Rusting of Iron (Cambridge International IGCSE Chemistry)

Spec point — 9.5.1 · State the conditions required for the rusting of iron and steel to form hydrated iron(III) oxide State some common barrier methods, including painting, greasing and coating with plastic Describe how barrier methods prevent rusting by excluding oxygen or water

Rusting of iron

What is rusting?

• Rusting is a chemical reaction between iron, water and oxygen

  • Rust is the reddish-brown compound product called hydrated iron(III) oxide

• Both oxygen and water must be present for rusting to occur

  • Steel, which contains iron, also rusts under the same conditions

• During rusting, iron is oxidised

iron + water + oxygen → hydrated iron(III) oxide

Investigating rusting

• To investigate the conditions required for rusting, prepare three test tubes as shown in the diagram
• The oil layer in the 2nd tube keeps out air and the water has been boiled so that no air is left in it
• The calcium chloride in the 3rd tube is used to remove any moisture in the air
• After a few days, the iron nail in the 1st tube will be the only nail to show signs of rust

Requirements for rusting

Diagram showing the requirements of oxygen and water for rust to occur: only the nail exposed to both air and water rusts

Rust prevention methods

• Rust can be prevented by coating iron or steel with barriers that prevent the iron from coming into contact with water and oxygen
• However, if the coatings are removed or scratched, the iron or steel is once again exposed to water and oxygen and will rust

Table to show common barrier methods

Exam Hint Only iron or steel (an alloy made from iron) can rust. If any other metal oxidises in air causing the metal to break down, you should say that the metal has corroded.