SOLVENTS AND DILUENTS 209
exert a partial pressure and contribute to the total pressure of the atmosphere they
pervade. When the vapor pressure in the air in a closed space above a liquid be-
comes a maximum, the air is saturated in respect to the liquid. When the tem-
perature of a liquid is raised, its vapor pressure increases and when it equals the
pressure of the atmosphere, the liquid boils. Evaporation rate is the rate at which
vapor evaporates freely and continuously at the surface of liquid in an open system
under a specified set of conditions. These conditions include surface exposed, rate
of air-flow over the surface, temperature, and other factors. Unlike vapor pres-
sures, evaporation rates are not readily capable of numerical expression but are
best shown by curves like those in figures I and 2. The simplest method of reaching
these determinations is gravimetric, the loss of weight of a fixed quantity of
solvent under fixed conditions being measured at regular intervals (see Gardner,
pp. 318—322). Evaporation rates of solvents are not in the same order as their
boiling points. Compounds that contain hydroxyl groups evaporate more slowly
than compounds of the same boiling point not containing hydroxyl; for example,
alcohols evaporate more slowly than their esters of higher boiling point (see
Hofmann, p. 135). Chemical structure and types of molecular aggregates are
influential.
When solvents are mixed together they behave somewhat inconsistently. If
they are members of a homologous series, as would be the case with toluene and
xylene, they evaporate at different rates simultaneously and do not affect each
other. Certain liquids, however, may form constant evaporating mixtures. The
solvent with the higher rate evaporates rapidly until, in a binary mixture, a
certain proportion of the two is reached,-and from that point on they evaporate
together at a constant rate. Such a mixture is apt to occur with dissimilar com-
pounds like that of an alcohol and a hydrocarbon. Hofmann gives the relative
proportions of certain constant evaporating mixtures as follows: benzene 47,
ethyl acetate 53; toluene 45, ethyl alcohol 55; there are also a few ternary mixtures
which strike a constant evaporating level. In the application of varnishes and
lacquers, rapidly evaporating solvents and diluents are not usually desired, for
they cause chilling at the surface with possible precipitation of moisture and a
blush in the film.
Solvents, flamxnability. When mixed with air, the vapors of most organic
solvents, diluents, and thinners are explosive. In general, the degree of flamma-
bility of hydrocarbons and compounds of carbon, hydrogen, and oxygen is in-
versely proportional to the boiling points and the vapor pressures. Those con-
taining sulphur are highly flammable; those with chlorine, much less. Unsaturated
compounds are generally more flammable than saturated compounds. The meas-
ure of flammability is the flash point—the lowest temperature at which a liquid
gives off enough vapor to be ignited. At this temperature the vapor molecules
are so concentrated that they can unite with oxygen, and combustion becomes
self-propellant. This point is determined by a special test and several types of