[Chap. VII CELLS AS BIOLOGICAL UNITS 61 



one. But when matter is dissolved in water or when, as in jelly, it is 

 dispersed in water without dissolving, such strikingly new properties 

 become evident that we must recognize two additional states of matter: 

 solutions and colloids. 



Solutions. When a substance dissolves in water its particles become 

 subdivided and separated as molecules. The molecules of salts, acids, 

 and bases are to some extent further separated into ions. The resulting 

 solutions are either colorless or colored homogeneous liquids, and the 

 particles remain equally dispersed throughout the solution because of 

 their constant motion. For example, if a gram of table salt (sodium 

 chloride ) dissolves in a glass of water the solution remains colorless, but 

 if a similar amount of copper sulfate dissolves in the water the solution 

 is blue. 



When salts, acids, and bases have dispersed in water as ions, each ion 

 has a characteristic positive or negative electric charge. Many of the 

 reactions which occur in solution are the result of these electric charges : 

 particles with unlike charges attract each other and unite, and those 

 with similar charges repel each other. For instance, if one pours the 

 above solution of copper sulfate into the solution of sodium chloride, 

 the resultant solution will contain temporary molecules, or compounds, 

 of NaCl, Nai.S04, CuCL, and CUSO4; and also the free ions: Na+, CI", 

 Cu^^, S04~ ~ . . ., etc. The solution is a stable system in which one or 

 more substances become so finely divided in the form of molecules and 

 ions that it disperses among the molecules of the water throughout the 

 system. 



Colloidal systems. When substances are almost completely insoluble 

 in water it is possible by various means to subdivide them into very 

 small particles which when dispersed in water continue to remain sepa- 

 rated and distributed throughout the water for a long time. If grains 

 of sand or pellets of clay are dropped into a beaker of water, they 

 immediately fall through the water to the bottom of the beaker. If the 

 pellets are crushed and the beaker is vigorously shaken for a moment, 

 the smaller particles of clay are held in suspension in the water and it 

 may remain turbid for an indefinite time. Streams and ponds that do not 

 become clear on long standing are excellent examples of turbid suspen- 

 sions. Some of the suspended particles in a drop of turbid water may 

 be seen through a microscope. 



Turbid water due to fine clay therefore illustrates a suspension in 

 which solid particles are dispersed in water. Each particle is a cluster of 



