Q1.
Do gases also have a fixed volume?
Answer
No. A gas has no fixed volume and no fixed shape — it takes both from its container.
Activity 7.5 shows this directly. Smoke is collected in Gas Jar A only. When Gas Jar B is placed over it and the glass plate is removed, the smoke spreads on its own until it fills the entire space in Gas Jar B as well (Fig. 7.7d). Nothing pushed it there; it went by itself.
The reason is that in a gas the particles move freely in all directions and the interparticle attractions are negligible. Nothing holds the particles back, so they keep spreading until the whole available space is occupied.
The other half of the proof: Activity 7.6 shows the same thing from the opposite side. Trapped air in a syringe can be squeezed into a much smaller volume just by pushing the plunger. A quantity of gas that can be made to fill a larger space, and be squeezed into a smaller one, plainly has no volume of its own.
Fluids: because both liquids and gases flow and do not keep a fixed shape, they are grouped together as fluids, and this is what distinguishes them from solids.