NCERT Solutions for Class 7th Science Chapter 5 .1 A Substance May Change in Appearance but Remain the Same! — Activity 5.2: Let us create and discuss
Book page 595 Updated on2026-09-19
Q1.
Take a few sheets of paper and fold them to create new objects (Fig. 5.1). Do you get the same paper back when you unfold these objects?
Fig. 5.1, page 59 — objects made by folding sheets of paper (redrawn sketch of the textbook photograph).
Answer
Yes. When the boat, the aeroplane, the bird or the butterfly is unfolded, we get back the same sheet of paper — the same size, the same colour, the same weight. Only some crease lines remain.
Why it happens: Folding only changes the shape of the sheet. The material — paper — is untouched. Since no new substance is formed, this is a physical change, and because we can undo it, it is also a reversible change.
Did you know? This art of folding paper without cutting or pasting is called origami. Every origami model is a beautiful demonstration that a physical change can alter appearance completely without altering the material at all.
Q2.
Take a balloon and inflate it. Now, loosen your grip and let the air escape out. Do you get the uninflated balloon back?
Answer
Yes. As soon as the air rushes out, the rubber shrinks back and we get the same uninflated balloon in our hand.
Inflating: rubber sheet stretches → balloon becomes big
Letting the air out: rubber relaxes → balloon becomes small again
Material before = material after = rubber
Why it happens: Blowing air in only stretches the rubber and changes the balloon's size and shape. No new substance is made, so it is a physical change — and a reversible one.
Q3.
Take another balloon; inflate it and grip the opening tightly. Now, prick it with a pin. What happens? Will you be able to get the uninflated balloon back?
Answer
The balloon bursts with a loud bang and tears into pieces of rubber. No — we cannot get the whole uninflated balloon back.
Why it happens: The stretched rubber is under great strain. The pin makes a tiny cut, the tear runs across the whole sheet in an instant, and the balloon splits. Even so, the pieces are still rubber — nothing new has been made. So bursting a balloon is still a physical change, only it is an irreversible one.
Tip: Do not mix up “physical” with “reversible”. Bursting a balloon, crushing chalk and tearing paper are all physical changes that cannot be reversed. The test for a physical change is only this — no new substance is formed.
Q4.
Crush a small piece of chalk into powder. Can you get the chalk piece back from the powder?
Answer
No — the powder cannot be pressed back into the original stick of chalk. But the powder is still chalk: it is the same white colour, it still writes on the blackboard, and it still tastes and feels the same.
Why it happens: Crushing breaks one big piece into millions of tiny particles. Only the size has changed — the substance has not. So this is a physical change that cannot be reversed at home.
Try This: Rub the powder on the blackboard with your finger. It writes just as well as the stick did. That is the proof that no new substance was formed.
Q5.
Is there any similarity in the changes listed in A, B, and C?
Answer
Yes — in all three cases only the appearance changed, while the material stayed exactly the same.
Change
What changed
Material before
Material after
A. Folding paper into objects
Shape
Paper
Paper
B. Inflating / bursting a balloon
Size and shape
Rubber
Rubber
C. Crushing chalk
Size
Chalk
Chalk
Since no new substance is formed in any of them, all three are physical changes. This is exactly the definition the book gives: changes in which only physical properties like shape, size and state change are called physical changes.