Gases may be distinguished from other forms of matter by their :

Gases are distinguished from other forms of matter primarily by their inability to form free surfaces. While both liquids and gases are fluids that can flow and take the shape of their container [t2, t3], liquids possess a definite volume and form a free surface when placed in a container due to cohesive forces [t8]. In contrast, gas particles move freely in all directions with negligible interparticle attractions [c2]. Consequently, gases lack a fixed volume and will expand indefinitely to fill all available space in a container, never forming a stable top surface [c3, t8]. Other options are not unique to gases: many liquids and solids are also colorless [t2], liquids also have the ability to flow [t3], and both liquids and gases (fluids) exert buoyant forces according to Archimedes' principle [t2]. Therefore, the lack of a free surface is the defining distinction.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 7: Particulate Nature of Matter > Safety first > p. 106
2. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 7: Particulate Nature of Matter > Prepare some questions based on your learnings so far ... > p. 115