Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Viscosity

Viscosity is the resistance to flow that is exhibited by all liquids and gases. The viscosity of a liquid can be obtained by measuring the time it takes for a given quantity to flow through a capillary tube or, alternatively, by the time it takes for a steel ball of given radius to fall through a column of liquid (Figure 11.20). Even without such quantitative measurements, you know from experience that the viscosity of syrup is greater than that of water; syrup is a viscous liquid. Viscosity is an important characteristic of motor oils. In the United States, the Society of Automotive Engineers (SAE) has established numbers to indicate the viscosity of motor oils at a given temperature (0_F, or _18_C); higher numbers indicate greater viscosities at that temperature. The viscosity of a simple oil increases as the temperature decreases, so an SAE 40 oil that is appropriate for an auto engine in the summer would be too viscous in very cold weather. An oil designated as SAE 10W/40 is an oil mixture that behaves like an SAE 10 oil in the winter (W), but like an SAE 40 oil in the summer. Column 5 in Table 11.2 gives the viscosities (in SI units) of some liquids at 20_C.