Physics – 2.2.3 Melting, boiling and evaporation | e-Consult

The density of a substance is defined as mass per unit volume. In solids, the particles (atoms, ions, or molecules) are arranged in a fixed, ordered structure. This arrangement means that the particles are packed closely together, leaving little empty space. The strong intermolecular forces between the particles also contribute to this close packing.

In liquids, the particles are still close together, but they are not held in fixed positions. They can move around and slide past each other. This allows for more empty space between the particles compared to a solid. The particles in a liquid are not as tightly packed as in a solid.

Since a solid has a greater number of particles packed into the same volume compared to a liquid of the same substance, the mass per unit volume (density) is greater for the solid. Therefore, solids are generally more dense than liquids of the same substance. The arrangement and the strength of intermolecular forces are key factors in determining the density of a substance.