MATTER 



11 



a suspension. If, however, the solid is reduced to particles of the size 

 of molecules or of atoms, the mixture will become as clear as the liquid 

 itself and such a mixture is termed a solution. The liquid is called the 

 solvent and the dissolved solid the solute. In the same way one liquid 

 may be mixed with another hquid and form a suspension or a solution, 

 depending on the size of the particles. A suspension of minute globules of 

 one liquid in another is termed an emulsion. Gases, too, go into solution 

 and also form suspensions, but such suspensions do not persist. A gas 

 and a hquid may be shaken up together and a suspension created, the gas 

 being distributed through the liquid in the form of bubbles, but the gas 

 quickly escapes from the mixture except for what becomes dissolved. 

 Gases also escape from solution unless there is just as much gas over the 

 liquid as there is in an equal volume within it. This passage of gas either 



Fig. 3. — Diagram showing the diffusion of a substance as salt, sugar or copper sulphate 

 in a glass of water. A, when the substance is first put into the glass of water. B, a few 

 minutes later the particles are leaving the main mass of the substance and moving in all 

 directions through the water. C, later (an hour, more or less) all the particles of the mass 

 have separated and become evenly distributed throughout the water. 



into solution or out of it, depending on whether the gas pressure is greater 

 without or within, explains why animals take in oxygen and pass out 

 carbon dioxide. This exchange, which is called respiration, takes place 

 through extremely thin membranes which separate the air from the hquids 

 in the body and which allow the gases to pass through freely. Differ- 

 ences in gas pressure also account for the constant entrance of oxygen into 

 water to replace what aquatic animals have taken from it in respiration, 

 and the constant escape into the water and then into the air of the carbon 

 dioxide which they have produced. 



18. Ionization. — Whenever acids, bases, or salts go into solution in 

 water, there is a tendency for the molecules to separate into the compo- 

 nent atoms or into radicals, which are groups of atoms. The atoms or 

 radicals then exist free in the solution. These solutions conduct elec- 

 tricity and are known as electrolytes. Free atoms or radicals in such a 

 solution are found to carry minute electrical charges and are called ions. 



