Chemical Reactivity of Gold
Gold (Au) is known for its remarkable chemical inertness, which is a primary reason for its high value and widespread use in jewellery and coinage. This noble metal stands apart from many other elements due to its resistance to chemical change.
Interaction with Water and Air
Gold exhibits extremely low reactivity with common environmental elements such as water and air.
- With Water: Gold does not react with water, regardless of its temperature or state (liquid or steam). It does not rust or corrode like iron.
- With Air: Gold does not tarnish or oxidize when exposed to air, even over long periods. Unlike silver, which darkens due to reaction with sulfur compounds in the air, gold retains its characteristic lustre indefinitely. This property makes it ideal for decorative items and long-lasting artefacts, as seen in ancient Indian temples and sculptures that maintain their shine over centuries.
General Inertness
The chemical inertness of gold stems from its electronic configuration and high ionization energy. Its outermost electrons are held very tightly by the nucleus, making it difficult for gold atoms to lose electrons and form positive ions, which is a prerequisite for most chemical reactions. This stability prevents it from readily combining with most other elements or compounds under ordinary conditions.
Other Properties of Gold
Toxicity
Elemental gold, in its pure metallic form, is generally considered non-toxic to humans and animals. It is biologically inert, meaning it does not readily react with biological systems. This characteristic allows its use in various applications, including dentistry (fillings, crowns), medicine (certain injectable gold compounds for arthritis treatment, though these are not elemental gold), and even as a food additive (e.g., gold leaf (varak) used to decorate sweets (mithai) like barfi and laddoo in India). While pure gold is safe, some gold compounds can be toxic if ingested or absorbed.
Radioactivity
Naturally occurring gold is not radioactive. The most common and stable isotope of gold is Gold-197. While synthetic radioactive isotopes of gold exist and are used in specialized medical treatments or research, these are not found naturally. Therefore, articles made of gold, such as traditional Indian mangalsutra or bangles, do not pose a risk of radioactivity.
Flammability
Gold is a metal and is not flammable in the conventional sense. It does not ignite or burn when exposed to flame or high temperatures. Instead, when heated to its melting point of 1064 °C, it transitions from a solid to a liquid state. It can be vaporized at much higher temperatures (around 2856 °C), but it does not support combustion.
A Notable Chemical Reaction: Aqua Regia
Despite its general inertness, gold can be dissolved by a highly corrosive mixture known as Aqua Regia. This Latin term, meaning “royal water,” reflects its ability to dissolve the “royal metal,” gold.
Aqua Regia is a fuming yellow or red solution formed by mixing concentrated nitric acid (HNO₃) and concentrated hydrochloric acid (HCl), typically in a molar ratio of 1:3.
The reaction proceeds in a two-step process:
- Nitric acid acts as a powerful oxidizing agent, oxidizing solid gold (Au) into gold(III) ions (Au³⁺): Au(s) + 3HNO₃(aq) → Au³⁺(aq) + 3NO₂(g) + H₂O(l) (Simplified representation)
- The hydrochloric acid then reacts with the gold(III) ions to form stable tetrachloroaurate(III) anions ([AuCl₄]⁻): Au³⁺(aq) + 4Cl⁻(aq) → [AuCl₄]⁻(aq)
This second step is crucial because it removes the gold ions from the solution, shifting the equilibrium of the first reaction forward and allowing more gold to be oxidized and dissolved. This unique property of Aqua Regia is historically significant and is still used today in laboratories and by goldsmiths (sunar) in India for refining gold or dissolving it for various processes.