Types of reaction · Classify
Which reaction is this?
Four types, and nobody labels them for you. What do you look at first — and what do you do with a reaction that fits none of them?
Start here
Four reaction types. One of them gets heavier, one gets lighter, one glows and one changes colour in a beaker.
You have met combustion, thermal decomposition, oxidation and displacement. In an exam nobody tells you which one you are looking at — you are given what happened and expected to name it.
Which single question sorts them fastest?
How many reactants there are, and what they are. One reactant means decomposition — nothing else does that. Two, one of which is oxygen means oxidation, and combustion if it burns. A metal and a compound of another metal means displacement. Count first, name second.
The four types are not four unrelated facts. Combustion is a special case of oxidation — the fast, flaming kind. Decomposition is the only one that runs a compound backwards. Displacement is the only one where a metal has a rival.
So the useful test is structural, not visual. Look at the equation, not the drama.
Reference · the decision rule
Four questions, in this order
Is there only one reactant?
Then it is thermal decomposition, if heat did it.
Is one reactant a metal and the other a compound containing a different metal?
Then it is displacement.
Is one reactant oxygen, and did it burn with a flame?
Then it is combustion.
Is one reactant oxygen, with no flame?
Then it is oxidation — and note that the answer to 3 is also oxidation, just a more specific name for it.
Mass is the tie-breaker when you are unsure. Gaining oxygen makes the product heavier. Losing a gas makes what is left lighter. Both of those are measurable, and neither depends on what the reaction looked like.
Your turn · eight reactions, rising difficulty
Name the type. Commit before you read the answer.
0 of 8 named
The last three are deliberately awkward. Two of them have a defensible second answer, and the eighth is not any of the four.
Reaction 1
Straightforward
Methane burns on a hob with a blue flame, making carbon dioxide and water.
Combustion.
A fuel and oxygen, a flame, energy out. It is also an oxidation — combustion is the specific name and the better answer.
Reaction 2
Straightforward
Green copper carbonate is heated in a test tube and turns black, giving off a gas that turns limewater milky.
Thermal decomposition.
One reactant, two products, heat needed continuously, and the mass in the tube falls. Nothing else in the unit has one reactant.
Reaction 3
Straightforward
A strip of zinc is dropped into blue copper sulfate solution. The blue fades and a brown solid collects on the zinc.
Displacement.
A metal and a compound of a less reactive metal. Zinc is above copper, so it takes its place, and the copper appears as a solid.
Reaction 4
Straightforward
An iron gate left outdoors for ten years is covered in orange flakes and is heavier than when it was new.
Oxidation.
Iron gaining oxygen, with water needed as well. No flame, so not combustion — the same gain of oxygen that burning makes in seconds, running slowly enough to take a decade.
Reaction 5
Harder
Limestone is heated in a kiln at 900 °C. Carbon dioxide comes off and a white solid is left that hisses when water is dropped on it.
Thermal decomposition.
No colour change to help you, and nothing burnt: one reactant broken apart by heat. The hissing white solid is calcium oxide, a new substance.
Reaction 6
Harder
Aluminium powder mixed with iron oxide is lit. It reaches 2500 °C and molten iron pours out.
Displacement.
Spectacular, and still a displacement: aluminium is more reactive than iron, so it takes the oxygen and the iron is pushed out. And yes — the aluminium gains oxygen, so oxidation is defensible too; displacement names the whole event, so it says more. The drama is not the diagnosis — the reactants are.
Reaction 7
Harder
Glucose reacts with oxygen inside your cells, releasing energy and producing carbon dioxide and water.
Oxidation.
The same reactants and products as burning glucose, at 37 °C and with no flame. Respiration is an oxidation, and biology gives it its own name for its own reasons.
Reaction 8
Awkward on purpose
Marble chips are dropped into hydrochloric acid. The mixture fizzes and a gas comes off that turns limewater milky.
None of the four. That is the correct answer, not a gap in what you know.
