Reversible and irreversible changes
Chemistry · Key Stage 2 · The School
Can you get it back?
Melting, freezing, dissolving and evaporating are REVERSIBLE — salt dissolved in water can be recovered by evaporating the water. Burning, rusting, cooking an egg and mixing vinegar with bicarbonate are IRREVERSIBLE: a NEW substance forms, and no amount of cooling brings the egg back.
Choosing a separation method
Sieving separates by particle size (sand from pebbles). Filtering separates an insoluble solid from a liquid (sand from water). Evaporating recovers a dissolved solid (salt from water). Magnetism pulls out iron. Pick the method by what the mixture actually is — that decision is the skill being assessed.
Signs a new substance has formed
Watch for: a gas given off (fizzing), a colour change that will not reverse, heat or light produced, or a solid appearing when two liquids mix. Spot one of these and you are looking at a chemical change, not a physical one.
Melting chocolate: it becomes runny, then sets again on cooling — same substance, reversible, physical. Dissolving salt: it vanishes, but evaporate the water and it comes back — reversible, physical. Burning a match: smoke, heat, a black stub, and no route back to a match — a NEW substance was made, so irreversible and chemical. Note that dissolving looked the most final and was the easiest to undo.