The Uniqueness of Water

When a liquid flows, the molecules slide around and past each other. A liquid s viscosity, its resistance to flow, results from intermolecular attractions that impede this movement. Both gases and liquids flow, but liquid viscosities are much higher because the intermolecular forces operate over much shorter distances. 

Viscosity decreases with heating, as Table 12.4 shows for water. When molecules move faster at higher temperatures, they can overcome intermolecular forces more easily, so the resistance to flow decreases. Next time you put cooking oil in a pan, watch the oil flow more easily and spread out in a thin layer as you heat it. 

O—H bonds and two lone pairs, one H2O molecule can engage in as many as four H bonds to surrounding H2O molecules, which are arranged tetrahedrally. 

CHAPTER 12 Intermolecular Forces Liquids, Solids, and Phase Changes 

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As an example of the usefulness of molecular visualization, Figure 1 shows a 128-molecule sample of ordinary hexagonal ice. The molecules are drawn in the Space Filling model style, commonly known to provide a reasonable representation of the effective size of most molecules. Figure la shows the ideal lattice structure (T = O K). It can be seen that the structure is exceptionally open, with channels that permeate the entire lattice. Essentially, the picture provides a hands-on molecular illustration of the uniqueness of water (the density of the solid is so low that it actually floats on the liquid). 

To draw together various items of chemistry that reveal the uniqueness of water for metabolic processes. 

The Uniqueness of Water Solvent Properties Thermal Properties Surface Properties... 

Chapter 11 is a new chapter including material from Chapters 5, 15, and 16 of the previous edition. The focus is on water, its unique properties and its importance for life, acid rainfall and water pollution, and water purification. There is a significantly expanded discussion illustrating the uniqueness of water by comparing it with several compounds of comparable size and molecular weight. The issue of water shortage problems worldwide is discussed. 

To demonstrate the uniqueness of water, some of its properties are compared with those of other compounds. In Table 4.2, this is done for the melting and evaporation temperatures (T and T ) and the heats of melting and evaporation... 

Summarizing, it is concluded that the uniqueness of water (liquid) is characterized by the strong association of its molecules, which gives rise to an extraordinary high boiling temperature and heat capacity. In spite of that association, liquid water molecules have normal mobilities as is reflected in the viscosity and the relaxation time. Furthermore, water is a good solvent for ions and polar components, whereas apolar substances are poorly soluble in it, if soluble at all. 

The Uniqueness of Water 371 Solvent Properties of Water 371 Thermal Properties of Water 371 Surface Properties of Water 372... 

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