How gases behave

Squeeze a water-filled syringe and almost nothing happens. Squeeze an air-filled one and the plunger moves easily , that difference tells you everything about how gases work at the particle level.

According to the particle model of matter, a gas is mostly empty space. The tiny particles (atoms or molecules) that make it up are spread so far apart that the gaps between them dwarf the particles themselves. Think of it like a near-empty football stadium: a handful of people scattered across thousands of seats. Because the "seats" (empty space) vastly outnumber the "people" (particles), you can push them closer together without the particles themselves shrinking at all , you're just reducing the gaps.

Those particles are never sitting still. They travel in straight lines, bouncing off each other and off container walls in constant, random motion. The forces pulling gas particles toward each other are so weak they're almost irrelevant , in the simplified ideal gas model, we treat those forces as zero except at the moment of a collision, and we treat each particle's own volume as negligible compared to the space it moves through.

Because gas particles spread out to fill whatever space they're given, gases have no fixed shape and no fixed volume , they always match the shape and volume of their container. This also makes gases fluids: like liquids, they flow and move around obstacles. Unlike liquids, though, they don't just pool at the bottom , they fill the entire container. That's why you can pump gas through a pipe, propel it through a spray nozzle, or breathe it in and out of your lungs.

particle model of matter: A framework for understanding substances by treating them as collections of tiny moving particles , where the arrangement, spacing, and motion of those particles explain the physical properties we observe.