An Introduction to Environmental Chemistry

4.6 Formation of soils

So far we have discussed the mechanismsand solid productsof chemical weathering

without precise consideration of the environment in which these reactions occur.

While chemical attack of exposed bedrock surfaces can happen, most weathering

reactions occur in (or under) soils. We have already noted that the oxidation of

soil organic matter causes acidity of natural waters (Section 4.4.2), promoting

chemical weathering. This acknowledges the important role of soils in environ-

mental chemistry. So what exactly are soils? A glance at a dictionary suggests that

soils constitute the upper layer of the Earth’s continental crust in which plants

grow, usually consisting of disintegrated rock with admixture of organic remains.

Soil formation is influenced by geological (G), environmental (E) and biolo-

gical (B) factors such that the product, soil (S), is a function of all of these factors

with respect to time (t), i.e.:

eqn. 4.15

In fact a number of key factors can be identified in soil formation, including

parent material (p), climate (cl), relief (r), vegetation (v) and the influence of

organisms (o). Thus, equation 4.15 above can be more precisely written:

Spclrvod=ft(),,,, eqn. 4.16

SGEBd=ft(),,

The Chemistry of Continental Solids 93

Box 4.7 Van der Waals’ forces

Non-polar molecules have no permanent
dipole and cannot form normal bonds. The
non-polar noble gases, however, condense to
liquid and ultimately form solids if cooled
sufficiently. This suggests that some form of
intermolecular force holds the molecules
together in the liquid and solid state. The
amount of energy (Box 4.8) required to melt
solid xenon is 14.9 kJ mol-^1 , demonstrating
that cohesive forces operate between the
molecules.
Weak, short-range forces of attraction,
independent of normal bonding forces, are
known as van der Waals’ forces, after the
19th-century Dutch physicist. These forces
arise because, at any particular moment, the
electron cloud around a molecule is not
perfectly symmetrical. In other words, there
are more electrons (thus net negative charge)
on one side of a molecule than on the other,
generating an instantaneous electrical dipole.

This dipole induces dipoles in neighbouring

molecules, the negative pole of the original

molecule attracting the positive pole of the

neighbour. In this way, weak induced dipole-

induced dipole attractions exist between

molecules.

Induced dipoles continually arise and

disappear as a result of electron movement,

but the force between neighbouring dipoles

is always attractive. Thus, although the

average dipole on each molecule measured

over time is zero, the resultant forces

between molecules at any instant are not

zero.

As the size of molecules increase, so do

the number of constituent electrons. As

a result, larger molecules have stronger

induced dipole-induced dipole attractions. It

must be stressed, however, that van der

Waals’ forces are much weaker than both

covalent and ionic bonds.