Q1.
What do you think about the interparticle spacing in solids?
Answer
It is the smallest of the three states — but it is not zero.
The constituent particles of a solid are held together by strong forces of attraction, so they do not move from one place to another and are closely packed. Even so, despite that close packing, some space is left between the particles, as Fig. 7.12a shows.
The correction the chapter makes: you might assume that this space is filled with air. It is not. The interparticle spaces contain nothing at all. Air is itself matter made of particles; if air were inside the gaps, those particles would need gaps of their own, and the question would never end.
Why a solid is still practically incompressible: the space is there, but it is far too little, and the attractions holding the particles in place are far too strong, for any ordinary push to close it up. Contrast this with the syringe full of air in Activity 7.6.