Writing and Balancing Equations
You'll be able to write word equations for synthesis, decomposition, combustion and displacement reactions.
In 2022, CSIRO's mineral processing teams balanced over 10 million chemical equations to optimise recovery rates at Australian mine sites, each one must be atom-perfect.
Printable Worksheets
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If you split water into hydrogen and oxygen, you get two hydrogen atoms for every oxygen atom in the formula H₂O. Now imagine writing a recipe for making water from those gases, how many molecules of each gas would you need to use, and why? Try to sketch or write your best guess.
The law of conservation of mass says atoms are never created or destroyed in a chemical reaction. So when you write a chemical equation, what rule must every correct equation obey? Why do you think chemists bother writing equations instead of just describing reactions in words?
● Know
- How to write word equations for chemical reactions
- The basics of symbolic equations using chemical formulas
- What state symbols are: (s), (l), (g), (aq)
● Understand
- Why equations must be balanced to obey conservation of mass
- That only coefficients (numbers in front) can be changed to balance equations
- How atom counting verifies that an equation is balanced
● Can do
- Write word equations for synthesis, decomposition, combustion and displacement reactions
- Balance simple symbolic equations by counting atoms
- Identify whether a given symbolic equation is balanced
Write H2 + O2 → H2O on a piece of paper and count the atoms: two hydrogens and two oxygens on the left, two hydrogens and only one oxygen on the right, one oxygen atom has vanished, which is impossible. A balanced chemical equation is a quantitative description of a reaction. It tells you not just what reacts and what forms, but in what proportions. Balancing equations is one of the most fundamental skills in chemistry.
The rules for balancing:
- Write the correct formulas for all reactants and products.
- Count the atoms of each element on both sides.
- Add coefficients (numbers in front of formulas) to make atom counts equal for every element.
- Never change subscripts within formulas - that changes the substance identity.
- Start with elements that appear in only one reactant and one product.
- Save hydrogen and oxygen for last, as they often appear in multiple compounds.
Balanced equations obey the law of conservation of mass because equal numbers of each type of atom appear on both sides. Atoms are rearranged, not created or destroyed.
Text description
- Step 1, unbalanced: H₂ + O₂ → H₂O. Left side H 2, O 2; right side H 2, O 1. O not balanced.
- Step 2, add a coefficient: H₂ + O₂ → 2H₂O. Left H 2, O 2; right H 4, O 2. H not balanced.
- Step 3, balanced: 2H₂ + O₂ → 2H₂O. Both sides H 4, O 2.
- Law of conservation of mass: four H and two O atoms on the left equal four H and two O on the right; same atoms in, same atoms out.
- Only change coefficients, never change subscripts.
Balance the equation for ammonia synthesis: N2 + H2 → NH3.
Nitrogen: 2 on left, 1 on right. Put coefficient 2 in front of NH3: N2 + H2 → 2NH3.
Now hydrogen: 2 on left, 6 on right. Put coefficient 3 in front of H2: N2 + 3H2 → 2NH3.
Final check: N: 2 = 2. H: 6 = 6. Balanced.
This tells us that one nitrogen molecule reacts with three hydrogen molecules to produce two ammonia molecules. In industrial practice, this ratio determines the stoichiometric feed rates for the Haber process.
Australian chemical engineering: Process engineers at plants like the Yara Pilbara ammonia facility use balanced equations to calculate exact feedstock requirements, predict product yields, and optimise reactor conditions. A 1% error in stoichiometric calculations could cost millions of dollars annually in wasted feedstock or lost production. Balanced equations are not just academic exercises - they are the foundation of industrial chemical accounting.
You can balance equations by changing the formulas of compounds. This is false and represents a fundamental misunderstanding. The formulas (like H2O or CO2) are determined by chemical bonding and valency. They describe what the substance is. You cannot change H2O to H2O2 to balance an equation because H2O2 is hydrogen peroxide, a completely different substance with different properties. Only coefficients can be adjusted.
Balance this equation for the combustion of propane.
