Misconception 1: "You can change the subscript to balance an equation"
Changing a subscript changes the substance entirely. H₂O is water; H₃O is a different compound (the hydronium ion). When balancing, you may ONLY add coefficients in front of whole formulae. Changing subscripts is the single most common error in balancing questions.
Misconception 2: "Mass is lost when a gas is produced"
Mass is never lost in a reaction. When a gas escapes an open container, the balance reads a lower mass because the gas is no longer sitting on it — but the gas still has mass. In a closed system the total mass would stay the same. Examiners frequently test whether students can explain this distinction.
Misconception 3: "All formulae are written with a positive ion first"
This is true for ionic compounds (Na⁺Cl⁻ → NaCl) but not for covalent ones. Carbon dioxide is CO₂ not OC₂. For covalent compounds the formula comes from the structural arrangement, not charge order.
This common misconceptions covers Common Misconceptions within Formulae, Equations and Conservation of Mass for GCSE Chemistry. Revise Formulae, Equations and Conservation of Mass in Quantitative Chemistry for GCSE Chemistry with 13 exam-style questions and 12 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 10 of 14 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Formulae, Equations and Conservation of Mass
Which of the following is the correct formula for oxygen gas?
Explain why mass is conserved in a chemical reaction.