A chemical change deserves a tidy way of writing it down. That's a word equation: the reactants on the left, an arrow meaning "turn into", and the products on the right.
Reading and Writing Word Equations
The arrow is the heart of it — it reads as "react to make", and it only ever points one way, from what you start with to what you end up with. Take a burning candle, or natural gas on a stovetop: methane + oxygen → carbon dioxide + water. Read it out loud: methane and oxygen are the reactants; they react to make carbon dioxide and water, the products. Rusting is just as neat: iron + oxygen → iron oxide.
To write one yourself, run three moves. One, list everything that went in (the reactants) joined with +. Two, draw the arrow. Three, list everything genuinely new that came out (the products), joined with + too. No numbers, no symbols yet — just the names, in the right places, pointing the right way.
The Law Underneath: Conservation of Mass
Here's the deep idea that makes all of this trustworthy. In a chemical change, atoms are never created and never destroyed — they're only rearranged. Every atom that goes into the reactants comes out again in the products, just bonded together differently. Bonds break, new bonds form, the same atoms shuffle into new partners.
And if no atoms are gained or lost, then the total mass can't change either. Weigh everything before, weigh everything after — sealed up so nothing escapes — and the scales read exactly the same. That's the law of conservation of mass, and it's iron-clad: 50 grams of reactants always make 50 grams of products.
A Worked One, Slowly
When hydrogen burns in oxygen to make water, 4 g of hydrogen reacts with 32 g of oxygen. What mass of water forms?
Start from the law: no atoms are lost, so the mass of the products equals the mass of the reactants. The reactants total 4 g + 32 g = 36 g. There's only one product, water, so all 36 g must end up as water — the answer is 36 g, no calculation cleverness needed. Press play in the toy and you'll watch it happen: the hydrogen and oxygen molecules come apart, the very same atoms snap together into water, and the balance underneath stays dead level the whole time.