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.
What is Chemistry Behind the Rusting of Iron?
Iron rusting is an oxidation reaction. During rusting, iron combines with oxygen in the air in the presence of water to generate Fe2O3.xH2O, a hydrated iron (III) oxide.
This hydrated iron (Ill) oxide is referred to as rust. Rust is largely hydrated Iron (III) Oxide, Fe2O3.xH2O. The color of rust is reddish-brown. We’ve all noticed reddish-brown rust on iron nails, screws, pipes, and railings.
When exposed to wet air, not just iron, but also steel, rusts. Steel, on the other hand, is more resistant to rust than iron.
Why is the rusting of iron considered to be a chemical change?
Rust is formed when iron (or an alloy of iron) is exposed to oxygen in the presence of moisture. This reaction is not instantaneous; rather, it takes place over a long period of time. Iron oxides are formed when oxygen atoms combine with iron atoms. The bonds between the iron atoms in the object/structure are weakened as a result.
The oxidation state of iron increases as a result of the rusting reaction, which is followed by the loss of electrons. Rust is primarily composed of two types of iron oxides that differ in the oxidation state of the iron atom. These are the oxides:
- Iron (II) oxide is also known as Ferrous Oxide. This substance has an oxidation state of +2 and the chemical formula FeO.
- Iron(III) oxide, often known as Ferric Oxide, is a compound in which the iron atom has an oxidation state of +3. Fe2O3 is the chemical formula for this substance.
Iron is a reducing agent, but oxygen is an excellent oxidizing agent. When exposed to oxygen, the iron atom easily gives away electrons. The chemical reaction is described as follows:
Fe → Fe2+ + 2e–
When water is present, the oxygen atom increases the oxidation state of iron.
4Fe2+ + O2 → 4Fe3+ + 2O2-
The iron cations and water molecules now undergo the following acid-base reactions.
Fe2+ + 2H2O ⇌ Fe(OH)2 + 2H+
Fe3+ + 3H2O ⇌ Fe(OH)3 + 3H+
The direct reaction between the iron cations and the hydroxide ions also produces iron hydroxides.
O2+H2O + 4e– → 4OH–
Fe2+ + 2OH– → Fe(OH)2
Fe3++ 3OH– → Fe(OH)3
The iron hydroxides that result are now dehydrated, yielding the iron oxides that makeup rust. Many chemical processes are involved in this process, some of which are given below.
- Fe(OH)2 ⇌ FeO + H2O
- 4Fe(OH)2 + O2 + xH2O → 2Fe2O3.(x+4)H2O
- Fe(OH)3 ⇌ FeO(OH) + H2O
- FeO(OH) ⇌ Fe2O3 + H2O
All of the chemical reactions listed above have one thing in common: they all require the presence of water and oxygen. As a result, the amount of oxygen and water surrounding the metal can be limited to prevent rusting.
Recognizing Chemical Reactions
How can you tell if a chemical reaction is taking place? Certain visual clues indicate that a chemical reaction is likely (but not necessarily) occurring, including the following examples:
- A change of color occurs during the reaction.
- Gas is produced during the reaction.
- Solid product, called a precipitate, is produced in the reaction.
- A visible transfer of energy occurs in the form of light as a result of the reaction.
When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced. The production of a gas is also an indication that a chemical reaction may be occurring.
When a colorless solution of lead (II) nitrate is added to a colorless solution of potassium iodide, a yellow solid called a precipitate is instantly produced. A precipitate is a solid product that forms from a reaction and settles out of a liquid mixture. The formation of a precipitate may also indicate the occurrence of a chemical reaction.
Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)
What is Physical Changes?
Properties such as shape, size, state of the substance and its colour are known as physical properties. Physical properties are affected by the physical change.
For Example, when ice changes to water, there is a change in the physical state of ice, i.e. it changes from solid to liquid. If we cool the water again, ice will be formed making it a reversible reaction. Also, no new substance is formed in the above physical change.
As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change. A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not.
Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible.
Other changes of the state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state. The salt may be regained by boiling off the water, leaving the salt behind.
When a piece of wood is ground into sawdust, that change is irreversible since the sawdust could not be reconstituted into the same piece of wood that it was before. Cutting the grass or pulverizing a rock would be other irreversible physical changes. Firewood also represents an irreversible physical change since the pieces cannot be put back together to form the tree.