Types of reaction · Process
Oxidation
A flash of burning magnesium and twenty years of a rusting gate are the same reaction. What is it, and what does it need?
Start here
Magnesium burns in two seconds. A gate rusts over twenty years. It is the same reaction.
One is a flash of white light and a puff of powder. The other is so slow that nobody has ever watched it happen. Both take a metal and turn it into a metal oxide, and both end up heavier than they started.
What do the two have in common?
Both are oxidation: a substance gaining oxygen. Speed is the only difference, and speed is not what a reaction is named after. Combustion is oxidation fast enough to produce a flame; rusting is oxidation slow enough to be somebody else's problem in ten years' time.
Oxidation is a reaction in which a substance gains oxygen. Metal plus oxygen makes a metal oxide, and the product always weighs more than the metal did, because the oxygen atoms are now part of it.
That makes oxidation the widest of the four reaction types in this unit. Every combustion is an oxidation. So is rusting, so is the browning of a cut apple, and so — inside every cell in your body, at a temperature you can survive — is respiration.
Your turn · four tubes, four weeks
An identical nail in each. Only what surrounds it is different.
Predict every tube before you open any of them. The tubes are designed so that the four results together answer the question no single tube could.
Tube 1
Air and water
An iron nail half in tap water, tube open to the air.
Rusted — orange and flaking at the water line.
Both requirements present. The worst of it is at the surface, where the nail meets air and water at once.
Tube 2
Dry air, no water
A nail in a bunged tube with a drying agent to take all moisture out of the air.
No rust. The nail is as shiny as the day it went in.
Oxygen on its own is not enough. This tube is the control that rules out air being the whole story.
Tube 3
Water, no air
A nail in water that was boiled to drive out dissolved air, with a layer of oil on top to keep more out.
No rust, or the faintest trace where the oil did not quite seal.
Water on its own is not enough either. Together with tube 2, this proves both are needed — and the faint trace is honest data, not a failure.
Tube 4
Salt water and air
A nail half in salt water, tube open to the air.
Rusted heavily — much worse than tube 1 in the same four weeks.
Salt is not a requirement; it is an accelerator. It is why cars rust faster near the coast and after a gritted winter.
Read the four together.
Tube 1 had air and water and rusted. Tube 2 had air and no water: no rust. Tube 3 had water and no air: no rust. So both are needed, and neither on its own will do it — which is a conclusion no single tube could have supported. Tube 4 had both plus salt and rusted fastest, so salt is not needed but it speeds the reaction up. Tubes 2 and 3 are the controls, and they are doing the actual work.
iron + oxygen + watermakeshydrated iron oxide (rust)
Key fact
Oxidation is a substance gaining oxygen, and the product is always heavier than what you started with. Rusting needs oxygen and water together; salt only makes it faster.
Five ways to stop it
Four keep the oxygen and water out. One does something cleverer.
Decide which kind each method is before you read it. If you know the rule, one of the five should feel wrong under it — that is the one worth understanding.
Painting a bridge
A continuous film of paint over every exposed surface.
A barrier. It works only while it is unbroken — a chip in the paint is where the rust starts, and it then spreads underneath.
Oiling a bicycle chain
A thin film of oil, renewed regularly.
A barrier, and a deliberately temporary one. It has to be renewed because it wears off, which is why chains need oiling rather than painting.
Stainless steel cutlery
Iron mixed with chromium and nickel.
A barrier, made by the alloy itself: the chromium oxidises into a tough invisible layer that seals the surface. Not paint, and the same trick aluminium performs.
Zinc blocks bolted to a ship
Lumps of zinc attached below the waterline and replaced every few years.
Not a barrier — the blocks do not cover anything. Zinc is more reactive than iron, so it corrodes instead of the hull. It is called sacrificial protection, and the blocks being eaten away is the method working.
A galvanised gate
Steel dipped in molten zinc, leaving a zinc coat.
Both, and the second one is why it is worth the money. The zinc is a barrier — and when the barrier is scratched the exposed zinc still corrodes in preference to the iron, so a scratch in galvanising does not start rusting.
Think again
“Aluminium does not corrode — that is why drinks cans and window frames are made of it.”
Aluminium really does last outdoors for decades without going orange and flaking. Commit before you read on.
Aluminium oxidises the moment it meets air, and faster than iron does — it is one of the more reactive metals you will meet. Every piece of aluminium you have ever touched was already covered in a layer of aluminium oxide before you got to it.
The difference is what the oxide does next. Aluminium oxide is tough, invisible and sticks tightly to the metal underneath, so it seals the surface and the reaction stops. Rust is crumbly and flakes off, exposing fresh iron, so the reaction never stops — a nail left long enough rusts all the way through. Both metals corrode; only one of them is protected by the result.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What happens to a substance during oxidation?
Rung 2 · The one that catches people
A nail in boiled water under a layer of oil does not rust. What does that tube on its own prove?
Rung 3 · Explain
Explain why the four-tube experiment needs tubes 2 and 3, when tube 1 already shows that a nail rusts. Then state the conclusion the whole set supports, and what tube 4 adds.
Rung 4 · Take it somewhere new
A boat owner is told that bolting blocks of a more reactive metal to the hull will protect it, and replies that adding more reactive metal to a corroding boat sounds like the worst possible idea. Explain why the method works, what happens to the blocks, and why they have to be checked.
Key note
Oxidation is a substance gaining oxygen, and the product weighs more than the starting material. Combustion is oxidation fast enough to burn; rusting is oxidation slow enough to take years, and it needs oxygen and water together — salt only speeds it up. Rust flakes off and exposes fresh iron, which is why iron keeps rusting while aluminium, whose oxide clings, does not.
Going further
Oxidation does not need a flame, a bench or a metal. Cut an apple and the surface browns within minutes as substances in the flesh oxidise in air; lemon juice slows it because vitamin C is oxidised first, which is what "antioxidant" means on a label. And the reaction that keeps you alive is an oxidation: glucose plus oxygen makes carbon dioxide and water, exactly the products of burning it, released slowly enough at 37 °C that your cells can use the energy instead of catching fire.
The cleverest anti-rusting trick is worth knowing in full. Bolt blocks of zinc to a steel ship's hull and the zinc corrodes instead of the steel — not by covering it, but because zinc is the more reactive of the two, and the more reactive metal corrodes in preference to the other. The blocks are inspected and replaced every few years, which is why they are called sacrificial. Galvanising is the same idea in a thin layer: a scratch in a galvanised gate does not start rusting, because the zinc around the scratch is still doing the corroding for it.
Before this lesson
Connects to
- Aerobic respiration
The reaction that keeps you alive is an oxidation, run slowly enough to be useful.
At GCSE this becomes
- Oxidation and reduction as electron transfer, corrosion and its prevention, and the reactivity series explained.
Where to next
Ask Mr Badmus AI
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