Chemical Properties and Why They Matter
The Sydney Harbour Bridge uses 52,800 tonnes of steel and spent $8.5 million on repainting in 2017, because iron's chemical reactivity makes corrosion inevitable.
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Q1 · Think about a nail left outside in the rain versus one kept dry indoors, what do you think would happen to each, and what is actually changing about the material?
Q2 · Why do you think chemical properties of a material are just as important as its physical properties when engineers choose what to build with?
● Know
- The key chemical properties of materials
- Examples of materials with high and low reactivity
- How to distinguish physical from chemical change
● Understand
- Why rust is a chemical change, not a physical one
- How chemical properties determine safe material use
- Why corrosion resistance matters in engineering
● Can do
- Identify chemical changes from physical changes
- Explain why certain materials corrode while others resist corrosion
- Evaluate material suitability based on chemical properties
Drop a small piece of sodium into a beaker of water and it instantly catches fire, skittering across the surface as it hisses and releases hydrogen gas; drop a copper coin into the same beaker and nothing happens at all. The reactivity series ranks metals in order of how readily they react with water, acids, and oxygen. At the top sit the alkali metals: potassium and sodium react explosively with water, releasing hydrogen gas and a huge amount of heat. In the middle sit iron and zinc, they corrode slowly. At the bottom sit copper, silver, and gold, so unreactive that they are found as pure metals in nature and have been used for thousands of years.
The position of a metal in the series determines almost everything about its engineering suitability. A metal high in the series needs expensive protective coatings. A metal low in the series can be left exposed, gold dental fillings don't corrode inside your mouth, but an iron filling would rust within weeks. Reactivity is a key chemical property that directly controls where and how a metal can be used.
Potassium (top of series) dropped into water produces a violent reaction and purple flame. Gold (bottom of series) can be submerged in seawater for centuries without changing, this is why gold jewellery recovered from shipwrecks still looks brand new after 400 years on the ocean floor.
Australia is the world's largest gold producer (300+ tonnes/year from Western Australian mines). Gold's extremely low reactivity, it sits near the bottom of the series, is precisely why it is used in electronics, space telescope mirrors, and medical implants where any corrosion would be catastrophic.
Corrosion is the gradual deterioration of a material through chemical reactions with its environment. Rusting is a specific type of corrosion affecting iron and steel: iron atoms react with oxygen and water in an electrochemical process to form hydrated iron(III) oxide (Fe₂O₃·xH₂O), the red-brown flaky solid we call rust. The reaction requires both oxygen and water together; removing either one stops rusting.
Different metals corrode differently. Aluminium oxidises rapidly in air, but its oxide (Al₂O₃) forms a hard, transparent layer that seals the surface, self-protective. Iron's rust is porous and flaky, it falls away, exposing fresh iron below, so corrosion continues until the metal is completely consumed. Copper forms a green patina (copper carbonate) that also self-protects. Understanding the mechanism explains why protection strategies must be tailored to each metal.
A steel car panel in a coastal NSW town corrodes visibly within 3 years without treatment. The same panel treated with a 15 µm zinc galvanising layer lasts 20+ years because zinc corrodes preferentially, protecting the steel beneath even if the zinc layer is scratched.
The Sydney Harbour Bridge's entire surface, 485,000 m², is continuously repainted by a team of workers using 30,000 litres of paint per year. The paint film is the only barrier preventing the bridge's 52,800 tonnes of steel from corroding into unusable rust within a decade.
Engineers use four main strategies to prevent corrosion. Galvanising coats steel with a thin zinc layer, zinc is higher in the reactivity series than iron, so it corrodes first, protecting the steel (sacrificial protection). Anodising uses electrolysis to thicken the natural oxide layer on aluminium, creating a hard, decorative coating. Paint and polymer coatings physically exclude oxygen and water. Alloying changes the metal's composition, stainless steel contains 10–12% chromium, which forms a self-healing chromium oxide barrier on the surface.
