1.1 Liquids and Gases
This section describes liquids and gases, emphasizing behavior of the molecules. This knowledge is useful for understanding the observable characteristics of fluids. Liquids and gases differ because of forces between the molecules. As shown in the first row of Table 1.1, a liquid will take the shape of a container whereas a gas will expand to fill a closed container. The behavior of the liquid is produced by strong attractive force between the molecules. This strong attractive force also explains why the density of a liquid is much higher than the density of gas (see the fourth row). The attributes in Table 1.1 can be generalized by defining a gas and liquid based on the differences in the attractive forces between molecules. A gas is a phase of material in which molecules are widely spaced, molecules move about freely, and forces between molecules are minuscule, except during collisions. Alternatively, a liquid is a phase of material in which molecules are closely spaced, molecules move about freely, and there are strong attractive forces between molecules. Table 1.1 Attribute Typical Visualization Solid COMPARISO OF SOLIDS, LIQUIDS, A D GASES Liquid Gas
Macroscopic Description Mobility of Molecules
Solids hold their shape; no need for a container Molecules have low mobility because they are bound in a structure by strong intermolecular forces Often high; e.g., density of steel is 7700 kg/m3 Small—molecules are close together Produces deformation Produces deformation that may associate with volume change; can cause failure NA Difficult to compress; bulk modulus of steel is 160 × 109 Pa
Liquids take the shape of the container and will stay in open container Liquids typically flow easily even though there are strong intermolecular forces between molecules Medium; e.g., density of water is 1000 kg/m3 Small—molecules are held close together by intermolecular forces Produces flow Produces deformation associated with volume change High; decreases as temperature increases Difficult to compress; bulk modulus of liquid water is 2.2 × 109 Pa
Gases expand to fill a closed container Molecules move around freely with little interaction except during collisions; this is why gases expand to fill their container Small; e.g., density of air at sea level is 1.2 kg/m3 Large—on average, molecules are far apart Produces flow Produces deformation associated with volume change Low; increases as temperature increases Easy to compress; bulk modulus of a gas at room conditions is about
Typical Density Molecular Spacing Effect of Shear Stress Effect of ormal Stress
