Physical and chemical change
Chemistry · Key Stage 3 · The School
The test is whether a new substance exists
Melt ice and you still have water — the same substance, rearranged. Burn magnesium and you have magnesium oxide, a white powder that behaves nothing like the shiny ribbon you started with. That is the whole distinction: a physical change rearranges what is there, a chemical change makes something that was not there before.
Four signs worth looking for
A permanent colour change, a gas given off when nothing was heated to boiling, a solid appearing when two clear liquids meet, and a temperature change with no heater. Any one of these suggests a reaction. None of them proves it alone — dissolving can look dramatic and is physical — so look for more than one, and ask the reversibility question below.
Reversibility is a strong clue, not a rule
Physical changes usually reverse easily: freeze the water again, let the salt solution evaporate and the salt returns unchanged. Chemical changes usually do not — you cannot un-burn the magnesium in a school laboratory. But some reactions genuinely do reverse, so treat this as evidence pointing one way rather than as a definition.
A Royal Society of Chemistry school practical gives a worked example. It starts with 2.39 g of shiny magnesium. This is heated in a lidded pot called a crucible until it is mostly a white powder. Step one: a colour change is only a clue, so weigh the powder. Step two: it weighs 3.78 g, a gain of 3.78 g minus 2.39 g, which is 1.39 g. Step three: atoms are only rearranged, never created or destroyed, so the extra 1.39 g joined from outside. It is oxygen from the air, which was always there but was not on the balance. Answer: magnesium joined 1.39 g of oxygen from the air. So 2.39 g became 3.78 g of mostly magnesium oxide. A perfect run would give about 3.98 g, because a little magnesium did not react. A powder with oxygen built in is a new substance, so this is a chemical change. Weigh a sealed container before and after any reaction and the reading does not move.