Physical vs. chemical changes
The idea
Crushing a can, melting butter, and burning toast all change matter — but not in the same way. In a physical change the substance itself survives: it may change shape, state, or get mixed in, yet the particles are still the same substance. In a chemical change the atoms regroup into new substances with new properties: burnt toast is no longer bread, and a fried egg is no longer raw egg. You already know some changes reverse easily and some do not, and this idea sharpens that instinct into a real test.
The reliable question is: did a new substance appear? Look for evidence — a color change that will not undo, a gas fizzing out of a liquid, a new smell, light, or an unexpected temperature change. Reversibility alone is a tempting but unreliable test. Sugar dissolving in tea looks dramatic and is annoying to undo, yet it is physical — the sugar is still there, sweet as ever, and evaporation brings it back. Meanwhile some chemical changes can be reversed with another reaction. Judge by what the substance is, not by how flashy the change looks.
Worked example
Sort these four kitchen events into physical or chemical changes, with a reason for each: ice melting in lemonade, an egg frying in a pan, sugar dissolving in hot tea, and a slice of toast burning black.
- Ice melting: the solid water becomes liquid water, but every particle is still H₂O — only the state changed, so this is a physical change.
- Egg frying: the clear runny white turns into a firm white solid and stays that way when cooled. The proteins have been rearranged into new substances, so this is a chemical change.
- Sugar dissolving: the crystals vanish from sight, yet the tea tastes sweet and gentle evaporation would leave the sugar behind unchanged — the substance survived, so it is a physical change.
- Toast burning: the bread blackens, smells different, and crumbles into charred material that no toaster setting can turn back into bread. New substances formed, so it is a chemical change.
- Interpret the pattern: the deciding question was never how dramatic the event looked, but whether a new substance was present at the end.
Answer. Melting and dissolving are physical changes; frying the egg and burning the toast are chemical changes, because only those two produce new substances.
Check your understanding
- Why is sugar dissolving classified as a physical change even though the crystals seem to disappear completely?
- What pieces of evidence would convince you a mystery change in a beaker was chemical rather than physical?
- Where does the rule 'chemical changes cannot be reversed' break down, and what is a better test to use instead?
- How could two changes look almost identical from the outside yet be different on the particle level?
Build the foundations first
Physical vs. chemical changes builds on these concepts. If any feel shaky, start there.