Two reactants, neither of them oxygen, and no metal displacing another — so none of the four names fits, and there was nothing you were meant to spot. The four are a useful set, not a complete one. This reaction has a name of its own: it is neutralisation, an acid reacting with a carbonate, and it is the whole of the next unit. Knowing that a case falls outside your rule is worth as much as knowing the rule.
Eight named. Notice how little the appearance helped: two of the eight involved no visible drama at all, and two of the most dramatic were the easiest to mislabel. Every one of them was settled by counting reactants and looking for oxygen or a metal.
Key fact
Name a reaction from its reactants, not from its appearance. One reactant means decomposition; a metal and another metal's compound means displacement; oxygen means oxidation, and combustion if it burns.
In your words
Write the rule you would give someone who missed this unit
One sentence per type, and each one has to say what you would look at rather than what it looks like. Nobody marks this. Compare it with the version underneath when you are done.
- Thermal decomposition: one reactant, two or more products, heat needed all the way through, mass left behind goes down.
- Combustion: a fuel and oxygen, a flame, energy given out, and carbon dioxide and water if the fuel contains carbon and hydrogen.
- Oxidation: something gains oxygen and the product is heavier. Combustion is the fast version of this.
- Displacement: a metal and a compound of a less reactive metal, and the less reactive one comes out as a solid.
If yours says the same things in worse English, yours is the more useful one — you will remember it.
Think again
“A reaction can only be one of the four types, so if it is oxidation it cannot be combustion.”
The four have four different names, which certainly suggests four separate boxes. Commit before you read on.
Burning magnesium is combustion and oxidation, and both answers are right. The names are not four boxes side by side: combustion sits inside oxidation, as the case where the reaction is fast enough to burn. Naming it combustion is more specific and therefore usually the better answer — it says more.
The two that genuinely cannot overlap are decomposition and the rest, because decomposition has one reactant and the others have two. So if you are ever torn between combustion and oxidation, you are not confused — you have noticed something true. Give the more specific name, and say why the other also applies.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
A reaction has exactly one reactant and two products, and stops when the flame is removed. Which type is it?
Rung 2 · The one that catches people
Magnesium ribbon burns in air with a bright white flame. A student writes "oxidation" and another writes "combustion". Who is right?
Rung 3 · Explain
Two reactions both leave a black solid in a test tube: heating copper carbonate, and heating copper in air until it goes black. Explain how you would tell which is which using only a balance and a limewater test, and name each type.
Rung 4 · Take it somewhere new
A student is told that a reaction between two colourless solutions produced a solid, with no heat and no gas. They try to fit it into the four types and fail. Explain why a classification failing is not the same as the student failing, and what they should do with the case.
Key note
Name a reaction from its reactants. One reactant broken apart by heat is thermal decomposition. A metal plus a compound of a less reactive metal is displacement. Oxygen as a reactant is oxidation, and combustion when it burns with a flame — so combustion sits inside oxidation rather than beside it. Mass is the tie-breaker: gaining oxygen makes it heavier, losing a gas makes it lighter.
Going further
There are more reaction types than four, and the two you have already met without a label are worth naming. Marble chips in acid is neutralisation, which is the whole of the next unit. Iron and sulfur heated together to make iron sulfide is synthesis — two elements joining to make one compound, the exact reverse of decomposition. Chemists also cut the same set a different way: exothermic reactions give out energy, endothermic ones take it in, which puts combustion and displacement on one side and thermal decomposition on the other.
None of these classifications is a fact about nature; they are all ways of grouping reactions so that a prediction becomes possible. That is what a type is for. Knowing a reaction is a displacement lets you predict the products from a reactivity order you can look up, and knowing one is a combustion lets you predict carbon dioxide and water without being told. A name that does not let you predict anything would not be worth learning.
Before this lesson
Next in this unit
At GCSE this becomes
- Reaction types extended with neutralisation, precipitation and redox — and classifying by energy change as well as by reactants.
Where to next
- Next: Acids and alkalis
Acids and alkalis
- Previous: Displacement
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