Why is rusting considered a chemical change?

Rusting is considered a chemical change because it involves a reaction that changes iron into a new substance, iron oxide.

Rusting is a process that occurs when iron or an alloy that contains iron, like steel, is exposed to oxygen and moisture for a long period of time. Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide and weakening the bonds of the metal itself. This process is commonly known as rusting and is a type of corrosion.

The reason why rusting is considered a chemical change rather than a physical one is because the iron is not merely changing its form or state, it is being transformed into a completely new substance. In a physical change, the substance may change its state or form, but its fundamental chemical structure remains the same. However, in a chemical change, the substance's basic structure is altered, creating a substance with different properties.

In the case of rusting, the iron reacts with oxygen in the air to form iron(III) oxide. This is a redox reaction, where the iron is oxidised (loses electrons) and the oxygen is reduced (gains electrons). The iron(III) oxide, or rust, is a different substance to the original iron - it has different properties, such as colour, hardness, and susceptibility to further corrosion.

This process is not easily reversible, which is another characteristic of chemical changes. Once the iron has rusted, it cannot be changed back into pure iron without undergoing another chemical reaction. This is in contrast to physical changes, which are often easily reversible.

In summary, rusting is considered a chemical change because it involves a reaction that changes the chemical structure of iron, creating a new substance, iron(III) oxide, with different properties.