The Classification of Matter
By definition, all material things are made of matter, and chemists are profoundly
interested in the nature of material stuff.
The Four Aristotelian Elements
Western chemistry grew up around old alchemical ideas of Earth, Air, Fire, and
Water – the so called Aristotelian elements – a concept that originated with the
ancient Greeks and others,
The Year 1800: Organic & Inorganic Matter
In the year 1800 some 27 chemical elements were known:
Ideas had moved on from the four Aristotelian elements, and it was thought that
there were two distinct types of matter: organic and inorganic.
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Organic matter was associated with living things (biological origin: flora,
forna, food, us) and was assumed to possess a vital-force, an indefinable
characteristic that separated living organisms and materials derived from
living organisms from inanimate inorganic matter:
Inorganic matter is of geological origin: minerals, rock, sea, air
New Ideas: Urea and an Unanswerable Challenge the Vital-Force
Theory
In the nineteenth century chemical knowledge increased dramatically:
In 1804, Dalton proposed that matter was constructed from identical, indivisible
atoms which combined with each other in constant, or stoichiometric, proportions.
In 1828 the classical distinction between organic and inorganic matter was
resolved as evidence accumulated that organic materials could be synthesised in
the laboratory from inorganic, non-living sources.
The crucial step occurred when the German chemist Friedrich Wohler heated
ammonium cyanate, an inorganic salt, and produced the substance urea,
H2NCONH2, that was identical to organic urea isolated from the urine of animals,
and so was organic.
Wohler's synthesis of an organic chemical from inorganic starting materials was
an unanswerable challenge to the vital-force theory.
By the end of the nineteenth century chemical methodology had become very
sophisticated.
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Scientists understood that atom types could be classified and grouped into
a periodic table of chemical elements, and by 1900 the only non-
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radioactive s, p or d-block element that remained to be discovered was
rhenium, Re.
Common pharmaceutical preparations such as opium, tobacco and coca
were shown to have active ingredients that were discrete molecular
entities (morphine, nicotine & cocaine, respectively) that could be purified
to white crystalline materials (usually as the .HCl salt) of known chemical
composition.
Three points:
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Most materials obtained from nature, organic and inorganic, are
chemically complex, heterogeneous mixtures or composites.
It is now generally recognised that many classically defined inorganic
materials, such as limestone, coal and the oxygen in our atmosphere are
actually of biological origin, produced over geological time scales.
The modern chemical classification system says that to be "organic" a
substances possess carbon hydrogen (C-H) chemical bonds, and that
"inorganic" substances do not possess C-H bonds. Under this system,
oxygen is inorganic. And, ironically, so is urea, H2NCONH2.
The Chemical Classification of Matter
Many chemistry textbooks provide a diagram In their introductory sections
showing how matter can be classified into mixtures and pure substances, and
then to heterogeneous and homogeneous mixtures, elements and compounds:
Matter, the stuff from which our physical world is formed, presents to us as
various types of material. On a first analysis, the possible phases are:
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gaseous, such as air
liquid, such as water
solid, such as rock
However, for classification purposes it is useful to divide materials into:
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mixtures: variable composition
pure substances: stoichiometric composition
Physical techniques, such as: distillation, filtration, crush-&-sort, selective
dissolution, chromatography, etc., can be used to separate the individual
components of a mixture into chemically pure substances, and physical methods
such as turbulent mixing can be used to blend pure substances together into
mixtures.
The Chemical Classification of Matter: Updated
However, the above graphic is a little over simple for our purposes, and can be
usefully expanded to a classification system is both derived from and is
compatible with the classification system employed in The Chemical Thesaurus
reaction chemistry database:
Mixtures can be sub classified into four types: homogeneous, heterogeneous, colloidal and
composite.
Homogeneous Mixtures can all be regarded as solutions, and they can form in various ways:
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mixture of two or more gases
gases dissolved in liquids
mixture of two or more miscible liquids
solid fully dissolved in a liquid By definition, any region of a homogeneous solution will
be chemically identical to any other region so sampling is not an issue. A common way
to insure that a homogeneous mixture remains homogeneous is by turbulent mixing.
Heterogeneous Mixtures are agglomerates. In the natural world, nearly all matter is
heterogeneous, apart from air, fresh clear water and various minerals: quartz, rock salt, sulfur
etc.
