PROPERTIES OF MATTER 19 



or even to materials li(]ui(l at room tcmperatuie il (he actual tem- 

 perature of the system is sufFic icntly above the boiling point. For other 

 cases special equations have been developed, but these are more com- 

 plex and even they cannot be accurately applied to all gaseous sys- 

 tems. Hence the ideal gas law is by far the most widely applied expres- 

 sion. 



Errors in the use of the gas law appear to arise from attractions be- 

 tween the individual molecules and from the volumes of the mole- 

 cules themselves. Thus the errors diminish when the molecules are far 

 apart. In this case the fraction of the total volume occupied by the 

 molecules themselves is very small. In addition, the farther apart the 

 particles, the less they affect one another. Hence a decrease in pres- 

 sure improves the accuracy. Recall that increasing temperature in- 

 creases molecular velocity, which in turn helps overcome the forces of 

 attraction between molecules. This effect likewise improves the appli- 

 cation of the ideal gas law. 



Transport of Gases in Nature 



Living cells utilize various gases in their physiological processes. 

 Depending upon the cell and the biological reaction concerned, a 

 gas may be consumed or given off by a cell. Hence cells depend in part 

 for their livelihoods on direct or indirect contact with gaseous systems. 

 Therefore, some sort of gas transport is required, the nature of the 

 mechanism depending on the gas and the requirements and location of 

 the cell. 



Certain cells have a portion of their external sinfaces in contact with 

 a gas phase, frequently air. In this simplest situation gases are ex- 

 changed through the cell wall by diffusion, a process depending upon 

 the random motion of molecules. Diffusion may be illustrated by the 

 distribution rapidly occurring when a sample of gas is introduced into 

 a system. Samples taken from all parts of the system reveal the pres- 

 ence of the newly introduced gas. Even though another gas be already 

 present, the new sample distributes itself almost as though the system 

 was originally empty. 



When a porous barrier is interposed, gas molecules diffuse through 

 the pores to reach and fill that portion of the system on the other side 

 of the barrier. As soon as some molecules pass through the barrier, 

 their random movements cause a few to return. However, the number 

 moving through the barrier in a given direction is proportional to the 

 number of molecides on the side from which the movement took place. 

 Thus, with a higher concentration on one side of the porous barrier, 

 a net transfer of gas takes place and continues until concentrations be- 



