Buffyyour study buddyHi! Ask me about physical changes, chemical changes, rusting or anything in this chapter.
Buffy is an AI helper and can be wrong — always check your NCERT textbook.
Melt it and you can freeze it back. Burn it and it's gone for good. The dividing line is one simple question — was a brand-new substance made?
Play with it
Tap an everyday change and ask two questions: can it be reversed? and is a new substance formed? Those two answers decide — physical or chemical.
Learn
In a physical change only the shape, size or state changes — the substance stays the same.
In a chemical change one or more new substances form, usually irreversibly.
Rusting is a slow chemical change: iron + oxygen + moisture → reddish-brown rust. We slow it by keeping oxygen and water away.
Worked example. You dissolve salt in water. Physical or chemical?
No new substance is made — it's still salt and water. Evaporate the water and the salt comes back. So it's a physical change (and reversible).
Where you'll meet it
Toasting bread, boiling an egg and baking a cake are chemical changes — new substances, new flavours, no going back. Melting butter, though, is just physical.
Melting old metal or plastic to remould it is a physical change — the material is unchanged, so it can be reused again and again.
Check yourself
Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.
Interactive built to the OpenMAIC approach (THU-MAIC, MIT). Content from the NCERT Class 7 Science textbook, Curiosity (ncert.nic.in).
Buffyyour study buddyBuffy is an AI helper and can be wrong — always check your NCERT textbook.