NCERT Solutions Curiosity Chapter 7 In-text Questions — How Does the Interparticle Spacing Differ in the Three States of Matter?

The level goes up first and then comes partly back down.

• A → B: adding two teaspoons of sugar raises the level, because the undissolved sugar is a solid lying at the bottom and occupies its own space in the vessel.
• B → C: after stirring dissolves the sugar, the level drops to some extent — C lies below B, though still above A.

This is the whole point of the activity: the volume of the solution is less than the sum of the volumes of the water and the sugar. That can only be true if there is some space already present between the water particles, and the particles of the dissolved sugar have gone into those spaces (Fig. 7.11).

No, the sand particles do not dissolve. Sand is a solid that does not dissolve in water. When it is added, the sand particles simply settle down at the bottom and occupy some space in the container.

Yes, the volume in the vessel does change — it increases, and stays increased. The sand keeps its own particles locked together in grains, so instead of slipping into the interparticle spaces of the water, each grain pushes the water aside and takes up room of its own. The total volume is therefore the volume of the water plus the volume of the sand.

Because the two solids differ in how strongly their own constituent particles are held together, compared with how hard the water particles can pull.

• In sugar, the interparticle attractions are weak enough for the moving water particles to pull the sugar particles out of the grain one by one. Freed, the sugar particles spread into the interparticle spaces of the water — the sugar dissolves.
• In sand, the constituent particles are held together so strongly that the water particles are unable to pull them out at all. Nothing is freed, so nothing spreads, and the sand stays as visible grains at the bottom. Substances like sand are insoluble in water.