Once you can spot the signs, you start seeing chemical change running the whole world — and the real skill is going backwards: you're handed the thing you noticed, and you reason back to "was that chemical?"
Chemical Change Is Everywhere
Combustion — fuels reacting with oxygen — powers cars, stoves and campfires: fuel + oxygen → carbon dioxide + water. Cooking is chemistry you can eat: a raw egg's clear goo turns solid and white and never goes back, bread browns, a steak sears — all new substances, none reversible. Rusting slowly eats iron gates and car panels as the metal grabs oxygen and water from the air. And the quietest one is happening inside you right now: respiration, your cells burning glucose with oxygen to release energy — glucose + oxygen → carbon dioxide + water, the very same word equation as a campfire, just gentler and wetter. You breathe out the products.
The Real Skill: Reasoning Backwards
Rung 1 went forwards — change first, then spot the signs. Mastery is going backwards. A cut apple turns brown on the bench. Work back: a colour change, hard to reverse, no boiling or mixing — chemical, the flesh reacting with oxygen. A puddle of water disappears overnight. Work back: still water, just gone to vapour, no new substance — purely physical. Two clear liquids are mixed and a cloudy solid appears. Work back: a new solid out of nowhere is a precipitate — chemical. Each time you take the sign and talk back through it to a verdict.
Try the scenario cards in the toy, then play with the sealed-versus-open candle: seal it and the gases are trapped, so the scales hold steady and you can see conservation of mass; open the lid and the products float off, so the reading drops even though nothing was destroyed.
A Depth-study Thread (optional)
This is a lovely launch pad for a Year 8 depth study (the scope sets aside time for one): design a fair test of what speeds up rusting — does water alone do it, does salt make it worse, does air-only keep iron shiny? Set up identical nails in different conditions, change one thing at a time, and watch them over a fortnight. It's real working scientifically (SC4-WS-04, SC4-WS-07) hanging off a single chemical change.