MYP Chemistry: Conservation of Mass — Questions Students & Parents Actually Ask

Conservation of mass says the total mass of reactants entering a chemical reaction always equals the total mass of products leaving it — atoms are rearranged, never created or destroyed. In MYP 4-5 you apply this law to check equations balance and to predict masses, often through practical work like precipitation or gas-producing reactions.

Mass only appears to change when a gas escapes or is absorbed from the air — the law itself never breaks down. Burning magnesium ribbon actually gains mass, because it combines with oxygen from the surrounding air to form magnesium oxide, and the oxide left behind weighs more than the original strip.

You balance an equation by changing the coefficients — the numbers placed in front of a formula — never the subscripts inside it, until the count of every element matches on both sides. Count each element left and right, adjust one coefficient at a time, and check hydrogen and oxygen last since they usually appear in several places.

You add up the masses of every reactant that goes in, because none of that mass disappears during the reaction — it's simply rearranged into products. If 4 g of magnesium reacts completely with 2.6 g of oxygen, the magnesium oxide produced must have a mass of exactly 6.6 g.

The standard MYP practical reacts calcium carbonate with dilute hydrochloric acid inside a sealed flask sitting on a balance, then reweighs the whole flask after the reaction finishes. Because the carbon dioxide produced can't escape the sealed container, the balance reading before and after stays identical, confirming the law directly.

In an open container, whatever gas the reaction produces simply escapes into the air, carrying its mass away with it — so the balance reads a lower final value even though no mass has actually vanished. Repeat the identical reaction in a sealed flask or a balloon-capped tube and the reading stays exactly the same.

Conservation of mass questions most often sit under MYP Sciences Criterion A, Knowing and Understanding, where you explain the law and apply it to balancing and mass calculations. If you're designing your own investigation — say, comparing sealed versus open reactions — that shifts into Criterion B, Inquiring and Designing.

Expect 'balance', 'explain', 'calculate', 'deduce' and 'justify'. 'Balance' wants correct coefficients only, not new products. 'Explain' wants reasoning tied to atoms rearranging, not disappearing. 'Calculate' wants a numerical answer with working and units shown — a bare final number usually loses marks even if it's correct.

Most MYP schools test this topic through short-answer questions — balancing equations, explaining mass changes — combined with a practical write-up assessed against Criterion C, Processing and Evaluating, where you analyse your own experimental data for anomalies like measurement error or escaped gas.

The single most common mistake is changing subscripts instead of coefficients when balancing — turning H₂O into H₂O₂ to fix a hydrogen count, which creates a completely different substance. The second is assuming mass 'lost' in an open reaction has vanished, rather than escaped as a gas.

A genuine top mark on this topic means balancing unfamiliar equations under time pressure, calculating masses without an arithmetic slip, and explaining a real anomaly in your own data using chemistry, not a vague excuse. Practise against the actual criterion descriptors your school uses, not just general revision notes.

MYP conservation of mass is the qualitative groundwork for DP Chemistry's quantitative stoichiometry. According to the IB, the current DP Chemistry guide (first exams 2025) builds directly on this law through mole calculations, limiting reagent and percentage yield questions at both SL and HL.

Most students who struggle aren't actually confused about the law itself — they're stuck on balancing equations or on why mass appears to change in open-container experiments, which are separate skills sitting on top of the concept. Ask your child to explain the difference between a sealed and an open reaction out loud; if they can't, that's the real gap.

Look for materials pairing short explanations with genuine equation-balancing practice and criterion-marked past-style questions, not just definitions to memorise. On revisionprep.com, the MYP Chemistry Revision Notes and Topical Worksheets cover conservation of mass with worked balancing examples and questions mapped to the actual MYP Sciences criteria.