GCSE · Chemistry · Edexcel · Spec 1CH0

Balanced chemical equations with state symbols

Chemistry has one rule it never breaks: nothing is made and nothing is lost. Atoms just move house. So how do you make an equation show that?

Chemistry · Reaction balancer

Make the atoms match

Tap + or − under a substance to change its big number (its coefficient). Watch the atom tallies underneath. The formulas never change — only the big numbers do. Magnesium first, then propane.

Reactants

C₃H₈

O₂

Products

CO₂

H₂O

C3→1✗

H8→2✗

O2→3✗

Conservation check: count atoms on both sides.Not yet.

Sort the carbon and the hydrogen first. Leave oxygen until last: add up the oxygen atoms on the right (some in the CO₂, some in the H₂O), then choose the O₂ number that matches. Predict it before you tap.

Tap + or − under each molecule to set its coefficient.

Watch out: Four O₂ is the tempting answer. Don't trust it — set it, then check the oxygen tally on both sides and see whether they really match.

From words to a full symbol equation

Problem

Zinc reacts with sulfuric acid to make zinc sulfate and hydrogen. Write the symbol equation, including state symbols.

Draw it, count it, add what's short

Tap a diagram to see the count it fixed.

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

Which idea sounds like you?

What are you allowed to change?

You're balancing a symbol equation and the atom counts don't match yet.

Which of these is closest to what you'd do?

I'd change the small number in a formula until the atoms match.I'd put a big number in front of a formula. It multiplies every atom in that formula.A big number in front only multiplies the first atom in the formula.A formula with no number in front of it isn't in the reaction, so I ignore it.

How sure are you?

I'm sureI think soI'm guessing

I'd change the small number in a formula until the atoms match.

This is a common misconception

Tempting, because it does change the atom count. But a different small number means a different substance: H₂O is water, H₂O₂ is hydrogen peroxide. The formula has to stay exactly as it is.

I'd put a big number in front of a formula. It multiplies every atom in that formula.

This matches the evidence

Yes. Coefficients are the only thing you change, and each one multiplies every atom in the formula it stands in front of. 3H₂O is 6 hydrogen atoms and 3 oxygen atoms.

A big number in front only multiplies the first atom in the formula.

This is a common misconception

Easy to think, but the coefficient covers the whole formula. In 2MgO there are 2 magnesium atoms and 2 oxygen atoms, not 2 and 1.

A formula with no number in front of it isn't in the reaction, so I ignore it.

This is a common misconception

No number in front means a coefficient of one: there is one of it, and it counts. In Mg + O₂ → MgO, each substance is there once.

Formulas stay fixed. Only the big numbers in front change, and each one multiplies every atom in its formula.

Check the method

Where did this student lose the mark?

A student answers: 'Write the balanced symbol equation for magnesium burning in oxygen to make magnesium oxide.' Pick the line where it goes wrong.

A student's answer — which line goes wrong?

Word equation: magnesium + oxygen → magnesium oxide
Formulas: Mg + O₃ → MgO
Count oxygen: 3 on the left, 1 on the right.
Add two more MgO on the right, so oxygen is 3 and 3. Now magnesium is 1 on the left and 3 on the right.
Add two more Mg on the left: 3Mg + O₃ → 3MgO

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Atoms only move house. Your job is to make the numbers add up.

