Changes Around Us: Physical and Chemical
Reversible and desirable changes
Find out which changes can be reversed, and which ones are useful to us.
Once something has changed, can you get it back (Activity 5.8)?
| Change | Can we get the original back? |
|---|---|
| ice melting | yes: freeze it again |
| water evaporating | yes: condense the vapour |
| chopping vegetables | no: a chopped lauki can't become whole |
| making popcorn | no |
| cooking an aloo paratha | no: you can't get the potatoes back |
So changes can be sorted into those that can be reversed and those that cannot.
We can also sort changes by whether we want them. Desirable changes: milk turning to curd (a spoonful of curd in warm milk overnight), fruit ripening, cutting fruit, cooking food. Undesirable changes: iron rusting, stored food going bad.
The same change can be wanted in one place and not in another. Raw mangoes ripening is what you want; ripe mangoes rotting before you eat them is not. And food rotting in the kitchen is a nuisance, but in a compost pit, peels rotting into compost is exactly what we want.
Some changes caused by people affect the environment for a long time. Burning more and more fuel in cars, trains and aeroplanes is raising the amount of carbon dioxide in the air. Even drying paint on walls and furniture gives off substances that pollute the air.
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