KS3 · Chemistry

Oxidation reactions

A firework is over in seconds. A copper coin in the ground changes for years. Chemically, they're doing the same thing.

Chemistry · Oxidation

Something reacts with oxygen. What comes out?

Pick what is reacting with oxygen, then pick the substance. Before its end opens, say out loud what you think it turns into, then check.

Oxidation starts → What is reacting? → Which substance?

  • Oxygen bonds to the substance, so a new substance forms. Which new substance depends on what was reacting.

9 routes, and every one ends in an oxide.

On A substance reacts with oxygen. 3 branches to choose from.

Walk a few routes, then notice what every single end has in common.

Watch out: Water counts as an oxide. It's an oxide of hydrogen, so when methane or ethanol reacts with oxygen, the water it makes is an oxide too.

Reading an oxide's name

Tap each chip. The prefix in a non-metal oxide's name tells you how many oxygen atoms it has.

Tap a molecule to see what it does in the reaction.

Two famous oxidations

CombustionvsRusting

Combustion and rusting are both oxidation reactions. Read across each row to see where they differ.

Focus

What has to be there

Combustion

Fuel, heat and oxygen. Take away any one and it won't happen.

Rusting

Iron (or a metal containing iron, like steel), oxygen and water.

The insight

Oxygen is on both lists. That's what makes them both oxidation.

How fast

Combustion

Can be very fast, over in seconds, like a firework going off.

Rusting

Much slower. In the nail experiment, iron is left in air and water for two weeks before you look for rust.

What it makes

Combustion

An oxide, plus whatever other products the fuel gives.

Rusting

Rust, which is iron oxide: iron + oxygen + water → iron oxide.

What's going on

Combustion

The substance combines with oxygen.

Rusting

A more complicated series of reactions between iron, oxygen and water. The iron corrodes: it reduces in size and its surface is damaged.

What do you think?

Do copper coins rust?

Copper coins can sit in the ground for many years, slowly oxidising.

Have those copper coins rusted? Pick the idea closest to what you think right now.
How sure are you?

The three-nail experiment

Which nail rusts?

Three iron nails are left for two weeks, each in different conditions. Tick what each nail has, and whether you think it rusted. Then check.

Nail 1: air and water
Nail 2: boiled water under a layer of oil
Nail 3: dry air, with anhydrous calcium chloride

Stopping rust

?

Reason it through

Why does a coat of paint stop iron rusting, and why does a scratch undo it?

Link 1 of 4

First link · your turn

What has to reach the iron for rusting to happen?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A firework that's over in seconds and a coin changing in the ground for years are doing the same kind of reaction.

What you need to know

  • Oxidation is when a substance reacts to combine with oxygen. Combustion and rusting are both examples.
  • An oxide is a compound containing oxygen formed in oxidation. One product is always an oxide; the other products vary.
  • Metals: metal + oxygen → metal oxide, e.g. magnesium + oxygen → magnesium oxide.
  • Non-metal elements make non-metal oxides: carbon → carbon dioxide, sulfur → sulfur dioxide, nitrogen → nitrogen dioxide.
  • Compounds make an oxide and other products: methane + oxygen → carbon dioxide + water; ethanol + oxygen → ethanoic acid + water. Water is an oxide of hydrogen.
  • In a non-metal oxide's name, the prefix counts the oxygen atoms: mono 1, di 2, tri 3, tetr 4, pent 5.
  • Combustion needs fuel, heat and oxygen. It won't happen without one of them.
  • Oxidation can be fast (fireworks, in seconds) or slow (copper coins in the ground, over many years).
  • Rust is iron oxide: iron + oxygen + water → iron oxide. Only iron, and metals containing iron such as steel, can rust. Other metal oxides, like copper oxide, look different.
  • Rusting needs both water and oxygen. Paint, oil or a metal coating stops it by acting as a barrier; changing the mixture of the metal can also reduce it.

The big picture

Oxidation is a substance combining with oxygen. One product is always an oxide; the others depend on what reacted. Metals make metal oxides, non-metals make non-metal oxides, and compounds such as methane make an oxide plus other products. Combustion and rusting are both oxidation. Rust is iron oxide and needs both water and oxygen, so a coating that keeps them off stops it.

Key points

1Follow the oxygen: whatever reacts, the oxygen ends up in an oxide.
2Metals give one product, the metal oxide. Compounds such as methane give an oxide plus other products.
3The prefix tells you the oxygen count: carbon monoxide (CO) has one, carbon dioxide (CO₂) has two.
4Combustion and rusting are both oxidation: one fast and needing fuel, heat and oxygen; one slow and needing iron, oxygen and water.
5Not all metals rust. Rust is iron oxide, and it forms only when water and oxygen both reach the iron.

Worked example

Problem

(a) Sodium reacts with oxygen. Name the product and say whether anything else forms. (b) A non-metal oxide is called sulfur trioxide. How many oxygen atoms does its formula have?

⚠ Watch out

Calling any metal oxide 'rust'. Rust is iron oxide. It can form only from iron, or metals containing iron such as steel, and only when both oxygen and water are there. Copper oxide, for example, is not rust.

🧠

Memory hook

Follow the oxygen. Wherever it ends up, that's your oxide. And rust is just the oxide iron makes when water is around too.

✓

Check yourself

Close the page. Name the product when aluminium reacts with oxygen, then explain why an iron nail kept in dry air doesn't rust.

Flashcards

(15)
What is oxidation?
When a substance reacts to combine with oxygen.
What is an oxide?
A compound containing oxygen that forms during an oxidation reaction.
What is always one product of an oxidation reaction?
An oxide. The other products vary, depending on what reacted.
Word equation for a metal reacting with oxygen
metal + oxygen → metal oxide (e.g. copper + oxygen → copper oxide)
What do the prefixes mono, di, tri, tetr and pent mean in an oxide's name?
The number of oxygen atoms: 1, 2, 3, 4, 5. So carbon monoxide is CO and carbon dioxide is CO₂.
Methane + oxygen → ?
Carbon dioxide + water. Both are oxides.
Ethanol + oxygen → ?
Ethanoic acid + water. Water is the oxide, because it's an oxide of hydrogen.
Which three things does combustion need?
Fuel, heat and oxygen. Without any one of them, it won't happen.
Is oxidation always fast?
No. It can take seconds (a firework) or many years (copper coins oxidising in the ground).
What is the chemical name for rust?
Iron oxide.
Word equation for rusting
iron + oxygen + water → iron oxide
What two things must be present for iron to rust?
Water and oxygen, both at the same time.
How does paint, oil or a metal coating stop rusting?
It's a barrier that keeps oxygen and water off the iron. If it's scratched, rusting can start.
What is corrosion?
When a metal reduces in size because of a chemical reaction. Rusting is one type of corrosion.
Do all metals rust?
No. Only iron, and metals containing iron such as steel, can rust. Other metals form oxides that look different from rust.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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