What is the significance of chemical formulae in stoichiometry?

Chemical formulae are crucial in stoichiometry as they provide the exact ratio of atoms in a compound, which is essential for calculating the amounts of reactants and products in a chemical reaction. Understanding formulae allows students to balance equations and perform mole calculations, which are key components of the Cambridge IGCSE Chemistry curriculum.

How do you determine the empirical formula from experimental data?

To determine the empirical formula from experimental data, you need to find the simplest whole-number ratio of moles of each element in a compound. This involves converting the mass of each element to moles using their atomic masses, then dividing by the smallest number of moles to find the ratio. This process is a fundamental skill in stoichiometry taught in the Cambridge IGCSE Chemistry syllabus.

What is the difference between empirical and molecular formulae?

The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of each type of atom in a molecule. For example, the empirical formula of glucose is CH2O, whereas its molecular formula is C6H12O6. Understanding this distinction is important for Cambridge IGCSE students studying stoichiometry.

How can you use a chemical formula to calculate the molar mass of a compound?

To calculate the molar mass of a compound using its chemical formula, sum the atomic masses of all the atoms present in the formula. For instance, the molar mass of H2O is calculated by adding the masses of two hydrogen atoms and one oxygen atom. This calculation is essential for stoichiometry problems in the Cambridge IGCSE Chemistry curriculum.

Why is it important to balance chemical equations in stoichiometry?

Balancing chemical equations is crucial in stoichiometry because it ensures the law of conservation of mass is upheld, meaning the same number of each type of atom is present on both sides of the equation. This allows for accurate calculations of reactants and products, a key skill for Cambridge IGCSE Chemistry students.

Why is "Changing a chemical formula to balance an equation." a common mistake in Formulae?

Why it happens: It seems like a quick way to make the atoms match. How to avoid it: Only change the COEFFICIENTS in front of formulae. Never change a subscript inside a formula — that would change the substance. Source: 0620/42 — every series

Why is "Omitting or muddling state symbols." a common mistake in Formulae?

Why it happens: Rushing, or not deciding whether a salt is solid or dissolved. How to avoid it: Add (s), (l), (g) or (aq) to every species; soluble salts in solution are (aq), precipitates are (s).

Why is "Forgetting brackets around a polyatomic ion." a common mistake in Formulae?

Why it happens: Writing the subscript straight after the last atom, e.g. CaNO_32. How to avoid it: When you need more than one polyatomic ion, bracket it first: Ca(NO_3)_2, (NH_4)_2SO_4, Al_2(SO_4)_3.

Why is "Charges not balanced in an ionic equation." a common mistake in Formulae?

Why it happens: Checking only the atoms and forgetting the total charge. How to avoid it: Make total charge on the LHS equal to total charge on the RHS, AND keep only aqueous species split into ions.

Cambridge IGCSE Chemistry (0620)

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Short Notes - Formulae

Detailed Study Notes

Detailed notes on Stoichiometry for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.

Formulae and Equations — Cambridge IGCSE 0620 Chemistry Extended (2026)

Writing chemical formulae from ion charges, balancing equations, and including state symbols. The base skill for everything stoichiometric.

At a glance

Charges balance: total positive = total negative in a formula.
Common polyatomic ions to memorise: NO₃⁻, OH⁻, CO₃²⁻, SO₄²⁻, NH₄⁺.
Balance equations by counting atoms on each side. Use coefficients only — never change subscripts.
State symbols: $(s)$ solid, $(l)$ liquid, $(g)$ gas, $(aq)$ aqueous.
Conservation: same number and type of atoms on each side.
Charge also balances in ionic equations.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

3.1 — Write the formulae of simple compounds from ionic charges.
3.1 — Balance chemical equations.
3.1 — Include appropriate state symbols.
3.1 — Construct ionic equations from full equations (Extended).

Writing chemical formulae

Total positive charge = total negative charge. Use subscripts to balance.

