Chemical Formulae and Equations

This lesson covers how to write chemical formulae and balanced chemical equations — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to be able to write formulae from names, construct word equations and balanced symbol equations, and understand the law of conservation of mass.

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Writing Chemical Formulae

A chemical formula shows the ratio of atoms of each element in a compound. To write a formula, you need to know the charges of the ions involved (for ionic compounds) or the number of bonds each atom forms (for covalent compounds).

Formulae of Ionic Compounds

In an ionic compound, the total positive charge must equal the total negative charge — the compound is electrically neutral overall.

Method: Using Ion Charges

1. Write down the ions and their charges.
2. Work out the ratio needed to balance the charges.
3. Write the formula using subscripts.

Worked Examples

Example 1: Sodium chloride

• Ions: Na⁺ and Cl⁻
• One Na⁺ balances one Cl⁻
• Formula: NaCl

Example 2: Magnesium chloride

• Ions: Mg²⁺ and Cl⁻
• One Mg²⁺ needs two Cl⁻ to balance
• Formula: MgCl₂

Example 3: Aluminium oxide

• Ions: Al³⁺ and O²⁻
• Two Al³⁺ (total charge +6) balance three O²⁻ (total charge −6)
• Formula: Al₂O₃

Example 4: Calcium hydroxide

• Ions: Ca²⁺ and OH⁻
• One Ca²⁺ needs two OH⁻
• Formula: Ca(OH)₂ — note the brackets around OH because there are 2 hydroxide groups

Example 5: Iron(III) sulfate

• Ions: Fe³⁺ and SO₄²⁻
• Two Fe³⁺ (total +6) balance three SO₄²⁻ (total −6)
• Formula: Fe₂(SO₄)₃

Exam Tip: When writing formulae, always check that the total positive and negative charges cancel out to zero. If they do not balance, your formula is incorrect. Also remember to use brackets when you have more than one polyatomic ion (e.g. (OH)₂, (NO₃)₂, (SO₄)₃).

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Common Formulae You Should Know

Substance	Formula
Water	H₂O
Carbon dioxide	CO₂
Hydrochloric acid	HCl
Sulfuric acid	H₂SO₄
Nitric acid	HNO₃
Sodium hydroxide	NaOH
Calcium carbonate	CaCO₃
Sodium chloride	NaCl
Magnesium oxide	MgO
Carbon monoxide	CO
Ammonia	NH₃
Methane	CH₄

Common Diatomic Elements

Some elements exist as diatomic molecules (two atoms bonded together). You must remember these:

H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

Exam Tip: Remember the diatomic elements with the mnemonic "Have No Fear Of Ice Cold Beer" — H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂. Always write these elements as diatomic molecules in equations.

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State Symbols

State symbols are written in brackets after each substance in a chemical equation to show the physical state:

Symbol	State	Meaning
(s)	Solid	The substance is a solid
(l)	Liquid	The substance is a liquid
(g)	Gas	The substance is a gas
(aq)	Aqueous	The substance is dissolved in water

Examples

• NaCl**(s)** — solid sodium chloride
• H₂O**(l)** — liquid water
• CO₂**(g)** — carbon dioxide gas
• HCl**(aq)** — hydrochloric acid (dissolved in water)

Exam Tip: The Edexcel specification requires you to use state symbols in all balanced symbol equations. You will lose marks if you forget them. Remember: (aq) means dissolved in water — any acid, alkali or salt solution is (aq).

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Word Equations

A word equation describes a chemical reaction using the names of the reactants and products.

Format

Reactants → Products

Examples

1. Magnesium + oxygen → magnesium oxide
2. Sodium + water → sodium hydroxide + hydrogen
3. Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
4. Methane + oxygen → carbon dioxide + water

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Balanced Symbol Equations

A balanced symbol equation uses chemical formulae instead of names and ensures that the number of atoms of each element is the same on both sides.

Why Must Equations Be Balanced?

The law of conservation of mass states that in a chemical reaction, no atoms are created or destroyed — they are simply rearranged. Therefore, the total mass of reactants equals the total mass of products, and the number of each type of atom must be the same on both sides of the equation.

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How to Balance Equations

Step-by-Step Method

1. Write the correct formulae for all reactants and products (do not change any formulae once written).
2. Count the number of each type of atom on both sides.
3. If they are not equal, add coefficients (large numbers in front of formulae) to balance.
4. Only change the large numbers in front — never change the subscripts within a formula.
5. Check that all atoms balance.
6. Add state symbols.

Worked Example 1: Magnesium + Oxygen → Magnesium Oxide

Step 1: Write the formulae:

Mg + O₂ → MgO

Step 2: Count atoms:

• Left: 1 Mg, 2 O
• Right: 1 Mg, 1 O — not balanced (oxygen is unbalanced)

Step 3: Balance oxygen by putting 2 in front of MgO:

Mg + O₂ → 2MgO

Now: Left: 1 Mg, 2 O | Right: 2 Mg, 2 O — Mg is now unbalanced

Step 4: Balance magnesium by putting 2 in front of Mg:

2Mg + O₂ → 2MgO

Step 5: Check: Left: 2 Mg, 2 O | Right: 2 Mg, 2 O ✓

Balanced equation: 2Mg(s) + O₂(g) → 2MgO(s)

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Worked Example 2: Sodium + Water → Sodium Hydroxide + Hydrogen

Step 1: Write formulae:

Na + H₂O → NaOH + H₂

Step 2: Count atoms:

• Left: 1 Na, 2 H, 1 O
• Right: 1 Na, 1 O, 1 H (in NaOH) + 2 H (in H₂) = 3 H total — not balanced

Step 3: Put 2 in front of NaOH and 2 in front of Na and 2 in front of H₂O:

2Na + 2H₂O → 2NaOH + H₂

Step 5: Check: Left: 2 Na, 4 H, 2 O | Right: 2 Na, 2 O, 2 H + 2 H = 4 H ✓

Balanced equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

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Worked Example 3: Calcium Carbonate + Hydrochloric Acid

Step 1: Write formulae:

CaCO₃ + HCl → CaCl₂ + H₂O + CO₂