Hinglish

Physical and Chemical Changes

Class 7 Science · Chapter 5: Changes Around Us: Physical and Chemical · NCERT Curiosity section 5.1–5.2

Learn

On a holiday morning Meher folds a paper boat, Ishaan blows up a balloon and Neel crushes a piece of chalk into powder for a rangoli. "We changed three things," says Ishaan. "But is the paper still paper and the chalk still chalk?" They check: yes, only the shape and size changed.

A change in which a substance changes its shape, size or state but no new substance is formed is a physical change. Melting ice, boiling water, drying clothes, cutting vegetables and dissolving sugar are all physical changes. Ice, water and steam are the same substance in three states.

Now Zoya blows gently through a straw into a glass of clear lime water. It slowly turns milky! Breath contains carbon dioxide, which reacts with calcium hydroxide (lime water) to form calcium carbonate, a white solid that does not dissolve. A new substance has formed: this is a chemical change.

calcium hydroxide + carbon dioxide → calcium carbonate + water

Kabir adds a spoon of baking soda to some vinegar. It fizzes! Passing the gas into lime water turns it milky, so the gas is carbon dioxide. Signs of a chemical change include gas bubbles, a change of colour or smell, heat or light given out, or a new solid forming. Making curd, cooking, ripening fruit and burning are chemical changes.

vinegar + baking soda → carbon dioxide + other substances

Explore: the lime water test.

  1. Test each gas and sort it.

Remember

  • Physical change: only shape, size or state changes; no new substance.
  • Chemical change: one or more new substances form; carbon dioxide turns lime water milky.

Quick check

Which of these is a chemical change?

  1. milk turning into curd
  2. ice melting
  3. folding paper
  4. dissolving sugar
Answer

milk turning into curd. Curd is a new substance with a new taste and smell.

Think about it

Is dissolving salt in water a physical or chemical change? How could you check?

One answer

Physical. If you evaporate the water, the same salt is left behind, so no new substance was formed.

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.