Chemical reactions · Model
Reactions rearrange atoms
A new substance appears where two old ones were. Nothing was added and nothing left — so what actually happened?
Start here
Two invisible gases. Set fire to them and you get something you can drink.
Hydrogen in one balloon, oxygen in another. Both colourless, neither of them heavy in the hand, neither of them wet. Light them and what condenses on the cold glass afterwards is water.
Where did the water come from?
Nothing arrived and nothing left. The same atoms that were in the two balloons are in the water — hydrogen atoms and oxygen atoms, joined up differently. A reaction does not make new matter. It rearranges the matter it is given. Every reaction you will ever meet is that sentence with different atoms in it.
Atoms are joined together in the substances you start with. A chemical reaction breaks those joins and makes different ones. The atoms are not changed, created or destroyed — they are the same atoms throughout, and only their partners are new.
That is why a new substance can appear out of two old ones, and why the atom count on each side is always identical. It has to be. There is nowhere else for an atom to come from.
Your turn · take a reaction apart
Break the joins, count the atoms, make the new joins
The reaction
hydrogen + oxygen makes water
magnesium + oxygen makes magnesium oxide
methane + oxygen makes carbon dioxide + water
Before · the substances you start with
The joins are broken · loose atoms on the table
After · the new substances
H2
H2
O2
H2O
H2O
Mg
Mg
O2
MgO
MgO
CH4
O2
O2
CO2
H2O
H2O
Atom count
| Atom | Before | After |
|---|---|---|
| Hydrogen | 4 | —4 |
| Oxygen | 2 | —2 |
| Magnesium | 2 | —2 |
| Oxygen | 2 | —2 |
| Carbon | 1 | —1 |
| Hydrogen | 4 | —4 |
| Oxygen | 4 | —4 |
Each box is one particle of a starting substance, and the atoms inside it are joined together. Nothing has happened yet.
Every join has been broken and nothing else has been done. This stage is not real — no chemist ever sees it — but it is the honest way to see what a reaction has to work with. Count what is on the table: that is your entire budget.
Two water particles, each one oxygen atom joined to two hydrogen atoms. Four hydrogens and two oxygens, exactly as before — and a liquid, from two gases.
Two magnesium oxide units, each one magnesium joined to one oxygen. The white powder weighs more than the ribbon did — because the oxygen atoms came from the air and are now part of it.
One carbon dioxide and two waters — and every one of the nine atoms is used, with none left over and none invented. This is what a gas hob does, several times a second.
The loose atoms are on the table. How many oxygen atoms will be in the water you make?
Two magnesium atoms and two oxygen atoms are loose. How many magnesium oxide units can you build?
One carbon, four hydrogens and four oxygens are loose. Which set of products uses all of them exactly?
All three reactions taken apart, and the atom count came out identical every time — because at the loose-atom stage there was nothing to add and nothing to take away. Whatever the products turn out to be, they can only be built from the atoms on the table.
The rule has teeth · try to break it
Ask the bench for a product it cannot build
You have hydrogen and oxygen on the table and nothing else. Pick something for the bench to make.
Built. Two hydrogens and one oxygen, and you had both.
Everything water needs was on the table. That is the only condition a reaction has to meet.
Built — and it is a different substance from water.
Two hydrogens and two oxygens. Same two elements as water, one extra oxygen atom, and a substance that bleaches hair rather than quenching thirst. The atoms decide, and so does how many of them.
Refused. There is no nitrogen on this table.
Ammonia is nitrogen joined to three hydrogens, and nothing in either balloon contains a nitrogen atom. No reaction can make one — atoms are not created. If you want nitrogen in your product, you have to put nitrogen in your reactants.
Refused, and not because the bench is not trying hard enough.
Gold is an element. The only way to have gold atoms at the end is to have had gold atoms at the start. Fifteen hundred years of alchemy is the evidence, and this rule is why it never worked.
Key fact
A chemical reaction breaks the joins between atoms and makes new ones. The atoms themselves are unchanged, and there are exactly as many of each kind afterwards as there were before.
Think again
“When magnesium burns, the magnesium atoms turn into magnesium oxide atoms.”
It sounds right, and it uses the right words in the right order. Commit before you read on.
There is no such thing as a magnesium oxide atom. Magnesium oxide is a compound, built from magnesium atoms and oxygen atoms held together — and both kinds of atom are still exactly what they were. Nothing in the flame changed one element into another; the periodic table has not been edited.
This is the same idea as thinking a copper atom is orange and conducts. An atom does not carry the properties of the substance it ends up in. Turning one element into another is possible, but not with a Bunsen burner and not in chemistry — it takes a nuclear reaction, and that is a different subject.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What happens to the atoms during a chemical reaction?
Rung 2 · The one that catches people
Methane burns in oxygen. There are four hydrogen atoms in the methane. How many hydrogen atoms are in the products?
Rung 3 · Explain
A ribbon of magnesium is weighed, burned in air, and the white powder left behind is weighed. It is heavier. Explain how the powder can be heavier than the ribbon if atoms are never created.
Rung 4 · Take it somewhere new
A student proposes making gold by reacting two cheap substances together, and says the reason nobody has done it is that no-one has found the right conditions. Use what you know about reactions and atoms to explain why the plan cannot work, and what would be needed instead.
Key note
A chemical reaction breaks the joins between atoms and makes new ones. The atoms are the same atoms all the way through: not created, not destroyed, and never changed into other kinds of atom. So the count of each kind of atom is identical before and after, and a reaction can only build products from the atoms it was given.
Going further
The reason a reaction takes or gives out energy sits exactly here. Breaking the joins between atoms always costs energy, and making new joins always gives energy back. If the new joins give back more than the old ones cost, the surplus leaves as heat and the reaction warms its surroundings; if they give back less, the reaction takes the difference from its surroundings and everything gets cold. Nothing else needs to be added to that story — it is bookkeeping on joins.
It also explains why some reactions need a match. A mixture of hydrogen and oxygen will sit in a balloon indefinitely, even though the reaction gives out an enormous amount of energy, because the first joins still have to be broken and nothing has paid for them. A flame pays for the first few, those release enough energy to pay for the next few, and the balloon goes off. Chemistry is full of things that are worth doing and still need starting.
Before this lesson
At GCSE this becomes
- Bond breaking and bond making as the energy story of a reaction, and balanced symbol equations with state symbols.
Where to next
Ask Mr Badmus AI
Still not sure why no reaction can make gold?
Lesson content © MrBadmusAI.