Choosing the right protection method depends on the application. Galvanising suits steel beams and fences exposed to weather. Anodising suits aluminium window frames and smartphone cases, it can be coloured. Paint suits the Sydney Harbour Bridge but needs constant renewal. Stainless steel suits cutlery, kitchen equipment, and surgical instruments where the surface must remain clean and the coating must not chip off.
An iPhone case made from 7000-series aluminium alloy is anodised to a depth of 10 µm. The anodised layer is 3× harder than unanodised aluminium, resists fingerprint acids and cosmetics, can be dyed any colour, and adds less than 0.01 mm to the device's dimensions.
BlueScope Steel's Port Kembla plant in NSW produces ZINCALUME steel, steel coated with an aluminium-zinc alloy, used in roofing across Australia. The coating lasts 3–4× longer than plain galvanising in coastal conditions because the aluminium component forms an additional self-healing barrier.
protects steel by coating it with a thin layer of zinc. Zinc is higher in the series than iron, so it corrodes first and acts as a sacrificial layer. thickens the natural oxide layer on aluminium to create a hard, protective coating. Paint and polymer coatings work by physically excluding and water. Stainless steel resists corrosion because it contains chromium, which forms a self-healing barrier.
At the start of this lesson, you heard that salt corrodes iron 10 times faster near the ocean than inland, and that the Sydney Harbour Bridge needs 30,000 litres of paint every year to stop the steel from rusting. That's the real cost of a chemical property, reactivity with oxygen and water, being ignored in material selection.
Now that you've worked through the lesson, how would you explain why salt speeds up corrosion? How has your understanding of chemical properties changed the way you'd think about picking a material for an outdoor structure near the sea?
Q1. Explain the difference between a physical change and a chemical change. Give one example of each involving iron.
Q2. A student says, 'Rust is just dirt that collects on iron surfaces.' Evaluate this claim using your knowledge of chemical properties and chemical change.
Q3. Stainless steel is used in kitchen utensils and surgical instruments. Explain which chemical properties make it suitable for these applications and why ordinary iron would be inadequate.
Model answers (click to reveal)
SAQ 1 (2 marks)
Model answer: A physical change alters the form or appearance of a substance but does not create a new substance, so the same material remains and the change is often reversible. A chemical change produces one or more new substances with different chemical properties, and it is usually difficult to reverse. For iron, a physical change is melting solid iron into liquid iron, it is still iron and will become solid again when cooled. A chemical change is iron rusting, where iron reacts with oxygen and water to form a new substance, iron oxide (rust), which is brittle and reddish-brown rather than strong and shiny.
SAQ 2 (3 marks)
Model answer: The claim is incorrect. Rust is not dirt that settles on the surface, it is a new substance, iron oxide, that is produced when iron undergoes a chemical change. The iron atoms in the metal react with oxygen and water from the environment, and this chemical reaction forms iron(III) oxide, which has completely different properties from the original iron, it is reddish-brown, flaky, and brittle rather than grey, strong, and shiny. We know it is a chemical change rather than dirt collecting because rust only forms when both oxygen and water are present, the iron is chemically consumed and loses mass of metal, and the change cannot simply be wiped or washed away like dirt. Therefore the student has confused a chemical change with the deposit of foreign material.
SAQ 3 (3 marks)
Model answer: The key chemical property that makes stainless steel suitable is its high corrosion resistance, meaning it has very low reactivity with oxygen and water. Stainless steel contains around 10 to 12 percent chromium, which reacts with oxygen to form a thin, hard, self-healing layer of chromium oxide on the surface. This invisible barrier seals the metal and prevents further reaction, so the surface stays clean and does not corrode even when repeatedly washed or sterilised. Ordinary iron would be inadequate because it is far more reactive, it readily reacts with oxygen and water to form rust. In kitchen utensils this would contaminate food, and in surgical instruments rust could enter wounds and cause infection, while the rust would also flake away and weaken the metal over time. Stainless steel therefore performs reliably where ordinary iron would chemically deteriorate.