Steps.

Write the symbol of each ion with its charge.
Find the lowest common multiple of the charges to balance.
Use subscripts to indicate how many of each ion. Drop the charges in the final formula.

Worked. Calcium chloride from Ca²⁺ and Cl⁻.

Need 2 Cl⁻ to balance 1 Ca²⁺.
Formula: $\text{CaCl}_{2}$ .

Worked. Aluminium oxide from Al³⁺ and O²⁻.

Need 2 Al³⁺ ( $+6$ total) to balance 3 O²⁻ ( $-6$ total).
Formula: $\text{Al}_{2}\text{O}_{3}$ .

Pick the ratio of ions that makes the positive and negative charges cancel exactly.

Worked. Ammonium sulfate from NH₄⁺ and SO₄²⁻.

Need 2 NH₄⁺ to balance 1 SO₄²⁻.
Formula: $(\text{NH}_{4})_{2}\text{SO}_{4}$ . (Brackets when more than one polyatomic ion.)

Common ions to memorise.

Ion	Charge	Notes
Na⁺, K⁺, Li⁺	$+1$	Group 1 metals
Mg²⁺, Ca²⁺	$+2$	Group 2 metals
Al³⁺	$+3$	Group 13
NH₄⁺	$+1$	Ammonium
Cl⁻, Br⁻, I⁻	$-1$	Halogens
O²⁻, S²⁻	$-2$	Oxide, sulfide
OH⁻	$-1$	Hydroxide
NO₃⁻	$-1$	Nitrate
CO₃²⁻	$-2$	Carbonate
SO₄²⁻	$-2$	Sulfate
PO₄³⁻	$-3$	Phosphate

Transition metals have variable charges; Cambridge usually quotes the charge in the question (e.g. iron(II) = Fe²⁺, iron(III) = Fe³⁺, copper(II) = Cu²⁺).

Total + = total − in a formula.
Memorise common polyatomic ions.
Brackets for multiple polyatomic ions.
Roman numeral = ion charge for transition metals.

Balancing equations

Same number of each type of atom on both sides. Use coefficients only.

Conservation of mass. Atoms are neither created nor destroyed in a chemical reaction. So the count of each atom must be the same on both sides.

Strategy.

Write the unbalanced equation (correct formulae for reactants and products).
Count atoms of each element on both sides.
Add coefficients (numbers in front of formulae) to balance.
NEVER change subscripts within a formula — that changes what the substance IS.

Worked. Combustion of methane.

Unbalanced: $\text{CH}_{4} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$ .
Carbon: 1 = 1 ✓.
Hydrogen: 4 vs 2 → put 2 in front of $\text{H}_{2}\text{O}$ : $\text{CH}_{4} + \text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$ .
Oxygen: 2 vs 4 → put 2 in front of $\text{O}_{2}$ : $\text{CH}_{4} + 2\text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$ .
Final check: C 1=1, H 4=4, O 4=4 ✓.

An equation is balanced when every element has the same atom count on both sides.

Worked. Aluminium + chlorine.

Unbalanced: $\text{Al} + \text{Cl}_{2} \to \text{AlCl}_{3}$ .
Need 2 Al + 3 Cl₂ → 2 AlCl₃ to balance.
Final: $2\text{Al} + 3\text{Cl}_{2} \to 2\text{AlCl}_{3}$ .

Tip. When you have an odd-numbered atom on one side (like the H in NH₃) and even-numbered on the other, multiply both sides by 2 to find a common ground.

Atoms must balance (conservation of mass).
Coefficients only, never subscripts.
Balance one element at a time.
Re-check at the end.

State symbols and ionic equations

Use $(s), (l), (g), (aq)$ to show physical state. Ionic equations show only the ions that change.

State symbols.

$(s)$ — solid.
$(l)$ — pure liquid.
$(g)$ — gas.
$(aq)$ — aqueous (dissolved in water).

