Q1.
Once something has undergone a change, can we get it back in its original form?
Answer
Sometimes yes, sometimes no. Changes fall into two groups — those that can be reversed and those that cannot.
| Can be reversed | Cannot be reversed |
|---|---|
| Melting of ice — it can be refrozen into ice | Chopping vegetables — the pieces cannot return to their original size and shape |
| Evaporation of water — the vapour can be condensed back into liquid water | Making popcorn — it can never go back to being a corn kernel |
| Folding a handkerchief, inflating a balloon, dissolving sugar in water | Burning of wood, rusting of iron, curdling of milk |
Why it happens: If the change only rearranged the substance — its shape, size or state — the substance is still there and can usually be brought back. If the change made a new substance, the old one has been used up, so it cannot simply be brought back. That is why most chemical changes are irreversible.
Tip: Reversible is not the same as physical. Bursting a balloon and crushing chalk are physical but irreversible. Always test for the new substance separately.