Distinguish physical from chemical properties and changes, and explain the law of conservation of mass in terms of atoms rearranging rather than being created or destroyed.
A physical property, like color, density, or melting point, can be observed without changing what a substance actually is. A chemical property describes how a substance behaves when it reacts and turns into something new, such as whether it burns or reacts with acid. A chemical reaction happens when starting substances, called reactants, transform into different substances, called products, with genuinely new properties -- because the atoms involved have rearranged into new combinations and new bonds.
This rearranging is exactly why the law of conservation of mass holds true: the same atoms that started the reaction are still present at the end, just bonded together differently, so the total mass of the products always equals the total mass of the reactants. Burning wood is a good test case: the visible ash left behind weighs much less than the original log, which can look like mass disappeared. It has not -- most of the original atoms escaped as carbon dioxide gas and water vapor, and if you could capture and weigh every product, including those gases, the total would match the mass of the wood and oxygen you started with.
Mastery looks like: They can distinguish physical from chemical properties and explain, using atoms, why mass is conserved even when a solid product appears to have lost mass.
Common stumbles: Believing mass is genuinely lost when a solid product weighs less than the reactants; confusing a physical change (same substance, new state) with a chemical change (new substance formed).