A change can be loud, hot and impressive and still be purely physical — and mass can look like it appears or vanishes when it never does. These are the traps that catch nearly everyone. Let's disarm them.
Trap One: Dramatic Doesn't Mean Chemical
Water boiling on the stove roars and steams and bubbles like mad — but every one of those bubbles is still water, just water as a gas. No new substance, so no chemical change. Dry ice "smoking" across a stage looks like sorcery and is only solid carbon dioxide turning straight to gas. Don't let the drama decide. Ask the one real question every time: is there genuinely new stuff at the end? Boiling and dissolving fail that test however spectacular they look.
Trap Two: the Disappearing (and Appearing) Mass
Burn a strip of steel wool and it gets heavier — which feels impossible. Burn a candle and it gets lighter, vanishing as it goes. Surely one of these breaks conservation of mass? Neither does. The steel wool grabs oxygen out of the air and locks it into new iron oxide, so the solid weighs more — but the air lost exactly that much. The candle turns into carbon dioxide and water vapour that float away invisibly, so what's left weighs less — but those escaped gases make up the difference precisely. Count the gases and the mass is always conserved. It only ever seems to change because a gas quietly joined the party or slipped out the door.
Trap Three: Reversible Is Not the Same as Physical
It's tempting to pair them up — physical = reversible, chemical = forever — but the pairing leaks. Plenty of physical changes are a one-way pain to undo (try un-mixing sand from sugar), and a few chemical changes can be reversed by another chemical change (charging a battery undoes the reaction that powered your phone). "Can I get my starting stuff back easily?" is a useful hint, but it is not the definition. The definition never changes: physical = no new substance, chemical = new substance.
And the Quiet One: Melting Versus Burning
Heat a candle gently and the wax melts — physical, same wax, you could let it set again. Heat it at the wick and the wax burns — chemical, gone for good. Same object, same source of heat, two completely different kinds of change happening centimetres apart. The trap is assuming "heat caused it" tells you which one it is. Heat drives both melting and burning; only one makes new substances.