Question

: Reena has been collecting silver coins and copper coins. One day, she observed a black coating on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coatings? Write the chemical name of green and black coatings.

Answer 1

This question is based on the experimental performance relating to the theoretical concept. Sometimes, we observe that the objects made up of iron are being covered with a coating or layers like reddish-brown colour forming a sulphide or oxide. That process is known as rusting which is considered to be a form of corrosion.

Complete step by step answer:
Now, we know that Reena has been collecting silver, and copper coins. As we know that silver, and copper both are metals or refined metals.
-According to the question, she observed a coating on silver and copper coins. This is all due to the corrosion phenomenon.
-Let us define the corrosion. In this phenomenon, refined metals or metals are being converted into compounds like oxides, sulphides or hydroxides of that metal.
-We can say that corrosion happens due to the reaction of metal with the water, or air present in the atmosphere.
-First, if we see the chemical reaction of silver coins having a black coating, it can be written as:
${\text{2Ag + }}{{\text{H}}_{\text{2}}}{\text{S}} \to {\text{A}}{{\text{g}}_{\text{2}}}{\text{S + }}{{\text{H}}_{\text{2}}}$
-In this reaction, we can see that the silver reacts with the hydrogen sulphide gas present in the atmosphere. Thus, here the product obtained is black silver sulphide.
-Second, we will see the chemical reaction of copper coins having a green coating. So, we can write the chemical reaction as:
${\text{2Cu + C}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{CuC}}{{\text{O}}_{\text{3}}}{\text{ + Cu(OH}}{{\text{)}}_{\text{2}}}$
-In this reaction, we can observe that copper reacts with moisture present in the air consisting of carbon-dioxide, oxygen, and water vapours. Here, the product obtained is copper carbonate and copper hydroxide.
-As we know that copper carbonate is green in colour.
Thus, in the last, we can conclude that the chemical phenomenon responsible for coatings is corrosion.
Thus, the chemical name of black coating is silver sulphide, and for the green coating, it is copper carbonate.

Note: Corrosion and rusting are two different phenomena. As we already mentioned that corrosion is the phenomenon for metals or refined metals. But the term rusting is only used for the iron. Rusting occurs when the iron reacts with the atmospheric moisture, and oxygen, and leads to the formation of iron oxides.