However, scale is important: a 1.0 m3 sample of air will be homogeneous but the atmosphere
as a whole is heterogeneous. Poorly stirred solutions where there is chemistry occurring, even
simple heating, are liable to become heterogeneous.
Generally, chemists dislike heterogeneous mixtures and materials. This is because chemists
are interested in the composition of a particular piece of matter and how it behaves chemically.
But, by definition, the composition of a heterogeneous material varies from region to region,
where the distance between regions may range from microns to kilometres.
A farmer may want to know the boron levels because B is an important trace element for crop
growth. Somebody will have to take samples from all over the farm, perform chemical analysis
of all the samples and perform a statistical analysis of the data because the soil is
heterogeneous and will vary in boron levels from place to place. On the other hand, if the
farmer wants to know the pH of the swimming pool only a single sample is required because
the pool will be homogeneous.
Chemists go to great lengths to homogenise heterogeneous matter. They grind and sort, but
the favoured methods are dissolution, distillation and filtration.
Colloids are defined thus:
"A colloid is a heterogeneous mixture composed of tiny particles suspended in another
material. The particles are larger than molecules but less than 1 µm in diameter. Particles this
small do not settle out and pass right through filter paper. Milk is an example of a colloid. The
particles can be solid, tiny droplets of liquid, or tiny bubbles of gas; the suspending medium
can be a solid, liquid, or gas (although gas-gas colloids aren't possible)."
Colloids often appear to be homogeneous in bulk, but when are examined under a microscope
are observed to be heterogeneous. Chemists must treat colloids as heterogeneous and
process colloids to homogeneous before analysis.
Many real world solid materials are composites:
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Many inorganic materials like rock are composite. Granite is a mixture of of feldspar (6590%) , quartz (10 to 60%) and biotite or mica (10 to 15%).
Wood is an organic composite of consisting of cellulose and lignin.
Yeast in block form looks rather like a pure substance, but it is of course an
extraordinarily complex, living biomaterial.
Glass Reinforced Plastic, GRP, is a composite of glass fibre in a crosslinked polymer
resin.
Many industrial chemical products may have names that make then appear to be pure
substances, but are actually highly complex mixtures of: active ingredient, binder,
stabilisers, accelerators, lubricants, etc. For example, aspirin is a tablet consisting of
many components including the active ingredient acetylsalicylic acid, calcium
carbonate, magnesium stearate, etc., and these may change with time. Likewise,
dynamite is not a substance, but a mixture of nitroglycerine, kieselguhr (diatomaceous
earth), stabilisers, etc.
Pure Substances have a fixed, stoichiometric composition: Ne, NaCl, O2, S8, CO2, C6H12O6,
etc. Pure substances are also materials.
Elemental substances are collections of atoms with the same proton number. Most elements
consist of a mixture of isotopes. This is not usually an issue, however, isotopes can be
separated (enriched or depleted) in various ways.
It is difficult to say how exactly many elements there are because:
There are 81 non-radioactive elements.
All elements heavier than barium, Ba, atomic number 83, are radioactive, are technetium, 43,
and promethium, 61.
Some radioactive elements have isotopes with half lives close to a billion years, and these still
exist on Earth: 235U and 238U are well known examples. Others, atomic number 93 to 118 (but
not 117) must be prepared synthetically and may exist for microseconds or less.
There are 92 naturally occuring elements.
Binary compound substances consist of just two elements with the constraint [used here]
that there is just one type of strong bond present: metallic, ionic or covalent. This definition
includes methane, CH4, but not ethane, CH3CH3, or the other hydrocarbons which possess
both C-H and C-C bonds.
The Laing Tetrahedron of bonding and material type, discussed in detail here, appears on
this page because pure elements and substances consisting of only two elements but with only
one type of strong chemical bond – exhibit four extremes of material type: metals, ionic salts,
molecular substances, network covalent materials, or they are intermediate between these four
extremes.
Ternary and polyelemental compound substances include chloromethane, CH3Cl,
methanol, CH3OH, and glucose, C6H12O6. There substances have multiple types of chemical
bonds of varying polarity.