Always include them in Cambridge mark-scheme answers.

Symbol	Meaning	Example
(s)	solid	Mg(s)
(l)	pure liquid	H₂O(l)
(g)	gas	H₂(g)
(aq)	aqueous — dissolved in water	HCl(aq)

Worked. $\text{Mg}(s) + 2\text{HCl}(aq) \to \text{MgCl}_{2}(aq) + \text{H}_{2}(g)$ .

Ionic equations (Extended). Show only the ions that actually change. Spectator ions (those that appear unchanged on both sides) are removed.

Steps.

Write the full balanced equation with state symbols.
Split aqueous compounds into their ions.
Cancel any ions that appear identically on both sides (spectators).
What remains is the ionic equation.

Worked. Reaction of sodium hydroxide with hydrochloric acid.

Full: $\text{NaOH}(aq) + \text{HCl}(aq) \to \text{NaCl}(aq) + \text{H}_{2}\text{O}(l)$ .
Split aqueous: $\text{Na}^{+}(aq) + \text{OH}^{-}(aq) + \text{H}^{+}(aq) + \text{Cl}^{-}(aq) \to \text{Na}^{+}(aq) + \text{Cl}^{-}(aq) + \text{H}_{2}\text{O}(l)$ .
Cancel Na⁺ and Cl⁻ (spectators).
Ionic: $\text{H}^{+}(aq) + \text{OH}^{-}(aq) \to \text{H}_{2}\text{O}(l)$ .

Cancelling the spectator ions leaves the equation common to every acid–alkali neutralisation.

This is the SAME ionic equation for ANY acid + alkali neutralisation — that's why neutralisation always involves H⁺ and OH⁻ combining to make water.

Charge must balance in ionic equations too. Sum of charges on left = sum on right.

$(s), (l), (g), (aq)$ .
Ionic equation: drop spectator ions.
Acid + alkali: $\text{H}^{+} + \text{OH}^{-} \to \text{H}_{2}\text{O}$ .
Charge AND atoms balance.

Quick recap

Charges balance in formulae.
Common polyatomic ions to memorise.
Balance equations: count atoms, use coefficients.
State symbols always.
Ionic equations: drop spectators.
Charge AND atoms balance.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Chemical formula — symbols and subscripts showing the elements and their ratios in a compound.
Coefficient — number in front of a formula in an equation, indicating quantity.
State symbol — $(s), (l), (g), (aq)$ indicating physical state.
Spectator ion — ion that appears unchanged on both sides of an equation.
Ionic equation — equation showing only the species that change in a reaction.

How it’s examined

Formulae and equations are foundational and appear EVERY paper. Paper 2: 3-5 marks for writing formulae and balancing. Paper 4: 5-7 marks combined with stoichiometric calculations. Examiner reports flag changing subscripts when balancing (you only adjust coefficients) and missing state symbols.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (3.1); 0620/42 Oct/Nov 2024 — Q7 (balancing, ionic equations); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

Take this whole topic with you

Step-by-step worked examples — Formulae

Step-by-step solutions to past-paper-style questions on formulae, written exactly the way a tutor would explain them at the board.