Balancing assumes you already know what forms. Often you do not, and you have to predict the products first — work out what the reaction makes, from what went in. The way to do it is to name the reaction type, because each type has one pattern.
Take zinc metal dropped into blue copper sulfate solution. This is a displacement, so the pattern is that one metal takes the other's place: Zn + CuSO4 → ZnSO4 + Cu. The prediction is testable, and it is what you see — the blue fades and a brown coating of copper appears on the zinc.
Why that way round, and not the reverse? Zinc gives up its outer electrons more readily than copper does. In a displacement, the metal that holds its electrons less tightly ends up as the ion in solution, and the other is left as the metal. That is why the reactivity series decides the direction: it is a ranking of how readily each metal gives up electrons, so it tells you which way the swap can go.
The commonest mistake is to swap the parts without checking the order. Copper dropped into zinc sulfate looks like the same kind of swap, so students write Cu + ZnSO4 → CuSO4 + Zn. Nothing happens in the beaker. Copper is less reactive than zinc, so it cannot take zinc's place. A pattern tells you the shape of the products; the reactivity series tells you whether the reaction runs at all.
Which type is this, and what is that type's pattern? It resolves all four types you have met:
- Synthesis — two reactants join: A + B → AB.
- Decomposition — one splits: AB → A + B.
- Displacement — one element takes another's place, if it is the more reactive: A + BC → AC + B.
- Neutralisation — acid plus base: acid + base → salt + water.
Name the type first and the products follow. Balance only once you have them, because balancing a formula you guessed wrong just gets you a tidy wrong answer.
Magnesium ribbon is placed in dilute hydrochloric acid and bubbles come off. Name the type, predict the two products, then write the balanced equation with state symbols. Check your answer against the reactivity series before you commit to it.
Find the error in this balanced equation.
- Hydrogen: 4 atoms on each side. Balanced.
- Oxygen: 2 atoms on left, 4 atoms on right. Balanced.
- Wait - the product is hydrogen peroxide, not water.
- Fixing both: the product should be H2O, and the equation then balances as 2H2 + O2 → 2H2O.
Here is a worked example for balancing the reaction between magnesium and oxygen:
Step 1: Mg + O2 → MgO
Count: Left = 1 Mg, 2 O. Right = 1 Mg, 1 O. Oxygen is unbalanced.
Step 2: Try 2MgO on the right: Mg + O2 → 2MgO
Count: Left = 1 Mg, 2 O. Right = 2 Mg, 2 O. Magnesium is now unbalanced.
Step 3: Add 2Mg on the left: 2Mg + O2 → 2MgO
Count: Left = 2 Mg, 2 O. Right = 2 Mg, 2 O. Balanced!
State symbols (introduced briefly)
State symbols tell us the physical state of each substance:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous (dissolved in water)
Example: 2Mg(s) + O2(g) → 2MgO(s)
Wrong: "You can change subscripts to balance an equation." No, changing subscripts changes the substance. H 2 O is water; H 2 O 2 is hydrogen peroxide. Only coefficients can be changed.
Right: Only coefficients, the numbers placed in front of a formula, can be adjusted when balancing. Changing a subscript changes the identity of the substance entirely; H₂O and H₂O₂ are completely different chemicals.
Wrong: "A balanced equation is just for show, it doesn't matter if it's unbalanced." No, an unbalanced equation violates the law of conservation of mass. Atoms cannot be created or destroyed in chemical reactions.
Right: A balanced equation is chemically necessary, it reflects the law of conservation of mass. An unbalanced equation is simply incorrect because it implies atoms are being created or destroyed, which cannot happen.
Wrong: "State symbols are optional and don't mean anything." State symbols give important information about the conditions of the reaction and help scientists predict how substances will behave.
Right: State symbols are chemically meaningful. (s), (l), (g) and (aq) tell you the physical state of each substance, and the state changes what the substance can do: magnesium ribbon reacts with acid at its surface only, while magnesium dissolved as ions is already spread through the liquid. Note the pair that catches people out — (l) is a pure liquid, such as water itself, and (aq) is something dissolved in water.
