Question

A metal which is unaffected by dilute or concentrated acid.

Answer 1

• Gold $\left(\mathrm{Au}\right)$ is a metal that is unaffected by dilute or concentrated acid.
• Gold is unaffected by air, moisture, alkalis or acids except for aqua regia which can dissolve gold. Aqua regia a solution of concentrated hydrochloric acid and concentrated nitric acid in the ratio $3:1$.

$$\begin{gathered} \mathrm{Au}\left(\mathrm{s}\right)+3{{\mathrm{NO}}_3}^{-}\left(\mathrm{aq}\right)+6{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{Au}}^{3+}\left(\mathrm{aq}\right)+3{\mathrm{NO}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{gold}\mathrm{nitrate}\mathrm{hydrogen}\mathrm{gold}\mathrm{nitrogen}\mathrm{water} \\ \mathrm{ion}\mathrm{dioxide} \\ {\mathrm{Au}}^{3+}\left(\mathrm{aq}\right)+4{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\rightleftharpoons {{\mathrm{AuCl}}_4}^{-}\left(\mathrm{aq}\right) \\ \mathrm{gold}\mathrm{chlorine}\mathrm{tetrachloroaurate} \end{gathered}$$

• So, it never rusts and is used in making jewellery.

Hence, Gold $\mathbf{\left(}\mathbf{Au}\mathbf{\right)}$ is a metal that is unaffected by dilute or concentrated acid.