The process of dissolving metal in acid is not a physical change, like dissolving sugar in water, but a chemical reaction known as corrosion or etching. This interaction transforms the solid metal into a soluble metal salt, or ion, within the liquid acid solution. Understanding which acids are effective and why they work is fundamental in fields ranging from industrial metal finishing to the refining of precious materials. The ability of an acid to dissolve a metal depends entirely on the specific chemical properties of both substances.
The Underlying Chemical Mechanism
The fundamental process that allows an acid to dissolve metal is a type of chemical exchange called an oxidation-reduction (redox) reaction. In this reaction, the metal atoms lose electrons (oxidation) to become positively charged metal ions that enter the solution. Simultaneously, the hydrogen ions (protons) present in the acid gain those electrons (reduction), typically forming hydrogen gas (\(\text{H}_2\)).
The likelihood of this reaction occurring is primarily determined by the metal’s position in the Activity Series. Metals that are positioned higher in this series are more reactive, meaning they have a greater tendency to lose electrons and are therefore more easily oxidized by common acids. Metals positioned below hydrogen in the series generally do not react with acids. This series acts as a reliable predictor for whether a metal will dissolve in a standard non-oxidizing acid.
Standard Acids Used for Dissolving Common Metals
For common, reactive metals like zinc, iron, and aluminum, the most frequently used acids are hydrochloric acid and sulfuric acid. Hydrochloric acid (\(\text{HCl}\)) and sulfuric acid (\(\text{H}_2\text{SO}_4\)) react with metals high on the Activity Series to produce the corresponding metal salt and hydrogen gas.
Nitric acid (\(\text{HNO}_3\)) is also a strong acid but acts differently because it is a potent oxidizing agent. When it reacts with most metals, the acid itself is reduced, not just the hydrogen ions, which means the reaction rarely produces hydrogen gas. Instead, the reduction of the nitrogen atom often yields various nitrogen oxides, such as nitrogen dioxide (\(\text{NO}_2\)).
This unique oxidizing property can also lead to a phenomenon known as passivation, particularly with metals like aluminum and stainless steel. In passivation, the nitric acid rapidly forms an extremely thin, dense, and highly protective metal oxide layer on the surface of the metal. This oxide layer shields the underlying metal from further corrosion, effectively preventing the acid from dissolving it.
The Exception: Acids Required for Noble Metals
Highly unreactive metals, known as noble metals, such as gold and platinum, resist dissolution by even the strongest single acids like concentrated nitric or hydrochloric acid. Their low reactivity means they are below hydrogen on the Activity Series and are extremely difficult to oxidize. To dissolve these metals, a specific combination of acids is required.
This special solvent is called Aqua Regia, which translates to “royal water,” and is a freshly prepared mixture of concentrated nitric acid and concentrated hydrochloric acid, traditionally in a 1:3 volume ratio. The combination works through a synergistic effect where the nitric acid acts as a powerful oxidizing agent, converting the noble metal atoms into metal ions. Simultaneously, the hydrochloric acid supplies chloride ions that immediately react with the newly formed metal ions to create highly stable complex ions.
For gold, the chloride ions form the tetrachloroaurate ion (\(\text{AuCl}_4^-\)). This complexing action removes the metal ions from the solution, preventing the reverse reaction and driving the dissolution process forward. The combined effect of strong oxidation and complex ion formation overcomes the metal’s inherent resistance.
Essential Safety Precautions
Working with strong acids used for metal dissolution necessitates strict adherence to safety protocols due to their corrosive and hazardous nature. Personal protective equipment is mandatory, including chemical splash goggles, chemical-resistant gloves, and a lab coat or apron. These concentrated chemicals can cause severe burns on contact.
All reactions involving strong acids, especially those that generate toxic or irritating fumes like nitrogen oxides or chlorine gas, must be performed under a properly functioning fume hood to ensure adequate ventilation. When preparing a dilute acid solution, always add the acid slowly to the water, never the other way around.
Adding water to concentrated acid can cause a violent, exothermic reaction that generates excessive heat, leading to boiling and dangerous splashing. Acid storage requires a dedicated, non-metallic cabinet, separated from other reactive chemicals like bases. These chemical processes should only be attempted by trained professionals.