Q1.
Think again about all the changes that we have discussed or talked about so far. In which of these can we get back the object or substance in the form we started with? Record your observations in Table 5.2.
Answer
Here is Table 5.2 completed with twelve of the changes discussed in this chapter.
| S.No. | Change | The original state can be brought back (Yes/No) | How, or why not |
|---|---|---|---|
| 1. | Melting ice cubes | Yes | Put the water back in the freezer |
| 2. | Chopping vegetables | No | The pieces cannot be joined back |
| 3. | Boiling water | Yes | Condense the steam back into water |
| 4. | Making popcorn from corn | No | New substances have formed; the kernel is destroyed |
| 5. | Folding paper into objects | Yes | Unfold and the same sheet is back |
| 6. | Inflating a balloon | Yes | Let the air out |
| 7. | Bursting an inflated balloon | No | The torn rubber cannot be made whole again |
| 8. | Crushing chalk into powder | No | The powder cannot be pressed back into a stick |
| 9. | Adding beetroot extract to water | Yes | Evaporate the water and the colouring matter is left behind |
| 10. | Blowing air into lime water | No | A new substance, calcium carbonate, has been formed |
| 11. | Burning of a magnesium ribbon | No | The white magnesium oxide cannot be turned back into the ribbon |
| 12. | Rusting of an iron nail | No | A new substance, iron oxide, has been formed |
Why it happens: Read the “No” rows. Rows 4, 10, 11 and 12 are chemical changes — the old substance no longer exists. Rows 2, 7 and 8 are physical changes that are still irreversible, because the pieces cannot practically be put back together. Both kinds of reason can make a change permanent.