Reagent Capable of Dissolving Gold and Platinum

The question asks to identify a specific chemical reagent that possesses the ability to dissolve gold and platinum. Gold (Au) and platinum (Pt) are known as noble metals, which means they are highly resistant to corrosion and chemical reactions under normal conditions. Finding a reagent that can effectively break them down requires a powerful chemical combination.

Understanding Noble Metals and Chemical Reactivity

Gold and platinum are valuable metals precisely because they are unreactive. This lack of reactivity makes them resistant to attack by most acids, alkalis, and other chemicals. Standard acids like hydrochloric acid (\(HCl\)) or nitric acid (\(HNO_3\)) alone cannot dissolve these metals.

• Nitric acid (\(HNO_3\)) is a strong oxidizing agent, capable of dissolving many metals, but it passivates gold and platinum by forming a thin protective oxide layer.
• Hydrochloric acid (\(HCl\)) is a strong acid that can dissolve metals forming metallic chlorides, but it lacks the oxidizing power to attack gold or platinum directly.

To dissolve gold and platinum, a reagent must be both a strong oxidizing agent and capable of forming stable complex ions with the metal atoms.

Analysis of the Reagent: Aqua Regia

Let's examine the provided options:

• Aqua guard, Aqua radar, and Aqua proof: These terms do not correspond to known chemical reagents used for dissolving noble metals in standard chemistry contexts. They appear to be distractors.
• Aqua regia: This is the correct answer. Aqua regia, which translates to "royal water" in Latin, is a highly corrosive mixture of freshly prepared concentrated nitric acid (\(HNO_3\)) and concentrated hydrochloric acid (\(HCl\)), usually in a molar ratio of 1:3.

Why Aqua Regia Dissolves Gold and Platinum

The effectiveness of aqua regia lies in the synergistic action of its components:

• Nitric Acid (\(HNO_3\)): Acts as the primary oxidizing agent. It oxidizes the metal atoms (like gold or platinum) into positive ions.
• Hydrochloric Acid (\(HCl\)): Provides chloride ions (\(Cl^-\)). These chloride ions react with the metal ions formed by nitric acid to create stable complex anions. This complex formation is crucial because it effectively removes the metal ions from the solution equilibrium, allowing the reaction to proceed further.

The reactions occurring in aqua regia are:

• Nitric acid and hydrochloric acid react to form nitrosyl chloride (\(NOCl\)) and nascent chlorine (\(Cl_2\)), both of which are powerful oxidizing agents.
• Dissolving Gold: Gold is oxidized by nitric acid and then complexed by chloride ions from hydrochloric acid. The reaction can be represented as:

  \(Au_{(s)} + HNO_{3(aq)} + 4HCl_{(aq)} \rightarrow H[AuCl_4]_{(aq)} + NO_{(g)} + 2H_2O_{(l)}\)

  Here, \(H[AuCl_4]\) is the tetrachloroaurate(III) ion complex.
• Dissolving Platinum: Platinum reacts similarly, forming a stable hexachloroplatinate(IV) complex. The reaction is typically represented as:

  \(Pt_{(s)} + 4HNO_{3(aq)} + 6HCl_{(aq)} \rightarrow H_2[PtCl_6]_{(aq)} + 4NO_{2(g)} + 4H_2O_{(l)}\)

  Alternatively, platinum can also react involving nitrosyl chloride and chlorine generated in situ:

  \(Pt_{(s)} + 2NOCl_{(g)} + 2Cl_{2(g)} \rightarrow PtCl_{4(s)}\)

  \(PtCl_{4(s)} + 2HCl_{(aq)} \rightarrow H_2[PtCl_6]_{(aq)}\)

  Here, \(H_2[PtCl_6]\) represents the hexachloroplatinate(IV) complex.

The formation of these stable chloro-complexes drives the dissolution process, making aqua regia one of the few reagents capable of dissolving gold and platinum.

Conclusion on the Reagent

Based on its chemical properties and established use in chemistry, Aqua regia is the correct reagent that can dissolve both gold and platinum.