Year 10 Science Unit 2 · Chemical Reactions Lesson 9 of 20 45 min

Writing and Balancing Equations

Chemical equations are the language of chemistry. They tell us exactly what reacts, what is produced, and in what proportions. In this lesson you will learn to write word equations, take your first steps into symbolic equations, and master the essential skill of balancing — making sure no atoms are created or destroyed.

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Think First

Before You Begin

Look at this symbolic equation:

H2 + O2 → H2O

Write down your answers before reading on:

  • How many hydrogen atoms are on the left side? How many on the right?
  • How many oxygen atoms are on the left side? How many on the right?
  • Is this equation balanced? If not, what could you change to balance it?
Write your thinking in your book before reading on.

Work mode: Digital — answers typed below

Learning Intentions

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
Key Terms
Word equation A way of describing a chemical reaction using the names of reactants and products.
Symbolic equation A chemical equation using chemical formulas and symbols to represent reactants and products.
Balanced equation An equation with the same number of each type of atom on both sides, obeying conservation of mass.
Coefficient A number placed in front of a chemical formula to show how many molecules or formula units are involved.
Subscript A small number written after an element symbol in a formula, showing how many atoms of that element are in the molecule.
State symbol A symbol showing the physical state of a substance: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Core Content
1

Word Equations

The first step in describing reactions

Exothermic Endothermic

Exothermic Endothermic

A word equation describes a chemical reaction using the names of substances. The reactants (starting substances) go on the left, and the products (new substances formed) go on the right, separated by an arrow:

reactant + reactant → product + product

Word equations are the foundation of chemical communication at Stage 5. Before using formulas, scientists always describe reactions in words to make sure everyone understands what is happening.

Examples for each reaction type

Reaction typeWord equation example
Synthesismagnesium + oxygen → magnesium oxide
Decompositioncalcium carbonate → calcium oxide + carbon dioxide
Combustionmethane + oxygen → carbon dioxide + water
Displacementzinc + copper sulfate → zinc sulfate + copper
Neutralisationhydrochloric acid + sodium hydroxide → sodium chloride + water
Remember Reactants always go on the LEFT. Products always go on the RIGHT. The arrow points from reactants to products.
2

Symbolic Equations and Balancing

Using formulas and counting atoms

A symbolic equation uses chemical formulas instead of names. For example, the reaction between hydrogen and oxygen to form water can be written as:

H2 + O2 → H2O

But is this balanced? Let's count atoms:

  • Left side: 2 hydrogen atoms, 2 oxygen atoms
  • Right side: 2 hydrogen atoms, 1 oxygen atom

The oxygen atoms are not balanced. We cannot change the formula H2O to H2O2 because that would be a different substance (hydrogen peroxide). Instead, we place a coefficient (a number in front) to show more molecules:

2H2 + O2 → 2H2O

Now count again:

  • Left side: 2 × 2 = 4 hydrogen atoms; 2 oxygen atoms
  • Right side: 2 × 2 = 4 hydrogen atoms; 2 × 1 = 2 oxygen atoms

Both sides have 4 hydrogen and 2 oxygen. The equation is now balanced!

Important You can only change COEFFICIENTS (numbers in front) to balance equations. Never change SUBSCRIPTS (small numbers in formulas) because that changes the substance into something else.
3

Balancing Simple Equations

Step-by-step atom counting

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)

Think about it Why is balancing equations important? (Hint: revisit the law of conservation of mass from Lesson 1.)

Common Misconceptions

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"You can change subscripts to balance an equation." No — changing subscripts changes the substance. H2O is water; H2O2 is hydrogen peroxide. Only coefficients can be changed.

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"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.

✗trong>"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.

Australian Context

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

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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)
Activities
Activity 1

Write Word Equations

Write a word equation for each reaction described.

1 Calcium carbonate decomposes when heated to produce calcium oxide and carbon dioxide.
Answer in your book.
2 Zinc metal displaces copper from copper sulfate solution.
Answer in your book.
3 Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water.
Answer in your book.
Activity 2

Balance by Atom Counting

Count the atoms on each side and determine if the equation is balanced. If not, write the balanced version.

1 N2 + H2 → NH3
Answer in your book.
2 Fe + O2 → Fe2O3
Answer in your book.
3 CH4 + O2 → CO2 + H2O
Answer in your book.
Multiple Choice
Q

Test Your Understanding

UnderstandBand 3

1. What does a word equation show?

AOnly the products of a reaction
BThe names of reactants and products
CThe temperature change during the reaction
DThe colour change during the reaction
UnderstandBand 3

2. In the equation H2 + Cl2 → 2HCl, what does the '2' in front of HCl mean?

AThere are two hydrogen atoms in one molecule
BTwo molecules of hydrogen chloride are produced
CThere are two chlorine atoms in one molecule
DThe equation is incorrect
ApplyBand 4

3. What coefficients balance the equation __Mg + __O2 → __MgO?

A1, 1, 1
B2, 1, 2
C2, 2, 2
D1, 2, 1
ApplyBand 4

4. How many oxygen atoms are on each side of 2H2 + O2 → 2H2O?

A1 on the left, 1 on the right
B2 on the left, 2 on the right
C2 on the left, 1 on the right
D4 on the left, 2 on the right
AnalyseBand 5

5. A student writes C + O2 → CO2 and says it is balanced. Another writes H2 + O2 → H2O and says it is balanced. Who is correct and why?

ABoth students are correct
BOnly the first student is correct — the second equation is not balanced
COnly the second student is correct — the first equation is not balanced
DNeither student is correct
Short Answer
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Short Answer Questions

UnderstandBand 3

1. Explain the difference between a word equation and a symbolic equation. Give one example of each for the same reaction. 4 MARKS

Answer in your book.
ApplyBand 4

2. Balance the equation for the reaction between nitrogen and hydrogen to form ammonia: N2 + H2 → NH3. Show your working by counting atoms on each side. 4 MARKS

Answer in your book with reasoning.
AnalyseBand 5

3. A student argues that you can balance H2 + O2 → H2O by changing the formula to H2O2. Explain why this is incorrect and demonstrate the correct way to balance this equation. 4 MARKS

Answer in your book.

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?
Update your thinking in your book.

Answers

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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

B — 2Mg + O2 → 2MgO is balanced: 2 Mg and 2 O on each side.

MCQ 4

B — Left side has 2 oxygen atoms (from O2). Right side has 2 × 1 = 2 oxygen atoms (from 2H2O). The equation is balanced.

MCQ 5

B — 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.

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Lesson Game

Equation Balancer Blaster

Test your equation balancing skills. Count atoms, fix coefficients and blast your way to a high score!

Mark lesson as complete

Tick when you have finished all activities and checked your answers.

Unit 2 overview · Science subject page