MEASUREMENT OF IONIZATION 97Such membranes do actually exist. Many animal membranes and
the cell walls of plants serve as more or less perfect semi-permeable
membranes. Many films of inorganic substance also can be pre-
pared which are permeable to water but not to dissolved substances.
Experiment 1 in the first part of this chapter illustrates quali-
tatively the effect of osmotic pressure. The very soluble salts
dropped into the sodium silicate solution at once begin to dissolve,
forming a layer of solution about the lumps. At the junction
between this layer and the sodium silicate solution a coherent film
of insoluble silicate of the metal is formed:
2Fe+++ 6C1"
3SiOr~ 6Na+
i
Fe 2 (Si0 3 )4This film is impermeable to salt molecules and ions and permeable
to water. The concentration of the nearly saturated salt within
is higher than that of the sodium silicate outside, and hence the
osmotic pressure from within is greater than that from without
and the film is forced to expand, allowing water to enter and dis-
solve more of the salt. The actual result is the same as if the
water were sucked in through the film. This creates an hydro-
static pressure which is, of course, uniform inside the sac and con-
sequently the film is expanded at the point where it is thinnest
which is at the top. Thus the osmotic pressure accounts for the
effect which we observed — the little colored sprouts shooting up
from the lumps of the salt towards the top of the solution.
- Find the osmotic pressure of 1 gram of sugar dissolved
in 10 grams of water at 0°. - — of 1 gram of urea, CO (NHs^, in 10 grams of water at
0°. - How many grams of sugar should be dissolved in 1000
grams of water to give an osmotic pressure of 1 atmosphere
at0°? - — to give the same pressure at 38°?
- What is the molecular weight of a substance, 5 grams
of which at 0° in 250 cc. of water has an osmotic pressure of
2.24 atmospheres?
Ionization. The electrical conductance of electrolytes is ex-
plained in terms of the ionic theory by the presence of independent