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NCERT Solutions Curiosity Chapter 7 In-text Questions — Solid state

Book page 102 Updated on2026-09-05

Q1.
How are constituent particles held together in solids?
Answer

Very tightly. In a solid the particles are closely packed and the interparticle attractions are very strong.

These strong forces hold every particle in a fixed position. A particle can still move — but only to and fro about its own position, which is called vibration or oscillation. It cannot move past its neighbours and travel from one place to another.

That single fact explains everything you observe in Activity 7.3:

  • Each object has a definite shape, because every particle is anchored where it is.
  • Each object has a definite volume, because the particles are packed as closely as their attractions can hold them.
  • Hammering can dent or crack the object, but the solid never flows the way water does.
Q2.
In the solid state, is there any way to move these particles apart?
Answer

Yes — heat the solid. That is the only thing needed.

Heat supplied → thermal energy of particles rises
→ vibrations become more vigorous (Fig. 7.4b)
→ particles begin to leave their fixed positions
→ interparticle distance grows, so the attractions weaken
→ the solid becomes a liquid (Fig. 7.4c)

The temperature at which this happens is the melting point of that solid — more exactly, the minimum temperature at which a solid melts to become a liquid at atmospheric pressure.

Why heating is enough: the interparticle force weakens drastically for even a slight increase in distance. So once the vibrations are large enough to push the particles slightly apart, the attractions collapse quickly and the particles are free to move around.
What melting does not mean: the particles themselves do not melt, soften, swell or change in any way. Only their spacing and their motion change. Ice and water are made of exactly the same particles.
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