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CHEMICAL ENGINEERING
all the different methods available is beyond the scope of this book; selected methods
are given for the more commonly needed properties. The criterion used for selecting
a particular method for presentation in this chapter was to choose the most easily used,
simplest, method that had sufficient accuracy for general use. If highly accurate values are
required, then specialised texts on physical property estimation should be consulted; such
as those by: Reid et al. (1987), Poling et al. (2000), Bretsznajder (1971) and Sterbacek
et al. (1979), and AIChemE (1983) (1985).
A quick check on the probable accuracy of a particular method can be made by using
it to estimate the property for an analogous compound, for which experimental values are
available.
The techniques used for prediction are also useful for the correlation, and extrapolation
and interpolation, of experimental values.
Group contribution techniques are based on the concept that a particular physical
property of a compound can be considered to be made up of contributions from the
constituent atoms, groups, and bonds; the contributions being determined from experi-
mental data. They provide the designer with simple, convenient, methods for physical
property estimation; requiring only a knowledge of the structural formula of the compound.
Also useful, and convenient to use, are prediction methods based on the use of reduced
properties (corresponding states); providing that values for the critical properties are
available, or can be estimated with sufficient accuracy; see Sterbacek et al. (1979).

8.6. DENSITY
8.6.1. Liquids
Valuesforthedensityofpureliquidscanusuallybefoundinthehandbooks.Itshouldbenoted
that the density of most organic liquids, other than those containing a halogen or other “heavy
3
atom”, usually lies between 800 and 1000 kg/m . Liquid densities are given in Appendix C.
An approximate estimate of the density at the normal boiling point can be obtained
from the molar volume (see Table 8.6)
M
b D 8.1
V m
3
where b D density, kg/m ,
M D molecular mass,
3
V m D molar volume, m /kmol.
For mixtures, it is usually sufficient to take the specific volume of the components as
additive; even for non-ideal solutions, as is illustrated by Example 8.1.
The densities of many aqueous solutions are given by Perry et al. (1997).

Example 8.1
Ž
Calculate the density of a mixture of methanol and water at 20 C, composition 40 per cent
w/w methanol.
Ž
Density of water at 20 C 998.2 kg/m 3
Ž
Density of methanol at 20 C 791.2 kg/m 3