Properties of Liquids and Gases

Abstract

Liquids are distinguished from solids by the fact that their particles are readily displaced. Whereas forces of finite magnitude are required to deform a solid, no force at all is required to alter the shape of a liquid, provided only that sufficient time is allowed for the change of shape to take place. When the shape is altered quickly, liquids do display a resistance, but this vanishes very quickly after the motion is finished. This ability of liquids to oppose a change in shape is called viscosity. We will discuss viscosity in depth in Section 4.2. As well as the usual liquids that are easy to move, there are also very viscous liquids whose resistance to change of shape is considerable, but which vanishes again at rest. Starting out from the viscous state, all phase transitions to (amorphous) solid bodies are possible. Heated glass, for example, passes through all possible transitions; in asphalt and similar substances these transitions occur at normal temperatures. For example, depending on the temperature, if a barrel of asphalt is tipped over, the asphalt will flow out within a few days or weeks. The mass that flows out forms a flat cake. Although it continually flows, one can walk on it without making footprints. Footprints will be left, however, if one stands still for a longer time on the asphalt. Hammering on the asphalt causes the mass to shatter like glass. In the study of the equilibrium of liquids, we consider states of rest or sufficiently slow motion. The resistance to change of shape may then be set to zero, and we obtain a definition of the liquid state: In a liquid in equilibrium, all resistance to change of shape is equal to zero.