This module covers the written language of chemistry: how to build a formula, balance an equation, show states, describe a change in words and strip an equation down to the ions that really react. These skills sit underneath almost every Chemistry calculation, because a wrong formula or an unbalanced equation breaks every later step.
All examples and questions in this module are original teaching examples, not past-paper items. The reactions are described at concept level only, to practise writing and reasoning on paper.
What is this topic, and why does it matter?
A formula tells you which atoms or ions are in a substance and in what ratio. An equation tells you which substances react, what forms, and in what ratio. The ratios matter because they carry straight into amount-of-substance work in relative masses and amounts.
Marks in this area go to accuracy on small details: a missing bracket, a changed subscript, a wrong state symbol. Each of those details is a rule you can learn to apply calmly.
What should I know before starting?
You will get more from these lessons if you already know what atoms, ions and compounds are, and why ions carry charge. The module on bonding and structure covers that background. You also need to be comfortable with small whole-number arithmetic, such as finding a common multiple of 2 and 3.
A quick orienting example
Sodium reacts with chlorine to form sodium chloride. The ions are Na⁺ and Cl⁻. One positive charge cancels one negative charge, so the formula is NaCl.
For the equation, write the formulas: Na + Cl₂ → NaCl. Chlorine exists as Cl₂, so there are 2 Cl atoms on the left and 1 on the right. Put a 2 in front of NaCl to give 2 Cl, then a 2 in front of Na to restore sodium.
2Na + Cl₂ → 2NaCl
Add states if the question supplies the evidence: 2Na(s) + Cl₂(g) → 2NaCl(s). That one example already uses four of the five lessons below.
In which order should I study the lessons?
- Construct a formula from ion charges: every later step needs correct formulas first.
- Balance atoms without changing subscripts: once formulas are fixed, balancing is pure counting.
- Include state symbols from supplied evidence: adds (s), (l), (g) and (aq) using clues in the question.
- Write a word equation from a described change: practises naming reactants and products before symbols.
- Identify a spectator species in an appropriate ionic equation: the most demanding step, which builds on all four above.
What are the common traps?
- Changing a subscript to make atoms match, instead of adding a coefficient.
- Forgetting brackets when a polyatomic ion appears more than once, as in Ca(OH)₂.
- Writing water as (aq) when it is a liquid, (l).
- Splitting a solid precipitate into ions in an ionic equation.
- Leaving a balanced equation with coefficients that could all be halved.
How do I use the practice set?
Attempt the mixed practice set on paper first, without looking at the answers. Mark yourself, then read each worked answer even when you were right, because the reasoning is what transfers to new questions. The equation balance reasoning trainer and the mole and equation-ratio tutor let you check atom counts and ratios afterwards.
Where does tuition fit?
Formula and equation errors are usually small and repeated, which makes them hard to spot on your own. A teacher who watches you write can see which rule slips. That is the kind of work we do in online one-to-one Chemistry tuition.