In liquids, the atoms or molecules are not as tightly bound as in solids, and due to that, they have some freedom to move around. The liquid state is a condensed state of matter, because even in it, as in solids, the particles are linked (weakly) to each other. On the other hand, as liquids and gases share the ability to flow, they are both called fluids.
They still experience attractive forces but not as strong as solids. Besides, they also begin to experience some repulsive forces. Among the molecules of a fluid act attractive forces of an entity sufficient to prevent their separation, but insufficient to immobilize the molecules in fixed and defined positions (such as those of a crystal lattice typical of a solid).
In a liquid at rest the motion of the molecules slide over each other is continuous and is identified with the motion of thermal agitation. In such motion cavities with diameters of the order of the molecular dimensions are formed between the molecules for very short times, which immediately close again by reforming themselves elsewhere; consequently, the liquids generally have a density slightly lower than that of the solid and are not very compressible.
The cohesion forces acting between the molecules of the liquids are of different intensity and vary with temperature, giving correspondingly different viscosity values according to the liquid considered and according to the temperature. So, depending on the type of liquid, the molecules may encounter greater or lesser resistance to moving within the liquid; the higher the resistance they encounter, the higher the viscosity of the liquid.
Such forces can also be of a different nature: predominantly electrostatic nature as in molten salts, in practice constituted by ions of opposite charge, or of the type of so-called van der Waals forces that act among molecules electrically neutral or even (for example, in the case of water) due to the polar structure of the molecules of the liquid which entails the existence, within each molecule, of predominantly positive charge zones alongside other predominantly negative charges.
Properties of liquids
Thus, liquids have a definite volume (almost invariably for any value of pressure) but not a definite shape; they assume the shape of the container. Due to their flexibility, the liquids can be poured from one container to another. They do not occupy the entire volume of the respective container, whatever its shape may be, deforming and depositing itself in the lower part of the container itself, they always present a “free surface” of contact (or border) with the overlying atmosphere.
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, and plasma), and is the only state with a definite volume but no fixed shape.
Most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.
A molecule of a liquid that at a given instant is not immediately adjacent to the surface that separates it from the overlying atmosphere is subject to the attractive forces exerted by the other molecules that surround it in all directions. The molecules that at a given instant constitute the surface of the liquid are instead adjacent to a smaller number of molecules and those of them that at that moment have higher kinetic energy can pass from the liquid mass to the atmosphere above, transforming itself into vapor.
It follows that all liquids, even at temperatures well below their boiling point, have a more or less marked tendency to evaporate: this tendency increases with temperature because with this increases the kinetic energy of the molecules. From the steam which is thus formed above the liquid surface, if this is not continuously removed (for example by ventilating the surface itself), the molecules show a certain tendency to return to the liquid mass, to which the attractive forces recall it before described.
At each temperature, exist a balance in which it is matching the number of molecules of a liquid that, in a specific instant, evaporate and the number of molecules that simultaneously condense, i.e., returning from the gaseous to the liquid state. In these conditions, the vapor pressure of the liquid increases with the temperature in a logarithmic way (for what concerns the static and the dynamics of the liquids). Static liquids in uniform gravitational fields also exhibit the phenomenon of buoyancy.