All substances (solid, liquid, gaseous) are composed of molecules and atoms, which are in continuous motion.
In solids, the forces of mutual attraction between molecules are strong. All molecules are arranged in a certain order and make up a crystal lattice. The molecules are in vibrational motion around the equilibrium position and cannot leave it. Solids retain their shape and volume.
In a liquid state, the forces of mutual attraction between molecules are weaker than in solids. Molecules are in a state of oscillatory motion around an equilibrium position and can jump to an adjacent equilibrium position. This structure explains the fluidity of fluids. They retain their volume and do not retain their shape.
In gases, the forces of interaction between molecules are negligible. Molecules can move freely throughout the volume. Gases do not retain their shape and volume.
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