Chemistry in Australian Industry
Australia's mining and metallurgical industries rely heavily on balanced chemical equations. When BHP extracts iron from iron ore, engineers use balanced equations to calculate exactly how much coke (carbon) is needed to reduce the iron oxide, and how much carbon dioxide will be produced.
At the Tomago aluminium smelter near Newcastle, chemists use symbolic equations to understand the electrolysis of aluminium oxide. Balanced equations ensure that raw materials are used efficiently and waste is minimised. Without balanced equations, industrial chemistry would be guesswork, costing money and creating unnecessary pollution.
✍ Copy Into Your Books
▾Word Equations
- reactant + reactant → product + product
- Use substance names
- Always check arrow direction
Balancing Rules
- Count atoms on each side
- Change only COEFFICIENTS
- Never change SUBSCRIPTS
- Recheck all atoms at the end
State Symbols
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous (dissolved)
Write Word Equations
Balance by Atom Counting
At the start of this lesson, the hook told you that a chemical equation is the most compact "recipe" in science, get even one atom wrong and an entire industrial plant produces waste instead of product.
Now that you can write and balance equations yourself, can you see why getting every coefficient right really matters? What was the trickiest part of balancing, and how has your picture of what a chemical equation actually represents changed since the start of this lesson?
Q1. 1. Explain the difference between a word equation and a symbolic equation. Give one example of each for the same reaction. 4 MARKS
Q2. 2. Balance the equation for the reaction between nitrogen and hydrogen to form ammonia: N 2 + H 2 → NH 3 . Show your working by counting atoms on each side. 4 MARKS
Q3. 3. A student argues that you can balance H 2 + O 2 → H 2 O by changing the formula to H 2 O 2 . Explain why this is incorrect and demonstrate the correct way to balance this equation. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Were your atom counts for H2 + O2 → H2O correct?
- Can you now balance this equation properly?
Model answers (click to reveal)
Answers
▾MCQ 1
B A word equation shows the names of the reactants and products in a chemical reaction.
MCQ 2
B The '2' in front of HCl is a coefficient, meaning two molecules of hydrogen chloride are produced.
MCQ 3
A2Mg + O2 → 2MgO is balanced: 2 Mg and 2 O on each side.
MCQ 4
C Left side has 2 oxygen atoms (from O2). Right side has 2 × 1 = 2 oxygen atoms (from 2H2O). The equation is balanced.
MCQ 5
D C + O2 → CO2 is balanced: 1 C and 2 O on each side. H2 + O2 → H2O is NOT balanced: 2 O on the left but only 1 O on the right. It should be 2H2 + O2 → 2H2O.
Short Answer 1
Model answer: A word equation uses the names of substances to describe a reaction. For example: hydrogen + oxygen → water. A symbolic equation uses chemical formulas and symbols instead of names. For the same reaction: H2 + O2 → H2O. Symbolic equations are more concise and allow chemists to balance reactions by counting atoms, while word equations are clearer for beginners and ensure everyone understands which substances are involved.
Short Answer 2
Model answer: In N2 + H2 → NH3, the left side has 2 nitrogen atoms and 2 hydrogen atoms. The right side has 1 nitrogen atom and 3 hydrogen atoms. To balance nitrogen, place a 2 in front of NH3: N2 + H2 → 2NH3. Now the right side has 2 nitrogen atoms and 6 hydrogen atoms. To balance hydrogen, place a 3 in front of H2: N2 + 3H2 → 2NH3. Final check: left = 2 N, 6 H; right = 2 N, 6 H. Balanced.
Short Answer 3
Model answer: Changing H2O to H2O2 is incorrect because H2O is water and H2O2 is hydrogen peroxide, they are completely different substances. In balancing, you can only change coefficients (numbers in front), never subscripts (small numbers in formulas). The correct way to balance H2 + O2 → H2O is: count atoms (2 H, 2 O on left; 2 H, 1 O on right). Place a 2 in front of H2O: H2 + O2 → 2H2O. Now right side has 4 H, 2 O. Place a 2 in front of H2: 2H2 + O2 → 2H2O. Now both sides have 4 H and 2 O.