What rusting needs and how to stop it, alloys and gold carats, glass and ceramics, the two kinds of polymer, and composites.
What you will learn
- Deduce the conditions for rusting from a test-tube experiment
- Explain sacrificial protection
- Convert gold carats to a percentage and a mass
- Compare thermosoftening and thermosetting polymers and describe composites
Rusting and how to stop it
Corrosion is the destruction of a material by reactions with substances around it.
Iron rusts only when both oxygen and water are present; salt speeds it up.
iron + oxygen + water → hydrated iron(III) oxide (rust)
- A nail in boiled water under a layer of oil does not rust: boiling removes dissolved air, and the oil stops more dissolving.
- A nail in a sealed tube with anhydrous calcium chloride does not rust: the drying agent removes water vapour.
- Barriers (paint, oil, grease, plastic, electroplating) keep oxygen and water out.
- Sacrificial protection: a more reactive metal (zinc, magnesium) is attached or coated on. It is oxidised instead of the iron, even through a scratch. Galvanising is a zinc coating.
A tin coating is only a barrier: tin is less reactive than iron, so once scratched the iron rusts.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Rusting and how to stop it
- Alloys and carats
- Glass, ceramics, polymers and composites
- Try it: Read the four tubes
9 practice questions, marked as you go; about 25 minutes in all.
Key terms
Barriers, Aluminium alloys, Clay ceramics, Thermosoftening, Thermosetting, Composites, Reinforcement, Matrix
In this unit: The Earth's atmosphere
The air now and how it formed, greenhouse gases and climate change, carbon footprints and the pollutants from burning fuels. Its later lessons on water, life cycles, materials and the Haber process stay here too; Using resources teaches them in full.