1Write formulae from ion charges
Getting started• formula
Question
Write the formulae of: (a) sodium chloride, (b) magnesium oxide, (c) calcium chloride.
Step-by-step solution
1. Step 1
  (a) $\text{Na}^{+}$ and $\text{Cl}^{-}$ — charges are $+1$ and $-1$ , so they balance one-to-one.
  $\text{Na}^{+} + \text{Cl}^{-} \to \text{NaCl}$
2. Step 2
  (b) $\text{Mg}^{2+}$ and $\text{O}^{2-}$ — both charges are size $2$ , so again one-to-one.
  $\text{Mg}^{2+} + \text{O}^{2-} \to \text{MgO}$
3. Step 3
  (c) $\text{Ca}^{2+}$ and $\text{Cl}^{-}$ — you need TWO $\text{Cl}^{-}$ to cancel the $2+$ .
  $\text{Ca}^{2+} + 2\text{Cl}^{-} \to \text{CaCl}_{2}$
Answer
(a) $\text{NaCl}$ (b) $\text{MgO}$ (c) $\text{CaCl}_{2}$
Examiner tip
The total positive charge must exactly cancel the total negative charge — the compound is always neutral overall.
2Deduce a formula from a model
Getting started• Paper 4 structured style• formula, diagram
Question
A ball-and-stick model of a molecule shows one carbon atom joined to four chlorine atoms. Deduce its formula.
Step-by-step solution
1. Step 1
  Count each type of atom shown in the model: 1 carbon (C) and 4 chlorine (Cl).
2. Step 2
  Write the symbol for each element, then a subscript for how many of that atom there are. A subscript of $1$ is not written.
  $1 \times \text{C} \;+\; 4 \times \text{Cl} \to \text{CCl}_{4}$
Answer
$\text{CCl}_{4}$
Examiner tip
Just count the atoms in the picture — there are no charges to balance here because it is a covalent molecule, not an ionic compound.
3Formulae with polyatomic ions
Building confidence• Adapted from 0620/42 May/Jun 2024 Q4• formula, polyatomic
Question
Write the formulae of: (a) aluminium oxide, (b) calcium nitrate, (c) ammonium sulfate.
Step-by-step solution
1. Step 1
  (a) $\text{Al}^{3+}$ and $\text{O}^{2-}$ . The lowest common multiple of $3$ and $2$ is $6$ : take two $\text{Al}^{3+}$ ( $+6$ ) and three $\text{O}^{2-}$ ( $-6$ ).
  $2\text{Al}^{3+} + 3\text{O}^{2-} \to \text{Al}_{2}\text{O}_{3}$
2. Step 2
  (b) $\text{Ca}^{2+}$ and the nitrate ion $\text{NO}_{3}^{-}$ . Two nitrate ions are needed; enclose the polyatomic ion in BRACKETS before the subscript.
  $\text{Ca}^{2+} + 2\text{NO}_{3}^{-} \to \text{Ca(NO}_{3})_{2}$
3. Step 3
  (c) The ammonium ion $\text{NH}_{4}^{+}$ and the sulfate ion $\text{SO}_{4}^{2-}$ . Two ammonium ions are needed — bracket $\text{NH}_{4}$ before the subscript.
  $2\text{NH}_{4}^{+} + \text{SO}_{4}^{2-} \to (\text{NH}_{4})_{2}\text{SO}_{4}$
Answer
(a) $\text{Al}_{2}\text{O}_{3}$ (b) $\text{Ca(NO}_{3})_{2}$ (c) $(\text{NH}_{4})_{2}\text{SO}_{4}$
Examiner tip
Whenever you need more than one of a polyatomic ion (nitrate, sulfate, carbonate, hydroxide, ammonium), put it in brackets first, e.g. $\text{Ca(NO}_{3})_{2}$ , NOT $\text{CaNO}_{32}$ .
4Balance and add state symbols
Building confidence• balance, states
Question
Write a balanced symbol equation, with state symbols, for the reaction of solid calcium carbonate with dilute hydrochloric acid to give calcium chloride solution, water, and carbon dioxide gas.
Step-by-step solution
1. Step 1
  Write the unbalanced equation using correct formulae: $\text{CaCO}_{3}$ , $\text{HCl}$ , $\text{CaCl}_{2}$ , $\text{H}_{2}\text{O}$ , $\text{CO}_{2}$ .
