MYP Chemistry: Balancing Chemical Equations

You need to know the Law of Conservation of Mass: atoms aren't created or destroyed in a reaction, so the same number of each atom must appear on both sides of the equation. You balance by changing coefficients (the numbers in front of formulas) — never by altering subscripts inside a formula.

Changing a subscript changes the substance itself — H₂O becomes H₂O₂, which is hydrogen peroxide, not water. Subscripts define a compound's identity; only coefficients (the big numbers out front) can legally change to balance atom counts, because they just mean "more molecules," not "a different molecule."

List atoms of each element on both sides, then adjust coefficients — usually starting with the most complex molecule — until every element matches. Check polyatomic ions (like sulfate or nitrate) as a single unit if they stay intact through the reaction; it's faster and less error-prone.

A word equation names the reactants and products in plain English — "magnesium plus oxygen produces magnesium oxide" — with no quantities shown. A balanced symbol equation uses chemical formulas and coefficients: 2Mg + O₂ → 2MgO, showing exact atom ratios that satisfy conservation of mass.

The top three: changing subscripts instead of coefficients, forgetting to balance polyatomic ions as a group, and stopping too early before checking every single element. A fourth, sneakier one — forgetting state symbols (s, l, g, aq) — costs marks in MYP Criterion A even when the equation itself is correctly balanced.

Yes, in most MYP 4-5 units and increasingly in assessed tasks — state symbols (s, l, g, aq) show whether a substance is solid, liquid, gas, or dissolved in water. Leaving them off usually isn't marked wrong at MYP 1-3, but by MYP 4-5 it starts appearing in Criterion A success criteria.

They usually sit under Criterion A (Knowing and understanding) or Criterion C (Processing and evaluating), depending on whether the task is a standalone skill check or part of a lab write-up. Full marks typically require the correct formulas, correct coefficients in lowest ratio, and correct state symbols where required.

Not conceptually — most students grasp the logic within a lesson or two. What makes it feel hard is arithmetic fluency: juggling three or four coefficients at once without losing track. Students who struggle with basic ratio and multiplication skills often struggle here too, regardless of how well they understand the chemistry.

MYP balancing work builds the exact skill DP Chemistry assumes on day one — writing balanced equations, including state symbols, for reactions covered in stoichiometry (Structure 1.4 in the current guide). Students who haven't consolidated this by the end of MYP 5 often lose early marks in DP1 on mole-ratio calculations that depend on a correctly balanced equation.

Look for materials with graded practice — simple diatomic reactions first, then combustion and displacement reactions, then equations involving polyatomic ions. On RevisionPrep, MYP Chemistry Topical Worksheets are organised this way, moving from single-step balancing to multi-step reactions with worked solutions your child can check against independently.

Don't reach for the answer key first — ask them to count atoms out loud, element by element, on each side of the equation. Most sticking points aren't about understanding the chemistry; they're arithmetic slips or confusion between coefficients and subscripts, both of which show up quickly once you talk through the counting together.

It usually means they're losing marks under Criterion A or C on tasks requiring balanced symbol equations — often not from misunderstanding the chemistry, but from rushing the atom count or forgetting state symbols. Ask your child's teacher which specific step in the process is dropping marks; it's rarely the whole skill that's weak.