Out of these three which particular are not in contact with each other: liquid, gas, solid

Gas particles aren't close to each other, liquids are kind of close, solids are very close to each other

Atoms in solid are very close(compact) to each other. Hence they have definite volume.

Atoms in liquid are a bit apart from each other. Hence they can take irregular shapes but their volume is constant.

Atoms in gas are far apart from each other which helps them to move freely between collisions. Gases expand quickly when released in surrounding and takes the shape of the container. It has infinite volume.

Hence gas is the correct answer.

Hope this helps!

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