  $\text{CaCO}_{3} + \text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$
2. Step 2
  Balance chlorine and hydrogen: the right has 2 Cl, so put a $2$ in front of $\text{HCl}$ . That also gives 2 H, matching the water.
  $\text{CaCO}_{3} + 2\text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$
3. Step 3
  Check all atoms: Ca 1=1, C 1=1, O 3 = (1+2), H 2=2, Cl 2=2. Add state symbols — solid (s), aqueous (aq), liquid (l), gas (g).
  $\text{CaCO}_{3}(\text{s}) + 2\text{HCl}(\text{aq}) \to \text{CaCl}_{2}(\text{aq}) + \text{H}_{2}\text{O}(\text{l}) + \text{CO}_{2}(\text{g})$
Answer
$\text{CaCO}_{3}(\text{s}) + 2\text{HCl}(\text{aq}) \to \text{CaCl}_{2}(\text{aq}) + \text{H}_{2}\text{O}(\text{l}) + \text{CO}_{2}(\text{g})$
Examiner tip
Balance by changing the big numbers (coefficients) in FRONT of formulae only — never alter a subscript inside a formula.
5Balance a combustion equation
Stretch• Adapted from 0620/22 May/Jun 2024 Q6• balance
Question
Balance the combustion of butane: $\text{C}_{4}\text{H}_{10} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$ .
Step-by-step solution
1. Step 1
  Balance carbon and hydrogen first: 4 C gives $4\text{CO}_{2}$ ; 10 H gives $5\text{H}_{2}\text{O}$ .
  $\text{C}_{4}\text{H}_{10} + \text{O}_{2} \to 4\text{CO}_{2} + 5\text{H}_{2}\text{O}$
2. Step 2
  Count oxygen on the right: $(4 \times 2) + 5 = 13$ O atoms. That needs $\tfrac{13}{2}\,\text{O}_{2}$ on the left.
  $\text{C}_{4}\text{H}_{10} + \dfrac{13}{2}\,\text{O}_{2} \to 4\text{CO}_{2} + 5\text{H}_{2}\text{O}$
3. Step 3
  Multiply every coefficient by 2 to clear the fraction, then verify: C 8=8, H 20=20, O 26 = (16+10).
  $2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$
Answer
$2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$
Examiner tip
For hydrocarbon combustion, always leave oxygen until last — balance C and H first, then fix the O atoms.
6Construct an ionic equation (Extended)
Stretch• Paper 4 (Extended) structured style• ionic equation
Question
Aqueous silver nitrate reacts with aqueous sodium chloride to form a white precipitate of silver chloride. Construct the ionic equation, with state symbols.
Step-by-step solution
1. Step 1
  Write the full balanced symbol equation first.
  $\text{AgNO}_{3}(\text{aq}) + \text{NaCl}(\text{aq}) \to \text{AgCl}(\text{s}) + \text{NaNO}_{3}(\text{aq})$
2. Step 2
  Split every aqueous compound into its separate ions. The solid $\text{AgCl}$ is NOT split. Identify the spectator ions that appear unchanged on both sides — here $\text{Na}^{+}$ and $\text{NO}_{3}^{-}$ .
  $\text{Ag}^{+} + \text{NO}_{3}^{-} + \text{Na}^{+} + \text{Cl}^{-} \to \text{AgCl}(\text{s}) + \text{Na}^{+} + \text{NO}_{3}^{-}$
3. Step 3
  Cancel the spectator ions to leave the net ionic equation. Check both atoms and charge: LHS charge $(+1) + (-1) = 0$ ; RHS charge $0$ — they balance.
  $\text{Ag}^{+}(\text{aq}) + \text{Cl}^{-}(\text{aq}) \to \text{AgCl}(\text{s})$
Answer
$\text{Ag}^{+}(\text{aq}) + \text{Cl}^{-}(\text{aq}) \to \text{AgCl}(\text{s})$
Examiner tip
An ionic equation must balance for BOTH atoms and total charge. Only soluble (aq) species split into ions; solids, liquids and gases stay as whole formulae.

