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In a pure gas, all the particles are the same. These may be either individual atoms (e.g. neon = Ne), molecules made from one type of atom (e.g. oxygen = O2), or molecules made of more than one type of atoms (e.g. carbon dioxide = CO2).
In a gas mixture, the particles are from various types: several pure gases are mixed up together (e.g. the air).
Unlike in liquids and solids, particles in gases are separated with a vast amount of space. This makes colorless gases invisible to humans.
A given quantity of a certain gas occupies the volume made available to it. If the volume becomes bigger, then the gas expands: the space between gas particles increases so that the same quantity of gas still occupies the whole bigger volume. Conversely, a gas can be compressed into a smaller volume so much more than liquids and solids.