Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

When metallic copper comes in contact with moisture, a green powdery/pasty coating can be seen over it. This is chemically known as (a) copper carbonate-copper sulphate (b) copper sulphate-copper sulphide (c) copper carbonate-copper hydroxide (d) copper sulphide-copper carbonate

Step-by-Step Solution

Answer

(c) copper carbonate-copper hydroxide.

Step 1: Understand the Problem

We need to identify the chemical name of the green coating formed on copper when it reacts with moisture. This happens due to a chemical reaction between copper, carbon dioxide, water, and oxygen.

Step 2: Analyze Possible Options

The options given are combinations of chemical compounds. We need to determine which of these combinations corresponds to the known chemical composition that forms on copper surfaces.

Step 3: Identify the Correct Compounds

The green coating that forms on copper in the presence of moisture is referred to as a patina. The most common patina on copper is a mix of copper carbonate (\( ext{CuCO}_3\)) and copper hydroxide (\( ext{Cu(OH)}_2\)).

Step 4: Select the Correct Answer

According to our analysis, the option that matches is (c) copper carbonate-copper hydroxide. This compound is formed as a result of the reaction between copper, water, carbon dioxide, and oxygen.

Key Concepts

Copper Reactions

Copper is a metal that might look shiny when it's new, but over time, it changes. This change happens because copper reacts with other substances in its environment. This process is called "oxidation." When copper reacts, it combines with elements like oxygen and carbon dioxide from the air. Moisture can speed up these reactions. Over time, these reactions cause copper to develop a patina. This patina is the green or bluish layer that forms on the surface. Though some might think it’s rust, this patina actually protects the metal beneath from further corrosion.

Copper Carbonate

Copper carbonate is one of the compounds that form the green patina on copper. It has the chemical formula \(CuCO_3\). When copper comes into contact with carbon dioxide from the air, this reaction occurs. The formation of copper carbonate not only contributes to the distinct color but also helps to protect the metal. This compound acts as a barrier, preventing more harmful interactions. It's an interesting paradox where exposure to the elements strengthens a weathered character in copper, rather than degrading it.

Copper Hydroxide

Copper hydroxide is another important player in the creation of copper patina. When water comes into contact with copper, it reacts with oxygen to form copper hydroxide ( Cu(OH)_2 ). This reaction results in a bluish-green layer. Copper hydroxide is slightly soluble in water, which means it can spread a bit, even reaching areas not directly hit by water. Together with copper carbonate, it helps form the overall patina, giving copper its aged and distinguished appearance.

Chemical Reactions with Moisture

Moisture is a significant factor in the formation of a patina on copper. When copper surfaces are exposed to moist environments, chemical reactions that include oxygen, carbon dioxide, and water become active. This is because moisture provides the necessary medium for these reactions to occur. As copper interacts with these chemical agents, a sequence of reactions is set off. Eventually, this leads to the formation of compounds like copper carbonate and copper hydroxide. These reactions contribute to the original bright color fading, replaced by the well-known protective green layer. These interactions highlight the fascinating ways metals can transform and adapt when exposed to everyday environmental conditions.