Gas : A substance is said to be gaseous if it neither possesses a definite volume nor a definite shape. This is because they fill up the whole vessel in which they are put. For eg. - hydrogen, oxygen, carbon dioxide, nitrogen, etc. Molecules in the gaseous state have high kinetic energy due to which they can move freely because of the weaker forces of attraction amongst them. And that is why they move endlessly. For eg.- hydrogen, oxygen, carbon dioxide, nitrogen, etc.