CHAPTER IIITHE THEORY OF IONIZATIONA substance dissolved in a liquid is in a state that resembles the
gaseous state in many respects. It is thoroughly dispersed, and
just as a gas expands into any available space, so a dissolved sub-
stance or solute expands or diffuses until its concentration is uni-
form throughout the solution. It is its tendency to expand which
makes a gas exert pressure. The similar tendency of a solute to
expand throughout the liquid causes osmotic pressure, which is
very similar to gas pressure and has approximately the same mag-
nitude as the pressure of an equal number of moles of a gas in the
same volume at the same temperature. The molecular weights
of gaseous substances can be determined when the weight, volume,
temperature, and pressure are known. Similarly the molecular
weight of a solute can be determined when its weight, volume of the
solution, temperature, and osmotic pressure are known. Although
osmotic pressure is difficult to measure there are other properties
of solutions, related to osmotic pressure, which are readily measured
and serve to determine the molecular weight of the solute.
Soluble substances fall into two classes: those that give solutions
which do not conduct electricity, called non-electrolytes; and those
that give solutions that do conduct electricity, called electrolytes.
In solution non-electrolytes behave normally, or in other words,
molecular weight methods show the same number of moles that one
would expect to find in the gaseous state of that substance if it
were volatile. Electrolytes, on the other hand, show a greater
number of moles than one would normally expect to find.
These facts are accounted for in the theory of ionization, ac-
cording to which electrolytes in solution are dissociated into ions.
These ions are the positively and negatively charged parts into
which the electrolytes dissociate. The individual ion has the same
effect as a complete unionized molecule in causing osmotic pres-
sure and related effects. Under electrical attraction the ions can
move through the solution towards the respective electrodes and
thus they can conduct a current.
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