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

FIGURE 11.8

Boiling of a liquid

A liquid boils when the vapor pressure inside a bubble of vapor equals the external pressure. The temperature at which this occurs is called the boiling point.

11.2 Phase Transitions

TABLE 11.1 Melting Points and Boiling Points (at 1 atm) of

Several Substances

Melting Boiling Melting Boiling

Name Type of Solid* Point, _ C Point, _C

Neon, Ne Molecular _ -249 _ -246

Hydrogen sulfide, H2S Molecular _ -86 _ -61

Chloroform, CHCl3 Molecular _ -64 -62

Water, H2O Molecular 0 100

Acetic acid, HC2H3O2 Molecular 17 118

Mercury, Hg Metallic _ -39 357

Sodium, Na Metallic 98 883

Tungsten, W Metallic 3410 5660

Cesium chloride, CsCl Ionic 645 1290

Sodium chloride, NaCl Ionic 801 1413

Magnesium oxide, MgO Ionic 2800 3600

Quartz, SiO2 Covalent network 1610 2230

Diamond, C Covalent network 3550 4827

Types of solids are discussed in Section 11.6.

Denver, Colorado, at 1609 m above sea level), the boiling point of water is 95_C. The normal boiling point of a liquid is the boiling point at 1 atm. < The temperature at which a pure liquid changes to a crystalline solid, or freezes, is called the freezing point. The temperature at which a crystalline solid changes to a liquid, or melts, is called the melting point; it is identical to the freezing point. The melting or freezing occurs at the temperature where the liquid and solid are in dynamic equilibrium.

S olid liquid

Unlike boiling points, melting points are affected significantly only by large pressure changes. Both the melting point and the boiling point are characteristic physical properties of a substance and can be used to help identify it. Table 11.1 gives melting points and boiling points of several substances.

If you want to cook a stew in less time than is possible at atmospheric pressure, you can use a pressur cooker. This is especially valuable at high altitudes, where water boils at a lower temperature. The steam pressure inside the cooker is allowed to build up to a given value before being released by a valve. The stew then boils at a higher temperature and is cooked more quickly.

. Heat of Phase Transition

Any change of state involves the addition or removal of energy as heat to or from the substance. A simple experiment shows that this is the case. Suppose you add heat at a constant rate to a beaker containing ice at _20_C. In Figure 11.9, we have plotted the temperature of the different phases of water as heat is added. The temperature of the ice begins to rise from _20_C, as you would expect; the addition of heat normally raises the temperature of a substance. At 0_C, the ice begins to melt, so that you get a beaker of ice in water. Note the flat region in the curve, labeled ice and water. Why is this region flat? It means that heat is being added to the system without a change in temperature; the temperature remains at 0_C. This temperature, of course, is the melting point of ice. The heat being added is energy required to melt ice to water at the same temperature. The intermolecular forces binding water molecules to specific sites in the solid phase must be partially broken to allow water molecules the ability to slide over one another easily, as happens in the liquid state. Note the flat regions for each of the phase transitions. Because heat is being added at a constant rate, the length of each flat region is proportional to the heat of phase transition.