NCERT Solutions Curiosity Chapter 7 In-text Questions — What Is Matter Composed of?
Book page 99 Updated on2026-09-05
Q1.
Where do these pebbles, stones, and sand come from?
Answer
They come from the rocks of the mountains.
In the mountains, rocks gradually break down due to erosion. The rivers flowing through those regions carry the eroded pieces along with them, and as they flow they keep breaking those pieces down further — into pebbles, then stones, then sand — and transport large quantities of them down to the plains. The pebbles you pick up on a riverbank or a beach are the small, rounded remains of rock that has travelled a long way.
Why it matters for this chapter: all of this is only physical change. A grain of sand is made of the same substance as the rock it broke off from — the breaking has changed the size of the pieces, not what they are made of. That is the very idea Activity 7.1 tests with a stick of chalk.
Q2.
Is this grain the smallest unit of a bigger rock or can these grains of sand and clay be broken down further?
Answer
It is not the smallest unit — a grain of sand or clay can be broken down further.
Just like a stick of chalk, each grain of sand is itself made up of a very large number of constituent particles. Grinding a grain only splits it into finer grains, each still made of the same enormous number of particles. If the grinding could be continued and continued, a stage would finally be reached where the pieces could not be broken down any further; those units — and only those — are the smallest units of the substance.
Keep the scale in mind: a speck of sand you can just see with a magnifying glass is still gigantic compared with a constituent particle. Constituent particles cannot be seen even through an ordinary microscope.
Q3.
Is every speck of this fine chalk powder still composed of the same substance, or has it changed into something else on breaking or grinding?
Answer
Every speck is still the same substance — chalk. Nothing new has been formed.
Look at the powder through a magnifying glass (Fig. 7.1d): each tiny grain is white, dusty and chalky, exactly like the stick you started with. Breaking and grinding only separate one group of chalk's constituent particles from another group. The particles themselves are untouched — they are not cut, changed or converted into anything else.
Why it happens: the properties of a substance come from the kind of particles it is made of. As long as the particles are unchanged, the substance is unchanged, however small you make the pieces. This is what makes grinding a physical change.
Q4.
Recall Curiosity, Grade 7 chapter ‘Changes Around Us: Physical and Chemical’— is grinding chalk a physical change or a chemical change?
Answer
It is a physical change.
A change is chemical only when a new substance with new properties is formed. Grinding chalk forms nothing new: the powder is chalk, it looks like chalk, it feels like chalk, and it behaves like chalk. The only thing that has changed is the size of each speck, which has become smaller and smaller.
Test to apply: ask yourself, "Is the material after the change the same material as before?" For grinding chalk the answer is yes, so the change is physical. Compare this with burning a candle or rusting iron, where the answer is no.