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Like melting, the process of dissolving is commonplace; yet, like melting,
it is quite remarkable. We are all familiar with the fact that a hard crystalline
solid like salt, when placed in contact with water, apparently disappears
in a short space of time; the crystalline structure breaks up and the atoms
enter into the water. How and why does this process occur? And how do the
atoms behave once they have dissolved? More puzzles soon arise when we
start to think more closely about dissolving and solutions. Salt, for example,
dissolves in water, but it will not dissolve in petrol; camphor (used in
moth balls) dissolves, however, quite easily in petrol but not in water.
While crystal like diamond and silicon will not dissolve in any liquid.
So what controls whether a solid dissolves and in what it dissolves?
Let's consider more carefully the example of salt dissolving in water.
We recall that salt (sodium chloride) has a simple crystal structure in
which positive sodium ions and negative chloride ions are stacked together
in a regular array. The electrical interactions between the
positive and negative ions means that they are strongly bound at their
sites in the crystal. To break up the crystal requires a large amount of
energy; or the energy of interaction between the ions must be replaced
by some other form of interaction. The latter is the key to understanding
what happens when the ions dissolve in water; the interaction between the
ions in the solid is replaced by the interaction between the ions and the
water molecules in the solution. Water is a polar liquid; the oxygen
atoms have a negative charge while the hydrogens have a positive charge.
When the sodium ion enters the liquid water, the water molecules cluster
round it so that the negative oxygens are next to the positive sodiums.
Similarly the water molecules group around the
chloride ions so that the positively charged hydrogens are directed toward
the negative charge of these ions. And it is these ion-water interactions
in the solution that replace the ion-ion interactions in the solid. The
ions can break away from their partners in the crystal because they have
found almost equally congenial ones in the solution. Such behavior is
quite general: high solubility requires that the interaction between the
atoms, ions or molecules in the dissolving solid be replaced by equivalent
interactions between these species and the molecules of the solvent (i.e.
the liquid into which the solid is dissolving). Normally, it still
costs some energy for a solid to dissolve in a solvent. But if this is
small enough, its effect can be outweighed by that of the increased disorder
of the solution compared with the solid. The process is driven by the entropy
of solution.
We are now in a position to understand an old chemical maxim: "Like
dissolves like.
Solids like salt which are built up from ions dissolve in polar solvents
like water in which the atoms have high charges, because the electrical
interactions between the ions and the solvent are needed to replace those
in the solid. In crystals built up out of molecules like camphor, the forces
are different; the molecules are bound in the crystal by weak 'van
der Waals" interactions. Similar forces predominate
between the molecule sin a solvent like petrol. So again the interactions
between the molecules in the solid can be nicely replaced by those between
the molecule and solvent and camphor dissolves in petrol.
Just as we saw that liquids, although disordered, have greater or lesser
degrees of structure, so do solutions. Indeed, this feature should be apparent
from our discussion of sodium and chloride ions in solution.
The structures of ions in solution may be very
well defined, as with the ions of, for example, the metallic elements iron
and nickel have a well
defined octahedron of water molecules around metal ions. Such hydrated
ions may also be found in solids. (Indeed, solids like copper sulfate
adsorb water from the atmosphere to form such species in the crystalline
state.)They are, however, an example of the general class of chemical species
known as "complex ions" involving metal ions and small molecules.
Much of what we have described so far relates to water as
a solvent. Water is, of course, the most widespread liquid on the surface
of our planet. And water is also an excellent solvent, as we have seen,
for polar solids. Polar solids include not only those that are constructed
from ions like sodium chloride, but those containing 'polar groups' like
OH and CO. So sugar molecules like glucose
and sucrose dissolve in water because the OH groups
of the molecule can interact strongly with those of the water molecules;
hydrogen bonding interaction
plays a major role here. Indeed, molecules or parts of molecules
(groups of atoms) can be classified as hydrophilic (water loving)
or hydrophobic (water hating) depending on whether they contain
polar groups. And the extent to which a molecule or group tends to either
of these extremes will, of course, strongly influence its behavior in
an aqueous environment (i.e. when surrounded by water). An important
class of molecules in this context are those present in soaps and detergents,
which have both hydrophilic and hydrophobic ends, leading to a range
of useful and remarkable properties. Of even
greater importance is the behavior, in the respect of biological molecules,
especially proteins, whose shape (and hence function) is to a large extent
controlled by the relative hydrophobicity of different parts of the molecule.
Solid Solutions
We do not normally think of solids like copper or silicon as being 'soluble'
(i.e. as being able to dissolve), because there are no common liquids
in which these solids will dissolve (although strong acids will "dissolve"
metals like iron, but this involves a chemical reaction). Remember that
for a solid to dissolve, the interaction between the atoms or molecules
in the solid must be replaced by comparable ones in the solution; and there
simply are not substances which are liquids at normal temperatures, in
which the atoms or molecules interact sufficiently strongly with those
of iron or silicon (without there being a chemical reaction). However,
it is possible for these solids to dissolve in other solids forming
solid solutions. So copper will dissolve in zinc to form an alloy
(simply a solution of one metal in another) known as brass. Like most alloys,
brass is crystalline, that is we have a regular arrangement of metal sites;
but some are occupied by copper and some by zinc atoms.
Alloy formation is very common, with other examples including pewter
(tin and zinc) and bronze (iron and copper). Dissolving a small amount
of one metal in another can also have drastic effects on physical and chemical
properties. Stainless steel is essentially iron into which a small amount
of chromium is dissolved, with drastic changes in the rate at which it
corrodes (i.e. reacts with oxygen in the atmosphere to form a metal
oxide). Alloying iron with copper in bronze results in a much tougher,
less brittle material.
Solid solutions are widespread. Silicon, as we have seen, will not dissolve
in any common liquid; but it will dissolve in germanium -a solid with the
same crystal structure - in much the same way that copper dissolves in
zinc. These solids further illustrate the point just made regarding alloys,
namely that dissolving small amounts of one solid substance in another
is a vitally important way of altering the properties of materials - and
one that is used on an enormous scale in contemporary technology. The classic
example is the semiconductor silicon: dissolving tiny amounts (less than
one part per million) of phosphorus has a drastic effect on its ability
to conduct electricity (making the material that is known as an 'n-type'
semiconductor); similar amounts of arsenic have equally large effects but
result in different electrical characteristics (the material becomes a
'p-type' semiconductor). Putting the two types of material together creates
the famous p/n junction which has 'rectifying action', that is, when included
in an electrical circuit, it allows electricity to flow only in one direction
- a vital feature of electrical circuitry. Silicon with tiny quantities
of deliberately introduced impurities is therefore the material basis of
the technology on which the modern electronics revolution is based.
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