Tanya says fast changes are chemical and slow ones are physical — so iron rusting (slow) is…
Tanya says fast changes are chemical and slow ones are physical — so iron rusting (slow) is physical, and wood burning (fast) is chemical. Her friend says speed does not decide this. Who is right?
Who do you think is right? Pick one.
The answer: Her friend is right, the real question is whether a new substance formed — rusting and burning are both chemical
Why
Tanya has invented a rule based on two examples that happen to fit — burning is fast and chemical, and she assumes rusting being slow must make it physical. But how quickly a change happens has nothing to do with whether it is physical or chemical. The only thing that matters is whether a new substance is formed. Rusting IS a chemical change even though it takes weeks: the iron combines with oxygen and moisture to make a brand-new, flaky, reddish-brown substance that is not iron any more. Burning is also chemical, just faster. Meanwhile ice melting is physical whether it happens in seconds or hours. Test it: scrape rust off an old gate — it is a different material from the shiny iron underneath, and you cannot turn it back into iron; slow did not make it physical. Her friend is right: rusting and burning are both chemical changes — a new substance forms in each — and speed does not decide the type of change.
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