Model Answers — Formulae

High-scoring sample answers for formulae on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
Write the formula of magnesium chloride. (1 mark)
Model answer
$\text{MgCl}_{2}$
Why this scores
One mark for the correct formula. $\text{Mg}^{2+}$ needs two $\text{Cl}^{-}$ ions to balance the $2+$ charge, so the subscript on Cl is 2.
Question 2
Paper 4 structured style2 marks
Deduce the formula of the ionic compound formed between aluminium ions ( $\text{Al}^{3+}$ ) and sulfate ions ( $\text{SO}_{4}^{2-}$ ). (2 marks)
Model answer
Two $\text{Al}^{3+}$ ions carry a total charge of $+6$ , and three $\text{SO}_{4}^{2-}$ ions carry a total charge of $-6$ , so these balance. Enclosing the polyatomic sulfate ion in brackets, the formula is $\text{Al}_{2}(\text{SO}_{4})_{3}$ .
Why this scores
One mark for balancing the charges (2 Al to 3 sulfate), one mark for the correctly bracketed formula $\text{Al}_{2}(\text{SO}_{4})_{3}$ .
Question 3
Paper 4 structured style3 marks
Balance the following equation: $\text{Fe}_{2}\text{O}_{3} + \text{CO} \to \text{Fe} + \text{CO}_{2}$ . (3 marks)
Model answer
Balancing the iron and oxygen by adding coefficients gives:

$\text{Fe}_{2}\text{O}_{3} + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_{2}$

Check: Fe $2 = 2$ ; C $3 = 3$ ; O $(3 + 3) = 6$ on the left and $(3 \times 2) = 6$ on the right.
Why this scores
Award the marks for the correct coefficients — $1, 3, 2, 3$ . Coefficients only go in FRONT of formulae; do not change any subscripts.
Question 4
Adapted from 0620/42 May/Jun 2023 Q34 marks
Solid zinc reacts with dilute sulfuric acid to form zinc sulfate solution and hydrogen gas. Write the balanced symbol equation, including state symbols. (4 marks)
Model answer
$\text{Zn}(\text{s}) + \text{H}_{2}\text{SO}_{4}(\text{aq}) \to \text{ZnSO}_{4}(\text{aq}) + \text{H}_{2}(\text{g})$

The equation is already balanced (Zn $1=1$ , H $2=2$ , S $1=1$ , O $4=4$ ). Zinc is a solid (s), the acid and the salt are aqueous (aq), and hydrogen is a gas (g).
Why this scores
Marks for: correct formulae of reactants; correct formulae of products; the equation balancing; and all four state symbols correct. A single missing or wrong state symbol typically loses the state-symbol mark.
Question 5
Paper 4 (Extended) structured style5 marks
Aqueous lead(II) nitrate reacts with aqueous potassium iodide to form a yellow precipitate of lead(II) iodide. Construct the ionic equation for this reaction, including state symbols. (5 marks)
Model answer
The full balanced equation is:

$\text{Pb(NO}_{3})_{2}(\text{aq}) + 2\text{KI}(\text{aq}) \to \text{PbI}_{2}(\text{s}) + 2\text{KNO}_{3}(\text{aq})$

The spectator ions, $\text{K}^{+}$ and $\text{NO}_{3}^{-}$ , are unchanged on both sides and are cancelled, leaving the ionic equation:

$\text{Pb}^{2+}(\text{aq}) + 2\text{I}^{-}(\text{aq}) \to \text{PbI}_{2}(\text{s})$

This balances for atoms (1 Pb, 2 I) and for charge: LHS $(+2) + 2(-1) = 0$ , RHS $= 0$ .
Why this scores
Marks for: a correct full balanced equation; identifying the spectator ions; the correct net ionic equation; the coefficient 2 on the iodide ion; and correct state symbols. The charges must balance as well as the atoms.
Question 6
Paper 4 (Extended) structured style6 marks
Propane ( $\text{C}_{3}\text{H}_{8}$ ) burns completely in oxygen to form carbon dioxide and water. Write the balanced symbol equation, with state symbols, and explain the steps you took to balance it. (6 marks)
Model answer
First write the unbalanced equation with correct formulae: $\text{C}_{3}\text{H}_{8} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$ . Balance the carbon next — 3 C needs $3\text{CO}_{2}$ — and then the hydrogen — 8 H needs $4\text{H}_{2}\text{O}$ . The right-hand side now has $(3 \times 2) + 4 = 10$ oxygen atoms, which requires $5\text{O}_{2}$ on the left. The balanced equation, with state symbols, is:

