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Chemistry · Key Stage 3 · The School
Two kinds of change
A PHYSICAL change alters form but not substance — melting, dissolving, snapping. A CHEMICAL change makes a new substance, usually with a sign: a colour change, a gas given off, heat released or taken in, or a solid appearing from two liquids. Melting a candle is physical; burning it is chemical.
Writing what happened
A word equation names the REACTANTS on the left, an arrow, and the PRODUCTS on the right: magnesium + oxygen → magnesium oxide. The arrow means 'becomes', never 'equals'. The pattern is the point: a metal plus oxygen always makes a metal oxide, whichever metal you use.
Nothing is lost
Atoms are rearranged, never destroyed, so the total mass before equals the total mass after — CONSERVATION OF MASS. When burning magnesium seems to gain mass, that is because oxygen from the air joined it. When a fizzing reaction seems to lose mass, the gas escaped. Weigh it in a sealed container and the mass does not change.
Burn 5 g of magnesium in air and the white ash weighs MORE — about 8.3 g — because oxygen has joined it: magnesium + oxygen → magnesium oxide. Burn a 5 g wooden splint and the ash weighs far LESS, because carbon dioxide and water vapour left as gases. One went up, one went down, and mass is conserved in both. Do the second in a sealed container on a balance and the reading does not move at all.