gas
/ GAS /
Open a bottle of perfume in one corner of a room and soon you can smell it everywhere. The scent does not stay where you poured it — it spreads until it fills the whole space. That eager spreading to fill any container is the hallmark of a gas.
In a gas the atoms or molecules are far apart and barely feel one another, so they fly in straight lines at high speed until they bounce off each other or the walls. Because there is so much empty space between them, a gas has no fixed shape and no fixed volume: it expands to fill its container and can be squeezed into a much smaller box. The pressure you feel from a gas is simply the drumming of countless tiny particles hitting a surface.
This matters because gases drive engines, fill our lungs, and make up the atmosphere and the stars' outer layers, all because they mix and expand so readily. A common misconception is that a gas is weightless or empty; in truth it has real mass and pushes back hard when compressed — a tank of compressed air is heavy and can be dangerously powerful.
Blow up a balloon and the air inside pushes evenly on every part of the rubber, holding it taut — that outward push is just gas molecules striking the inner wall billions of times a second.
A balloon stays inflated because gas molecules hammer the walls from inside.
Gas and vapour mean almost the same thing; vapour usually refers to the gas form of something that is liquid at room temperature, like water vapour, while gas is the general word.