Chemical reactions · Model
Word equations
You can describe a reaction in a paragraph and still not have said what reacted. What has to be in the sentence for it to count?
Start here
Twenty-two words, and not one of them says what the reaction was.
“I held the magnesium ribbon in the flame with tongs and it burned with a really bright white light and left a white powder behind.” Every word is true. A chemist reading it still does not know what reacted with what.
What is missing from that sentence?
The other reactant, and the name of the product. The sentence describes what it looked like; it never says that the magnesium joined with the oxygen in the air, or that the white powder is magnesium oxide. A word equation says all of it in six words and cannot avoid saying it: magnesium + oxygen magnesium oxide.
A word equation puts the substances you start with — the reactants — on the left, and the substances you end with — the products — on the right. The arrow is read as “makes” or “turns into”. It is not an equals sign, and the two sides are not interchangeable: a reaction runs one way, and so does the arrow.
The plus sign is read as “and”. It never means add up.
Your turn · build the equation
Read what happened. Put each substance where it belongs.
On the bench · not everything here belongs in the equation
Your equation
nothing yetmakesnothing yet
MagnesiumOxygenMagnesium oxideNitrogenHeatYour equation
nothing yetmakesnothing yet
Calcium carbonateHydrochloric acidCalcium chlorideWaterCarbon dioxideLimewaterBubblesYour equation
nothing yetmakesnothing yet
MethaneOxygenCarbon dioxideWaterEnergyFlameThat is the equation.
Every reactant on the left, every product on the right, nothing in it that is not a substance, and nothing in it that was only watching.
One thing in there does not belong.
Nitrogen was in the air and took no part in the reaction. A substance that is only present does not go in the equation — only what reacts and what is made.
Heat is not a substance. It is a condition, and conditions are written above the arrow if they are written at all.
Something is missing.
Magnesium + OxygenmakesMagnesium oxide
That is the equation.
Every reactant on the left, every product on the right, nothing in it that is not a substance, and nothing in it that was only watching.
One thing in there does not belong.
Limewater was used to test the gas in a different tube. It never met the marble, so it is not part of this reaction.
Bubbles are what you saw, not a substance. The substance is carbon dioxide — name it.
Something is missing.
Calcium carbonate + Hydrochloric acidmakesCalcium chloride + Water + Carbon dioxide
That is the equation.
Every reactant on the left, every product on the right, nothing in it that is not a substance, and nothing in it that was only watching.
One thing in there does not belong.
Energy is given out, and it is still not a substance. It has no place on either side of a word equation.
The flame is the reaction, seen. Nothing is made of flame.
Something is missing.
Methane + OxygenmakesCarbon dioxide + Water
Key fact
Reactants on the left, products on the right, and the arrow means “makes”. Only substances go in a word equation — never conditions, never energy, and never anything that was only watching.
Read it back
Which sentence is the equation actually saying?
zinc + copper sulfatemakeszinc sulfate + copper
The first. "Plus" and "equals" describe an arithmetic sum; this is a reaction, so it is read as "and" and "makes". Say it out loud the wrong way and you will write it the wrong way.
copper carbonatemakescopper oxide + carbon dioxide
One reactant, two products: the copper carbonate has been broken apart. Nothing joined it and nothing needed to — this kind of reaction has a name of its own, thermal decomposition.
iron + sulfurmakesiron sulfide
The arrow means a reaction happened, so the product is a compound, not a mixture. A mixture of iron and sulfur would still be separable with a magnet; iron sulfide is not.
Think again
“The arrow is like an equals sign, so you could write the equation the other way round.”
The two sides do have the same atoms in them, which is exactly why this is tempting. Commit before you read on.
The two sides balance in atoms and they are not the same thing. Written the other way round, “magnesium oxide makes magnesium and oxygen” claims that white powder spontaneously turns back into burning metal and air, which it does not — it takes a furnace, and it is a different reaction with a different equation.
So the arrow carries information an equals sign does not: which way it went. Two other habits follow from the same mistake. You cannot cancel a substance that appears on both sides the way you cancel in algebra — it is doing something on each side. And “heat”, “energy” and “a flame” are not substances, so they never appear in the equation; if the conditions matter, they are written above the arrow.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
In a word equation, what does the arrow mean?
Rung 2 · The one that catches people
A student writes: magnesium + oxygen + heat makes magnesium oxide. What is wrong with it?
Rung 3 · Explain
Write the word equation for marble chips reacting with hydrochloric acid, then explain how someone reading only your equation would know that a gas was given off and that a new substance was made.
Rung 4 · Take it somewhere new
A student says word equations are pointless because you could just describe the reaction in a sentence. Give two things a word equation forces you to say that a description can leave out, and one thing a word equation cannot say at all.
Key note
A word equation puts the reactants on the left, the products on the right, and reads the arrow as “makes”. Plus means “and”, never add. Only substances appear — not heat, not energy, not a flame, and not anything that was merely present. The arrow runs one way, because so does the reaction.
Going further
Word equations run out of road quickly, and it is worth knowing exactly where. They can say which substances react and which are made. They cannot say how many particles of each — and that turns out to be most of chemistry. Two hydrogen particles react with one oxygen particle; "hydrogen + oxygen makes water" cannot tell you that, so it cannot tell you how much oxygen to supply or how much water to expect.
They also depend on the name being agreed. "Iron oxide" names three different compounds, and rust is a hydrated one, so a word equation for rusting is honest only if everyone reading it knows which iron oxide is meant. Symbols fix both problems at once, and that is the next lesson but one: formulae name the substance exactly, and the numbers in front of them say how many.
Before this lesson
Next in this unit
At GCSE this becomes
- Balanced symbol equations with state symbols, ionic equations, and half equations.
Where to next
Ask Mr Badmus AI
Still not sure why heat cannot go in the equation?
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