Hinglish

Reversible, Permanent and Desirable Changes

Class 7 Science · Chapter 5: Changes Around Us: Physical and Chemical · NCERT Curiosity section 5.4–5.6

Learn

Ishaan lets the air out of his balloon: it shrinks back. Meher unfolds her boat: it is a flat sheet again. Some changes can be reversed: we get back the same object or substance. Melting ice can be frozen again; water vapour can condense back to water.

But chopped vegetables cannot be joined again, idli cannot become batter, and ash cannot become paper. These are irreversible changes. Most chemical changes, like cooking and burning, are irreversible, but so are some physical ones, like cutting, grinding wheat into flour, or bursting a balloon.

Some changes help us and some harm us. Setting curd, ripening fruit, cooking food and turning kitchen waste into compost are desirable. Rusting of tools, food rotting, and the extra carbon dioxide and smoke from burning fuels in vehicles and factories are undesirable. We paint iron to stop rust and keep food cold to slow spoiling.

Explore: can it be undone?

  1. Sort each change.

Remember

  • Reversible changes can be undone; irreversible ones cannot, whether physical or chemical.
  • Desirable changes are useful (curd, compost); undesirable ones cause harm (rust, rotting, pollution).

Quick check

Grinding wheat into flour is …

  1. a physical change that cannot be reversed
  2. a chemical change
  3. a reversible change
  4. not a change
Answer

a physical change that cannot be reversed. No new substance forms, but you cannot get the grains back.

Think about it

Ripening of fruit is desirable, but over-ripening is not. What does this tell you?

One answer

Whether a change is desirable depends on how far it goes and on our needs; the same change can be useful up to a point and harmful beyond it.

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.