PROPERTIES OF MATTER 31 



ot the total water or its solvent properties are not much affected by 

 the dynamic association. Hence, "bound water" may not be as im- 

 portant as was once thought in explaining the winter hardiness of 

 certain plants or water retention by others under dry conditions. 



SOLIDS 



This third state of matter is represented by those rigid materials 

 whose subunits are ions or molecules. These subunits are characteris- 

 tically organized into fixed, regular arrangements called crystals in 

 which motion is confined largely to movements about average fixed 

 sites. Thus molecules (or ions) do not wander randomly but vibrate 

 about definite locations. Hence, materials in the solid state do not 

 flow readily and are relatively rigid, holding a position until broken. 



Phase Diagrams 



Even solids, however, possess a few molecules of stitficient energy 

 to escape from the surface as vapor and give rise to a vapor presstire 

 in the same way as that of liquids. Naturally the forces between the 

 molecules of a solid exceed those in liquids, thus reducing the rate 

 of escape and hence the vapor pressure. Indeed the vapor pressures 

 of many solids are exceedingly small, and the rate of evaporation is 

 negligible. Others do evaporate slowly, as illustrated by ice, which 

 passes into the gas phase on exposure to dry air. This transfer without 

 involving a liquid phase is called sublimation. It is the basis of the 

 use of moth balls and gives rise to the odors characteristic of so many 

 solids. 



Thus substances may exist in any of the three physical states of 

 matter, and any one state may be converted into either of the others. 

 The effects of temperature and pressure on these transitions are shown 

 in plots called phase diagrams. The diagram for water (Figure 2-4) 

 is taken as an illustration, being the most important biological case. 

 In this system of curves, two characteristic points appear. The critical 

 point represents the upper limit of distinction between liquid and 

 vapor, for above the temperature at this point the vapor cannot be 

 liquefied, no matter what pressure is applied. For water this point is 

 reached at 374 °C. The triple point represents the temperature and 

 pressure where the three states are in equilibritim with each other. 

 For water the point is at 0.0099°C. and a pressure of 4.58 mm. of mer- 

 cury. The cunes themselves indicate regions of stability between two 

 different states in contact with each other. From such a diagram it 



