MATTER 11 



a suspension. If, however, the sohd 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 liquid and form a suspension or a solution, 

 depending on the size of the particles. A suspension 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 

 liquid 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 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 

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

 Differences 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. 



17. 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. 

 Those ions which are metallic in nature carry positive charges, and those 

 which are nonmetallic carry negative charges; they are termed, therefore, 

 positive or negative ions. Table salt (NaCl) in solution separates into 

 sodium (Na) and chlorine (CI) ions; sodium sulphate (Na2S04), into 

 Na and SO4 ions, SO4 being a radical. Na is a positive ion; CI and SO4 

 are negative. When, by evaporation of the solution or by precipitation, 

 the substance which is in solution is made to reappear again in solid form, 

 the ions combine, and the charges neutralize one another and disappear. 

 This separation of ions in solution is known as electrolysis, or dissociation; 

 different substances show great differences in the degree to which this 

 occurs. Sugar and other substances which are non-conductors do not 

 show much dissociation. Acids by dissociation produce H ions; bases, 

 OH ions. 



18. Colloids. — Many thin membranes occur in the bodies of animals 

 in which openings exist of very minute size; similar membranes can be 

 artificially produced. Whenever two different liquids are in contact 