$\text{C}_{3}\text{H}_{8}(\text{g}) + 5\text{O}_{2}(\text{g}) \to 3\text{CO}_{2}(\text{g}) + 4\text{H}_{2}\text{O}(\text{l})$

Final check: C $3=3$ , H $8=8$ , O $10 = (6+4)$ .
Why this scores
Marks for: balancing carbon; balancing hydrogen; balancing oxygen last; the fully correct coefficients ( $1, 5, 3, 4$ ); correct state symbols; and a clear final atom check. Balancing carbon and hydrogen before oxygen is the method examiners expect.

Key Formulae — Formulae

The formulae you need to memorise for formulae on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Ion-charge swap (criss-cross)
$M^{a+} + X^{b-} \to M_{b}X_{a}$
$a+$
charge on the positive ion (cation)
$b-$
charge on the negative ion (anion)
When to use
Quickly write the formula of an ionic compound — swap the charge sizes to become the subscripts, then cancel to the simplest ratio.
Example
$\text{Al}^{3+}$ + $\text{O}^{2-}$ → $\text{Al}_{2}\text{O}_{3}$

Key Definitions and Keywords — Formulae

Definitions to memorise and the exact keywords mark schemes credit for formulae answers — sharpened from recent examiner reports for the 2026 0620 sitting.

State symbols
Examiner keyword
Letters in brackets after a formula showing the physical state: $(\text{s})$ solid, $(\text{l})$ liquid, $(\text{g})$ gas, $(\text{aq})$ aqueous (dissolved in water).
Balanced equation
Examiner keyword
An equation with equal numbers of each type of atom (and equal total charge) on both sides, achieved by adjusting the coefficients in front of formulae.
Polyatomic ion
Examiner keyword
A charged group of atoms acting as a single ion, e.g. nitrate $\text{NO}_{3}^{-}$ , sulfate $\text{SO}_{4}^{2-}$ , carbonate $\text{CO}_{3}^{2-}$ , hydroxide $\text{OH}^{-}$ , ammonium $\text{NH}_{4}^{+}$ .
Ionic equation
Examiner keyword
An equation showing only the ions and atoms that actually take part in the reaction, with the spectator ions removed.
Spectator ion
Examiner keyword
An ion that appears unchanged on both sides of an equation and is therefore cancelled when writing the ionic equation.

Common Mistakes and Misconceptions — Formulae

The traps other students keep falling into on formulae questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Changing a chemical formula to balance an equation.
0620/42 — every series
Why it happens
It seems like a quick way to make the atoms match.
How to avoid it
Only change the COEFFICIENTS in front of formulae. Never change a subscript inside a formula — that would change the substance.
✕Omitting or muddling state symbols.
Why it happens
Rushing, or not deciding whether a salt is solid or dissolved.
How to avoid it
Add $(\text{s})$ , $(\text{l})$ , $(\text{g})$ or $(\text{aq})$ to every species; soluble salts in solution are $(\text{aq})$ , precipitates are $(\text{s})$ .
✕Forgetting brackets around a polyatomic ion.
Why it happens
Writing the subscript straight after the last atom, e.g. $\text{CaNO}_{32}$ .
How to avoid it
When you need more than one polyatomic ion, bracket it first: $\text{Ca(NO}_{3})_{2}$ , $(\text{NH}_{4})_{2}\text{SO}_{4}$ , $\text{Al}_{2}(\text{SO}_{4})_{3}$ .
✕Charges not balanced in an ionic equation.
Why it happens
Checking only the atoms and forgetting the total charge.
How to avoid it
Make total charge on the LHS equal to total charge on the RHS, AND keep only aqueous species split into ions.

