Word & Balanced chemical Equations

A word equation is the simplest way to describe a reaction. It uses the names of the substances — no symbols, no formulae and no numbers.

• The substances you start with (the reactants) go on the left.
• The substances you make (the products) go on the right.
• An arrow (→) means "turns into" / "reacts to make" — it is not an equals sign.
• A plus sign (+) separates two reactants, or two products. Read it as "and" / "reacts with".

Worked examples.

• magnesium + oxygen → magnesium oxide
• hydrochloric acid + sodium hydroxide → sodium chloride + water
• zinc + copper sulfate → zinc sulfate + copper

Watch the wording in the question. Phrases like "burns in oxygen" mean oxygen is a reactant; "neutralises an acid" tells you a salt + water are products. Pull the names straight out of the description.

Before you can write a symbol equation you need the correct formula of every substance. For an ionic compound, the rule is simple: the compound is neutral, so the total positive charge must equal the total negative charge.

The cross-over method.

1. Write the positive ion (cation) with its charge, then the negative ion (anion) with its charge.
2. Swap (cross over) the size of each charge so it becomes the subscript of the other ion.
3. Simplify to the smallest whole-number ratio. (A subscript of 1 is never written.)

Worked examples.

• Sodium chloride: Na⁺ + Cl⁻ → NaCl (charges already cancel, 1 : 1).
• Calcium chloride: Ca²⁺ + Cl⁻ → CaCl₂ (need 2 Cl⁻ to balance one Ca²⁺).
• Magnesium oxide: Mg²⁺ + O²⁻ → MgO (2+ and 2− cancel).
• Aluminium oxide: Al³⁺ + O²⁻ → Al₂O₃ (2 × 3 = 6 = 3 × 2).
• Sodium oxide: Na⁺ + O²⁻ → Na₂O.

Common ions to learn.

Ion	Symbol	Charge
Sodium / potassium	Na⁺, K⁺	+1
Magnesium / calcium	Mg²⁺, Ca²⁺	+2
Aluminium	Al³⁺	+3
Chloride / bromide	Cl⁻, Br⁻	−1
Oxide / sulfide	O²⁻, S²⁻	−2
Hydroxide / nitrate	OH⁻, NO₃⁻	−1
Carbonate / sulfate	CO₃²⁻, SO₄²⁻	−2

Brackets for groups of atoms. When you need more than one of a polyatomic ion (like OH⁻ or NO₃⁻), wrap it in brackets: calcium hydroxide is Ca(OH)₂, calcium nitrate is Ca(NO₃)₂, ammonium sulfate is (NH₄)₂SO₄.

A balanced symbol equation has the same number of atoms of every element on both sides of the arrow. This must be true because of the law of conservation of mass — atoms are simply rearranged in a reaction, never made or lost.

The golden rule: you may only change the coefficients — the big numbers written in front of a formula. You must NEVER change a subscript (the small numbers inside a formula), because that would change the substance into something completely different.

A reliable method.

1. Write the equation with the correct formula for every substance (use ion charges if needed).
2. Count the atoms of each element on both sides.
3. Balance one element at a time, using coefficients. Leave O and H until last — they often appear in several places.
4. Re-check every element after each change — fixing one can unbalance another.
5. Reduce all coefficients to the smallest whole-number ratio.

Worked example — combustion of methane.

Unbalanced: CH₄ + O₂ → CO₂ + H₂O

• Carbon is already balanced (1 each side).
• Balance H: 4 H on the left, so put 2 in front of H₂O → CH₄ + O₂ → CO₂ + 2H₂O.
• Now count O on the right: 2 (in CO₂) + 2 (in 2H₂O) = 4, so put 2 in front of O₂.
• Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O.

The illegal move. Never "balance" oxygen by changing CO₂ into CO — that turns carbon dioxide into carbon monoxide, a totally different gas. Adjust the front numbers only.

A fully marked symbol equation usually needs state symbols — a small bracket after each formula telling you the physical state of that substance under the reaction conditions.

Symbol	Meaning
(s)	solid
(l)	liquid (a pure liquid, e.g. water)
(g)	gas
(aq)	aqueous — dissolved in water (a solution)

The (l) vs (aq) trap. These are not the same:

• (l) = a substance that is a pure liquid itself (e.g. water, H₂O(l); molten metal).
• (aq) = a substance dissolved in water to form a solution (e.g. NaCl(aq), HCl(aq)).

So salt water is NaCl(aq), but the water it is dissolved in is H₂O(l).

Worked example — neutralisation.

$\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$

• Both the acid and the alkali are solutions → (aq).
• The salt formed stays dissolved → NaCl(aq).
• The water produced is a pure liquid → H₂O(l).

Tip: decide the state from the description — a gas given off is (g), a precipitate that forms is (s), anything in solution is (aq), and pure water made is (l).