A formula tells you which atoms are in a substance and in what ratio. An equation shows what happens in a reaction, with the same atoms on both sides. Writing ionic equations with state symbols is Extended content.
In an ionic compound the total positive charge equals the total negative charge, so Al3+ and O2− combine 2 : 3 as Al2O3. A balanced symbol equation has the same number of each type of atom on both sides. Balance it by changing only the big numbers in front of formulae, never the small numbers inside them. State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
A model or diagram of a molecule lets you count the atoms directly. For an ionic compound, use the charges on the ions and choose the smallest numbers of each ion that make the total charge zero.
| Ions | Charge balance | Formula |
|---|---|---|
| Mg2+ and Cl− | +2 needs two −1 | MgCl2 |
| Al3+ and O2− | 2 × (+3) = +6 and 3 × (−2) = −6 | Al2O3 |
| Ca2+ and OH− | +2 needs two −1 | Ca(OH)2 |
| NH4+ and SO42− | two +1 balance one −2 | (NH4)2SO4 |
Write the word equation, replace each name with its correct formula, then balance by changing only the numbers in front. Add state symbols at the end.
Never balance an equation by changing a small number inside a formula. Writing H2O2 to make H2 + O2 → H2O balance turns water into hydrogen peroxide, a different substance. Change only the numbers in front: 2H2 + O2 → 2H2O.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the formula of magnesium chloride? (Mg2+ and Cl−)
Which equation is correctly balanced?
The state symbol (aq) means the substance is:
A model of a molecule shows one nitrogen atom joined to three hydrogen atoms. What is its formula?
Extended content. An ionic equation shows only the ions that react. Leave out the spectator ions, which are in the solution before and after and do not change.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
What is the formula of aluminium oxide? (Al3+ and O2−)
Iron reacts with chlorine: xFe + yCl2 → 2FeCl3. What are x and y?
Extended: aqueous silver nitrate is added to aqueous sodium chloride. What is the ionic equation?
Extended: a diagram of an ionic lattice shows two K+ ions for every O2− ion. What is the formula of the compound?
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