Question

a gas fills completely he vessel in which it is kept

Answer 1

The molecules of the gas are present in a free state. There is more intercellular spaces between the molecules of the gas . There is less force of attraction between the molecules of the gas . Hence , a gas fills completely the vessel in which it is kept .
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Answer 2

The particles of gas move freely.
Also , the force of attraction between the particles is less.
The Gases move in all direction and they also do not have a fixed volume.
Hence , it gets filled completely in the vessel in which it is kept