In this lab, we've seen that the space between particles is a result of the battle between particles' intrinsic motion and the attractive and repulsive forces between them.
Particles in a liquid or solid phase are moving slowly enough so that the attractive forces are able to hold particles next to each other. If the particles are compressed any more, however, (like David squeezing the plunger of the water-filled syringe), the shorter distances between them cause the repulsive forces to take over. When this happens, there is no change in volume on a macroscopic scale.
Particles in a gas phase are moving too rapidly for the attractive forces to hold them together. An outside force (like Joana pushing on the plunger of the air-filled syringe) will squeeze them closer together, but when they get close enough the repulsive forces take over and the gas becomes harder to compress.