top of page

C1.2 Chemical Equations

GCSE Combined Science · AQA · Chapter C1: Atomic Structure

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: word equations, formulae, conservation of mass, balancing symbol equations and state symbols. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Write word equations for reactions, naming the reactants and products
  • Recall the formulae of common substances, including the diatomic elements
  • Use the law of conservation of mass to work out a missing mass
  • Balance symbol equations by counting atoms (Higher tier: harder equations)
  • Add state symbols and explain why mass seems to change when a gas enters or leaves an open container

Key words

  • Reactant: A substance that you start with in a chemical reaction.
  • Product: A new substance made in a chemical reaction.
  • Word equation: An equation that uses the names of the reactants and products.
  • Symbol equation: An equation that uses formulae instead of names.
  • Balanced equation: An equation with the same number of each type of atom on both sides.
  • Conservation of mass: No atoms are lost or made in a reaction, so the total mass of products equals the total mass of reactants.
  • Precipitate: An insoluble solid formed when two solutions react together.
  • State symbol: Letters in brackets that show the state of each substance: (s), (l), (g) or (aq).

Word equations

In a chemical reaction, atoms are rearranged to make new substances. A word equation names the reactants on the left and the products on the right. The arrow means “react to make”; the + sign means “and”.

magnesium + oxygen → magnesium oxide

ethene + oxygen → carbon dioxide + water

iron + hydrochloric acid → iron chloride + hydrogen

Exam tip: Never write an equals sign (=) in a chemical equation. Use an arrow (→).

Formulae you need to know

A symbol equation uses formulae. Seven elements exist as diatomic molecules (two atoms joined): H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Other elements, such as metals and carbon, are written as just the symbol: Mg, Fe, C.

  • water; Formula H₂O; Substance hydrochloric acid; Formula HCl
  • carbon dioxide; Formula CO₂; Substance sulfuric acid; Formula H₂SO₄
  • methane; Formula CH₄; Substance sodium hydroxide; Formula NaOH
  • ammonia; Formula NH₃; Substance sodium chloride; Formula NaCl
  • magnesium oxide; Formula MgO; Substance calcium carbonate; Formula CaCO₃

Conservation of mass

Atoms are not created or destroyed in a reaction. They are only rearranged. So the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass.

Worked example: finding a missing mass

7.2 g of magnesium burns completely in oxygen to make 12.0 g of magnesium oxide. What mass of oxygen reacted?
mass of reactants = mass of products, so 7.2 + mass of oxygen = 12.0
mass of oxygen = 12.0 − 7.2 = 4.8 g

Answer: 4.8 g

You can test this in the lab. Mix two solutions that react to make a precipitate (an insoluble solid that forms when two solutions react) in a closed flask on a balance. For example, copper sulfate solution and sodium hydroxide solution make a blue precipitate of copper hydroxide:

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

Key fact: If a reaction happens in an open container, the mass on the balance can change. It goes up if a gas from the air (e.g. oxygen) joins on, and down if a gas product (e.g. CO₂) escapes. Mass is still conserved – the gas is just not being weighed.

Balancing symbol equations

A balanced equation has the same number of atoms of each element on both sides. To balance, you put big numbers in front of formulae. You must never change the small numbers inside a formula, because that would make a different substance.

Worked example: balancing the burning of ethene

C₂H₄ + O₂ → CO₂ + H₂O
C: 2 left, 1 right ✗ → put 2 in front of CO₂ · H: 4 left, 2 right ✗ → put 2 in front of H₂O
C₂H₄ + O₂ → 2CO₂ + 2H₂O · O: 2 left, 4 + 2 = 6 right ✗ → put 3 in front of O₂
Check: C 2 = 2, H 4 = 4, O 6 = 6 ✓

Answer: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Worked example: a harder one (Higher tier)

Fe + O₂ → Fe₂O₃
O: 2 on the left and 3 on the right. The lowest common multiple is 6, so use 3O₂ and 2Fe₂O₃.
Now there are 4 Fe on the right, so put 4 in front of Fe.

Answer: 4Fe + 3O₂ → 2Fe₂O₃

State symbols

  • (s); Meaning solid; Example Fe(s), MgO(s)
  • (l); Meaning liquid; Example H₂O(l), Br₂(l)
  • (g); Meaning gas; Example CO₂(g), H₂(g)
  • (aq); Meaning aqueous: dissolved in water; Example NaCl(aq), HCl(aq)

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Exam tip: Pure water is H₂O(l), not (aq). Use (aq) only for something dissolved in water.

Common mistakes

  • Changing a small number to balance, e.g. turning H₂O into H₂O₂. That is a different substance (hydrogen peroxide).
  • Writing O or H for oxygen or hydrogen gas. They are diatomic: O₂ and H₂.
  • Forgetting that a big number multiplies the whole formula: 2H₂O has 4 H atoms and 2 O atoms.
  • Saying mass is “lost” when a gas escapes. The atoms still exist; they have just left the container.
  • Writing (aq) for water itself, or (l) for a solution.

Revision questions

Try these, then check your answers below each one.

Question 1. Which equation is balanced? (Choose from: Mg + O₂ → MgO, 2Mg + O₂ → 2MgO, Mg + O₂ → 2MgO, 2Mg + 2O₂ → 2MgO.)

Answer: 2Mg + O₂ → 2MgO

Working: 2Mg + O₂ → 2MgO has 2 Mg and 2 O on each side.

Question 2. 2.4 g of magnesium burns completely to make 4.0 g of magnesium oxide. What mass of oxygen reacted?

Answer: 1.6 g

Working: 4.0 − 2.4 = 1.6 g of oxygen.

Question 3. The diagram shows a reaction between hydrogen and chlorine molecules. Which balanced equation does it show? (Choose from: H + Cl → HCl, H₂ + Cl₂ → H₂Cl₂, H₂ + Cl₂ → 2HCl, 2H₂ + Cl₂ → 2HCl.)

Answer: H₂ + Cl₂ → 2HCl

Working: One H₂ molecule and one Cl₂ molecule react to make two HCl molecules: H₂ + Cl₂ → 2HCl.

Related lessons

Continue in the full course

This lesson is section C1.2 (AGCSC1802) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

bottom of page