The Liquid State
Polar liquids typically exhibit capillary action, the spontaneous rising of a liquid in a narrow tube. Two different types of forces are responsible for this property: cohesive forces, the intermolecular forces among the molecules of the liquid, and adhesive forces, the forces between the liquid molecules and their container. We have already seen how cohesive forces operate among polar molecules. Adhesive forces occur when a container is made of a substance that has polar bonds.
For example, a glass surface contains many oxygen atoms with partial negative charges that are attractive to the
positive end of a polar molecule such as water. This ability of water to “wet” glass makes it creep up the walls of the tube where the water surface touches the glass. This, however, tends to increase the surface area of the water, which is opposed by the cohesive forces that try to minimize the surface. Thus because water has both strong cohesive (intermolecular) forces and strong adhesive forces to glass, it “pulls itself” up a glass capillary tube (a tube with a small diameter) to a height where the weight of the
column of water just balances the water’s tendency to be attracted to the glass surface.
The concave shape of the meniscus (Fig. 10.7) shows that water’s adhesive forces toward the glass are stronger than its cohesive forces. A nonpolar liquid such as mercury (see Fig. 10.7) shows a convex meniscus. This behavior is characteristic of a liquid in
which the cohesive forces are stronger than the adhesive forces toward glass.
viscosity Another property of liquids strongly dependent on intermolecular forces is viscosity, a measure of a liquid’s resistance to flow. As might be expected, liquids with large intermolecular forces tend to be highly viscous. For example, glycerol, whose structure is has an unusually high viscosity due mainly to its high capacity to form hydrogen bonds using its OOH groups (see margin).
Molecular complexity also leads to higher viscosity because very large molecules can become entangled with each other. For example, gasoline, a nonviscous liquid, contains hydrocarbon molecules of the type CH3O(CH2)nOCH3, where n varies from
about 3 to 8. However, grease, which is very viscous, contains much larger hydrocarbon molecules in which n varies from 20 to 25.
Structural Model for LiquidsIn many respects, the development of a structural model for liquids presents greater challenges than the development of such a model for the other two states of matter. In the gaseous state, the particles are so far apart and are moving so rapidly that intermolecular forces are negligible under most circumstances. This means that we can use a relatively simple model for gases. In the solid state, although the intermolecular forces are large, the molecular motions are minimal, and fairly simple models are again possible. The liquid state, however, has both strong intermolecular forces and signifcant
molecular motions. Such a situation precludes the use of really simple models for liquids. Recent advances in spectroscopy, the study of the manner in which substances interact with electromagnetic radiation, make it possible to follow the very rapid
changes that occur in liquids. As a result, our models of liquids are becoming more accurate. As a starting point, a typical liquid might best be viewed as containing a large number of regions where the arrangements of the components are similar to those found in the solid, but with more disorder, and a smaller number of regions where holes are present. The situation is highly dynamic, with rapid fluctuations occurring in both types of regions.
Polar liquids typically exhibit capillary action, the spontaneous rising of a liquid in a narrow tube. Two different types of forces are responsible for this property: cohesive forces, the intermolecular forces among the molecules of the liquid, and adhesive forces, the forces between the liquid molecules and their container. We have already seen how cohesive forces operate among polar molecules. Adhesive forces occur when a container is made of a substance that has polar bonds.
For example, a glass surface contains many oxygen atoms with partial negative charges that are attractive to the
positive end of a polar molecule such as water. This ability of water to “wet” glass makes it creep up the walls of the tube where the water surface touches the glass. This, however, tends to increase the surface area of the water, which is opposed by the cohesive forces that try to minimize the surface. Thus because water has both strong cohesive (intermolecular) forces and strong adhesive forces to glass, it “pulls itself” up a glass capillary tube (a tube with a small diameter) to a height where the weight of the
column of water just balances the water’s tendency to be attracted to the glass surface.
The concave shape of the meniscus (Fig. 10.7) shows that water’s adhesive forces toward the glass are stronger than its cohesive forces. A nonpolar liquid such as mercury (see Fig. 10.7) shows a convex meniscus. This behavior is characteristic of a liquid in
which the cohesive forces are stronger than the adhesive forces toward glass.
viscosity Another property of liquids strongly dependent on intermolecular forces is viscosity, a measure of a liquid’s resistance to flow. As might be expected, liquids with large intermolecular forces tend to be highly viscous. For example, glycerol, whose structure is has an unusually high viscosity due mainly to its high capacity to form hydrogen bonds using its OOH groups (see margin).
Molecular complexity also leads to higher viscosity because very large molecules can become entangled with each other. For example, gasoline, a nonviscous liquid, contains hydrocarbon molecules of the type CH3O(CH2)nOCH3, where n varies from
about 3 to 8. However, grease, which is very viscous, contains much larger hydrocarbon molecules in which n varies from 20 to 25.
Structural Model for LiquidsIn many respects, the development of a structural model for liquids presents greater challenges than the development of such a model for the other two states of matter. In the gaseous state, the particles are so far apart and are moving so rapidly that intermolecular forces are negligible under most circumstances. This means that we can use a relatively simple model for gases. In the solid state, although the intermolecular forces are large, the molecular motions are minimal, and fairly simple models are again possible. The liquid state, however, has both strong intermolecular forces and signifcant
molecular motions. Such a situation precludes the use of really simple models for liquids. Recent advances in spectroscopy, the study of the manner in which substances interact with electromagnetic radiation, make it possible to follow the very rapid
changes that occur in liquids. As a result, our models of liquids are becoming more accurate. As a starting point, a typical liquid might best be viewed as containing a large number of regions where the arrangements of the components are similar to those found in the solid, but with more disorder, and a smaller number of regions where holes are present. The situation is highly dynamic, with rapid fluctuations occurring in both types of regions.
