Yes, nitric acid does dissolve copper, but this process is a rapid and highly vigorous chemical reaction, not a simple dissolution like salt in water. This chemical transformation converts the solid metal into dissolved ions and gaseous byproducts. The outcome of this reaction, including the specific gases produced, depends directly on the concentration of the acid being used.
The Underlying Chemical Mechanism
The dissolution of copper in nitric acid is classified as a reduction-oxidation (redox) reaction, meaning electrons are transferred between the reactants. Copper, a solid metal, acts as the electron donor, losing two electrons to become a dissolved copper(II) ion. This process is known as oxidation, as the copper’s oxidation state increases from zero to positive two.
Because copper is below hydrogen on the reactivity series, it does not react with many common acids to produce hydrogen gas. However, nitric acid is a strong oxidizing agent, readily accepting the electrons released by the copper. The nitrate ion within the acid gains these electrons, undergoing reduction. The strong oxidizing power of the nitrate ion drives the dissolution of the copper metal, facilitating the transformation of the solid metal into a dissolved salt.
How Acid Concentration Determines the Outcome
The specific product formed from the reduction of the nitrate ion is directly dependent on the concentration of the nitric acid solution. This variable dictates which nitrogen-containing gas is released into the atmosphere, resulting in two distinct reaction pathways.
When concentrated nitric acid is poured over copper, the primary reduction product is nitrogen dioxide. This gas is readily identifiable because it is a dense, reddish-brown gas that evolves rapidly from the reaction mixture. The formation of this toxic gas is a highly exothermic process, releasing a significant amount of heat, which accelerates the reaction further.
A different pathway is observed when using dilute nitric acid; here, the reduction primarily yields nitric oxide. Nitric oxide is a colorless gas, but it is unstable when exposed to the oxygen naturally present in the air. Upon contact with the atmosphere, the colorless nitric oxide quickly reacts with oxygen to form the familiar reddish-brown nitrogen dioxide. This color change from colorless to brown just above the solution surface indicates the presence of the less oxidized nitric oxide gas.
Identifying the Reaction Products
Regardless of the acid concentration used, the copper metal is uniformly converted into a dissolved salt called copper(II) nitrate. This compound is the source of the distinctive color change in the liquid solution. The presence of the dissolved copper(II) ions causes the solution to turn a vivid blue or blue-green.
In a highly concentrated nitric acid solution, the color may initially appear more green due to the interaction of the copper ions with the high concentration of nitrate ions. However, as the reaction progresses and the solution becomes more dilute with water, the characteristic blue color of the hydrated copper(II) ion emerges. This dissolved copper nitrate is the non-gaseous product of the complete reaction.
The other major products are the nitrogen-containing gases, which are collectively referred to as nitrogen oxides. These gases, whether initially produced as nitric oxide or nitrogen dioxide, are the reduced forms of the nitrate ion.
Critical Safety Considerations
The reaction between copper and nitric acid necessitates strict safety protocols due to the corrosive nature of the acid and the toxicity of the gaseous products. The primary safety concern is the generation of nitrogen dioxide, the reddish-brown gas formed in both concentrated and dilute reactions. Nitrogen dioxide is highly toxic and can cause serious respiratory damage if inhaled.
The reaction must always be performed in a well-ventilated area, preferably within a chemical fume hood, to effectively capture and remove the hazardous gases. Personal protective equipment, including chemical splash goggles and appropriate gloves, is necessary to prevent contact with the highly corrosive nitric acid. Nitric acid is a strong oxidizer and can cause severe chemical burns upon skin contact.
Proper handling and storage of the acid and the resulting copper nitrate solution are also important. The resulting solution contains heavy metal ions and corrosive material, requiring specialized disposal procedures rather than being poured down a standard drain.