Gold has always been a highly prized metal, widely used from the beginning of history as a store of value. It doesn't rust like iron and doesn't become tarnished like silver. It is so chemically inert that it won't react even with the strongest concentrated acids. But it can be dissolved in aqua regia- a fresh-prepared mixture of concentrated {eq}HNO_3 {/eq} and HCl (1:3).

When Germany invaded Denmark in World War II, the Hungarian chemist George de Heresy dissolved the gold Nobel Prizes of Max von Laue and James Franck in aqua ragia to prevent the Nazis from stealing them. He placed the jar with the solution on a shelf in his laboratory, and after the war, precipitated the gold out of the add and returned it to the Royal Swedish Academy of Sciences and the Nobel Foundation who recast the medals and again presented them to Laue and Franck.

The unbalanced equation for the reaction of gold with aqua regia is given below. Add the stoichiometric coefficients to the equation to balance it.

{eq}Au(s)+HNO_3(aq)+HCl(aq) \rightarrow HAuCl_4(aq) + NO_2(g) +H_2O(l) {/eq}

What is the function of HCI?

a. It supplies chloride ions to form a complex ion with the oxidized gold.

b. It acts as an oxidizing agent.

c. It acts as a reducing agent.