Formulae — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Ion

Charge

Notes

Na⁺, K⁺, Li⁺

+1

Group 1 metals

Mg²⁺, Ca²⁺

+2

Group 2 metals

Al³⁺

+3

Group 13

NH₄⁺

+1

Ammonium

Cl⁻, Br⁻, I⁻

-1

Halogens

O²⁻, S²⁻

-2

Oxide, sulfide

OH⁻

-1

Hydroxide

NO₃⁻

-1

Nitrate

CO₃²⁻

-2

Carbonate

SO₄²⁻

-2

Sulfate

PO₄³⁻

-3

Phosphate

Conservation of mass. Atoms are neither created nor destroyed in a chemical reaction. So the count of each atom must be the same on both sides.

Strategy.

Write the unbalanced equation (correct formulae for reactants and products).
Count atoms of each element on both sides.
Add coefficients (numbers in front of formulae) to balance.
NEVER change subscripts within a formula — that changes what the substance IS.

Worked. Combustion of methane.

Unbalanced: $\text{CH}_{4} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$ .
Carbon: 1 = 1 ✓.
Hydrogen: 4 vs 2 → put 2 in front of $\text{H}_{2}\text{O}$ : $\text{CH}_{4} + \text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$ .
Oxygen: 2 vs 4 → put 2 in front of $\text{O}_{2}$ : $\text{CH}_{4} + 2\text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$ .
Final check: C 1=1, H 4=4, O 4=4 ✓.

An equation is balanced when every element has the same atom count on both sides.

Worked. Aluminium + chlorine.

Unbalanced: $\text{Al} + \text{Cl}_{2} \to \text{AlCl}_{3}$ .
Need 2 Al + 3 Cl₂ → 2 AlCl₃ to balance.
Final: $2\text{Al} + 3\text{Cl}_{2} \to 2\text{AlCl}_{3}$ .

Tip. When you have an odd-numbered atom on one side (like the H in NH₃) and even-numbered on the other, multiply both sides by 2 to find a common ground.

Symbol

Meaning

Example

(s)

solid

Mg(s)

(l)

pure liquid

H₂O(l)

(g)

gas

H₂(g)

(aq)

aqueous — dissolved in water

HCl(aq)

Formulae

Short Notes - Formulae

Detailed Study Notes

At a glance

What you’ll learn

Writing chemical formulae

Balancing equations

State symbols and ionic equations

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Write formulae from ion charges

2Deduce a formula from a model

3Formulae with polyatomic ions

4Balance and add state symbols

5Balance a combustion equation

6Construct an ionic equation (Extended)

Ion-charge swap (criss-cross)

State symbols

Balanced equation

Polyatomic ion

Ionic equation

Spectator ion

✕Changing a chemical formula to balance an equation.

✕Omitting or muddling state symbols.

✕Forgetting brackets around a polyatomic ion.

✕Charges not balanced in an ionic equation.

Formulae — frequently asked questions

Formulae

Short Notes - Formulae

Detailed Study Notes

At a glance

What you’ll learn

Writing chemical formulae

Balancing equations

State symbols and ionic equations

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Write formulae from ion charges

2Deduce a formula from a model

3Formulae with polyatomic ions

4Balance and add state symbols

5Balance a combustion equation

6Construct an ionic equation (Extended)

Ion-charge swap (criss-cross)

State symbols

Balanced equation

Polyatomic ion

Ionic equation

Spectator ion

✕Changing a chemical formula to balance an equation.

✕Omitting or muddling state symbols.

✕Forgetting brackets around a polyatomic ion.

✕Charges not balanced in an ionic equation.

Formulae — frequently asked questions