Question

An iron metal, solid iron metal, rusts. What type of reaction is it?
A. Decomposition
B. Synthesis
C. Combustion
D. Single replacement

Answer 1

The classification of rusting of iron as a chemical reaction can be done keeping in mind the number of reactants that are involved in the reaction and the type and number of products being formed. The rust formed is an oxidized form of iron metal.

Complete answer:
Rusting of iron is an example of the corrosion reactions. The iron metal degrades and loses its shine and luster by the action of moisture and oxygen from the surroundings. The metal surface is known to get oxidized and an iron oxide is formed in the process.
The chemical equation for the reaction involving rusting of iron is written as follows:
$4Fe(s) + 3{O_2}(g)\xrightarrow{{x{H_2}O(moisture)}}2F{e_2}{O_3}.x{H_2}O$
Even though the metal surface decays in this reaction, there is no breaking down of any compound, rather the reaction starts with iron in its elemental form and therefore it cannot be a decomposition reaction.
The reaction involves the combination of two reacting species that results in the formation of ferric oxide. Thus, this reaction can be classified as a Synthesis reaction that involves the synthesis of iron oxide.
Hence, the correct option is (B) synthesis.

Note:
Combustion is also a type of chemical reaction that involves reaction with oxygen and results in the formation of oxides. But the rusting of iron cannot be classified as combustion as iron metal is not a combustible substance, it does not burn or release large amounts of heat on reaction with oxygen.