Question

On exposure to air Some articles made of silver, copper and iron get coloured coating over them. Identify the colour along with the chemical name of the substance of coating in each case.

Answer 1

Hint:Silver, Iron and Copper react with atmospheric oxygen or in some cases sulphur dioxide to form its oxides. These oxides of respective metals have their characteristic colours which form a coloured coating over the article.

Complete step-by-step answer:The coloured coating formed is a result of corrosion of the metal in presence of oxygen or sulphur dioxide or other compounds. The coating is generally a thin layer. It is also called tarnish. Let’s see what is formed in each case one by one.
i) Silver:
The silver from the surface of articles may react with hydrogen sulphide of sulphur in other compounds to form silver sulphide. Silver sulphide forms a thin layer over the whole article and it has a different colour than silver metal. Silver sulphide formed a black coloured layer over silver. The reaction can be given as below.
     \[2AgCl+{{H}_{2}}S\to A{{g}_{2}}S+2HCl\]
ii) Copper:
The copper on the surface of its articles react with atmospheric oxygen to give its oxide. But this oxide further react with the moisture and carbon dioxide gas to give a mixture of copper hydroxide and copper carbonate. This both compounds from the layer on copper articles and the layer is green in colour due to both of them. The overall reaction of formation of layer can be given as below.
     \[2C{{u}_{(s)}}+{{H}_{2}}O+{{O}_{2}}+C{{O}_{2}}\to Cu{{(OH)}_{2}}+CuC{{O}_{3}}\]
iii) Iron:
Iron on reaction with the atmospheric oxygen and moisture, undergoes corrosion and iron on the surface gets oxidized to its oxide form. Thus, ferric oxide gets formed and this forms a layer on the surface of the article. This layer is reddish brown in colour. The reaction of formation of this layer can be given as under.
     \[2Fe+3{{O}_{3}}+x{{H}_{2}}O\to F{{e}_{2}}{{O}_{3}}\cdot x{{H}_{2}}O\]
Thus, we can conclude that layers formed on silver, copper and iron is black, green and reddish brown in colour respectively. Black colour of silver articles is due to silver sulphide, green colour of copper articles is due to copper hydroxide and copper carbonate and reddish brown coloured layer formed on iron is due to ferric oxide layer.

Note:Remember that most of the metals react with atmospheric oxygen to form their oxides but silver is an exception there. It reacts with the hydrogen sulphide to form silver sulphide. So, note that silver oxide is not formed in this process.