

What is seen in the test tube: brisk effervescence of a colourless, odourless gas that burns with a squeaky pop, and a solution that turns pale green because of the Fe2+ ions formed.

Cross-check against the other outcomes

• Chlorine gas would require the chloride ion to be oxidised, which needs a strong oxidising agent with concentrated hydrochloric acid (for example MnO2 warmed with concentrated HCl). A metal reacting with dilute acid does the opposite — it supplies electrons rather than removing them.

• An iron hydroxide such as Fe(OH)2 needs a source of OH- ions, for example sodium hydroxide solution added to an iron(II) salt. Dilute hydrochloric acid supplies H+ and Cl-, not OH-.

• “No reaction” is what is recorded for metals that lie below hydrogen — copper, silver and gold show no effervescence with dilute hydrochloric acid.

• Exactly one of the three listed outcomes occurs here, so neither “more than one of the above” nor “none of the above” describes the observation.

Result: adding dilute hydrochloric acid to iron filings produces hydrogen gas and iron(II) chloride.