Reversible and irreversible changes
Chemistry · Key Stage 2 · The School
Can you get it back?
Some changes can be undone. Melting, freezing, dissolving and evaporating are REVERSIBLE. Dissolve salt in water and you can get it back. Just evaporate the water away. Other changes cannot be undone. Burning. Rusting. Cooking an egg. Mixing vinegar with bicarbonate. These are IRREVERSIBLE. A NEW substance forms. No amount of cooling brings the egg back.
Choosing a separation method
Sieving separates things by particle size, like sand from pebbles. Filtering separates a solid that will not dissolve from a liquid, like sand from water. Evaporating gets back a dissolved solid, like salt from water. A magnet pulls out iron. Pick your method by what the mixture actually is. That decision is the skill being assessed.
Signs a new substance has formed
Watch for four signs. A gas given off, so it fizzes. A colour change that will not reverse. Heat or light being produced. Or a solid appearing when two liquids mix. Spot any one of these and you are looking at a chemical change. It is not a physical one.
Melt chocolate. It goes runny, then sets again as it cools. Same substance. Reversible, and physical. Dissolve salt. It vanishes. But evaporate the water and it comes back. Reversible, and physical. Now burn a match. You get smoke, heat and a black stub. There is no route back to a match. A NEW substance was made. So it is irreversible, and chemical. Notice that dissolving looked the most final and was the easiest to undo.