What you need to know

A word equation uses names. A symbol equation uses formulas, with reactants left of the arrow and products on the right.
A formula shows the elements in a substance and the ratio of their atoms, so symbol equations work in any language.
State symbols tell you each substance's physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Have a goSalt is stirred into water until you can't see it any more. Which state symbol goes after its formula?
(aq)
It is dissolved in water, which is exactly what (aq) means. (l) is for a substance that is itself a liquid.
In a reaction, atoms can't be created, destroyed or turned into different atoms. They're just rearranged.
A balanced equation has the same number of atoms of each element on both sides. Coefficients get you there.
Never change a formula to balance. A different subscript is a different substance: H₂O is water, H₂O₂ is hydrogen peroxide.
Have a goA friend 'balances' an equation by turning H₂O into H₂O₂ so the oxygen counts match. Talk them out of it.
H₂O₂ isn't water. It's hydrogen peroxide, a different substance.
Changing a subscript changes which substance you're writing, so the equation stops describing the reaction you started with.
Only the big numbers in front (coefficients) can change. A coefficient multiplies every atom in its formula; no number means 1.
Have a goA classmate reads 2MgO as 'two magnesium atoms and one oxygen atom'. How many of each is it really?
2 magnesium atoms and 2 oxygen atoms.
The 2 in front multiplies every atom in the formula, not just the first one.
Coefficients give the ratio of substances. In 2Mg + O₂ → 2MgO it's 2 : 1 : 2, whatever amounts you use.
To balance, draw each substance and count the atoms. Where one side is short, add another substance diagram, then recount.
Check every formula first, because oxygen gas is O₂, not O₃ (that's ozone). Then balance.

The big picture

A symbol equation writes a reaction with formulas and state symbols. Atoms are only rearranged, so a balanced equation has the same number of atoms of each element on both sides. You balance by changing coefficients only, never the formulas.

Key points

1A symbol equation uses formulas: reactants on the left of the arrow, products on the right.

2State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

3Atoms are rearranged, never created, destroyed or changed into other atoms.

4Balance with coefficients only. A coefficient multiplies every atom in its formula.

5Correct formulas first, then balance.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Show that Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) is balanced.

⚠ Watch out

Changing a small number inside a formula to make the atoms match. H₂O becomes H₂O₂, which is a different substance. Only the big coefficients in front of the formulas may change.

🧠

Memory hook

Big numbers are how many boxes. Small numbers are what's inside a box. You can add boxes, but you can't tamper with what's inside one.

✓

Check yourself

Pick any formula, put a 3 in front of it and count every atom you now have. Then say out loud why you can't fix an unbalanced equation by editing the formulas themselves.

Flashcards

(14)

Word equation or symbol equation: which one uses formulas?

A symbol equation. A word equation uses names. Both have reactants on the left of the arrow and products on the right.

What does a chemical formula tell you?

Which elements are in the substance and the ratio of their atoms. That's why a symbol equation is understood in any language.

What do (s), (l), (g) and (aq) stand for?

(s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water.

What happens to atoms in a chemical reaction?

They're rearranged. They can't be created, destroyed or changed into a different type of atom.

When can a reaction make a particular product?

Only if the reactants contain all the atoms that product needs.

What makes a symbol equation balanced?

The same number of atoms of each element on the reactant side and the product side.

When balancing, what are you allowed to change?

Only the coefficients, the big numbers in front of formulas. The formulas themselves stay as they are.

Why can't you turn H₂O into H₂O₂ to balance an equation?

They are different substances: H₂O is water and H₂O₂ is hydrogen peroxide. Changing a subscript changes the substance.

A formula has no number in front of it. What is its coefficient?

One.

What does a coefficient multiply?

Every atom in the formula it stands in front of, not just the first one.

What do the coefficients in 2Mg + O₂ → 2MgO tell you?

The ratio: 2 magnesium atoms react with 1 oxygen molecule to make 2 magnesium oxide formula units. The ratio stays the same whatever the amounts.

In the diagram-and-count method, what do you do on the side with fewer atoms of an element?

Add another diagram of a substance containing that element, then recount the atoms every time.

Oxygen gas in an equation: O₂ or O₃?

O₂. O₃ is ozone, a different substance.

What comes first: balancing or checking formulas?

Checking formulas. Every substance needs its correct formula before you balance.

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.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

Make the atoms match

What are you allowed to change?

Where did this student lose the mark?

More Edexcel GCSE Chemistry topics