Chemical Substance Types
Network covalent materials have atoms arranged in an extended lattice of strong, "shared
electron pair" covalent bonds. Materials are generally hard, refractory solid substances. They
are poor electrical conductors, and they are not soluble in any solvent. Very high melting point
(>1500°C). Chemically intractable materials.
Metallic elements are modelled as a single type of atom arranged as a lattice of cations
immersed in a sea of mobile valence electrons delocalised over the entire crystal. Electrons
are the agents responsible for the conduction of electricity and heat. Metals have a
characteristic lustre, are often ductile and exhibit a huge range of melting points, from mercury,
-39°C, to tungsten at 3200°C.
Alloys are metallic materials consisting of a melt of two or more metals that is cooled to the
solid phase. If can only be determined if an alloy is heterogeneous, homogeneous or
stoichiometric by microscopic, physical and chemical examination, see here.
Molecular substances consist of discrete molecules. Materials held together internally by
strong intramolecular (within molecule) "shared electron pair" covalent bonds, but when
forming condensed solid or liquid phases, the molecules interact via weak intermolecular
(between molecule) van der Waals forces:
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There are several types of van der Waals attraction: dipole/dipole, dipole/induced-dipole
and spontaneous-dipole/induced-dipole. It is tempting to consider these forces to be of
different strengths, but it is the distance range that is more important. The spontaneousdipole/induced-dipole attractions – also known as London dispersion forces (LDF) – are
surprisingly strong but only act at very short range. (It is as if the surface of even
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neutral, non-polar molecules like methane are 'sticky'.)
All molecules have London dispersion forces and the strength increases with the
size/surface area of the molecule. This logic is used to explains the increasing boiling
and sublimation temperatures of the halogens: F2 < Cl2 < Br2 I2.
In addition, some molecules have dipole-dipole, hydrogen bonding, etc., which increase
the total amount of interaction between the molecules. Consider iodine chloride, ICl and
bromine, Br2. Both are 70-electron systems, but ICl is polar and Br2 is non-polar, yet
they have rather similar boiling points of 97° and 59° respectively, showing that the
dipole/dipole attraction makes only a minor contributionMolecular materials may also be
hydrogen bonded, where a hydrogen bond involves a proton being shared between two
Lewis bases, usually with oxygen, nitrogen or fluorine atomic centres.
Molecular materials exhibit a vast array of properties, but they are generally mechanically
weak, have low electrical conductivity, have low melting and boiling points, and/or a
susceptibility to sublime. Molecular materials usually soluble in (or miscible with) non-polar
solvents. Hydrogen bonded molecular solids are often soluble in water.
Simple ionic salts, like sodium chloride, Na+ Cl–, have a crystal lattice with anions
electrostatically attracted to adjacent cations and cations electrostatically attracted to adjacent
anions. Simple ionic materials are insulators as solids, but are electrical conductors when
molten and when dissolved in aqueous solution. Simple ionic materials may be soluble in
water (and sometimes in dipolar aprotic solvents such as DMSO), but they are insoluble in
non-polar solvents like hexane. Simple ionic materials have moderately high melting points,
usually 300-1000°C.
Molecular and complex salts have a crystal lattice anions and cations electrostatically
attracted to each other, but the cations and anions are compound entities. Some properties of
molecular and complex salts are dominated by the ionic nature of the material. For example,
substances are more soluble in water than organic solvents, indeed, many complex ions are
only stable in aqueous solution. Other properties are dominated by the molecular nature of the
ions. For example, melting points tend to be low or substances decompose on heating.
Solubility is often pH dependent. Examples include:
sodium acetate Na+ CH3COO–
ammonium nitrate [NH4]+[NO3]–
hexaaquacopper(II) chloride [Cu(H2O)6]2+ 2Cl–
Intermediate materials are between ionic, molecular and network. Examples include metal
oxides, such as magnesium oxide and calcium oxide, as well as metal sulfides and
phosphides.
Polymers consist of a large number of identical monomer components linked together in a
chain, and there maybe cross linking between chains. Properties such as melting point and
crystallinity are determined more by chain length and the degree of cross linking than by the
nature of the monomer entities or their bonding.
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Polymers consisting of long chains, such as low density polyethylene, are essentially
molecular and are often thermoplastic and melt on heating.
Extensively crosslinked polymers, such as the and melamine-formaldehyde are network
covalent materials that do not melt. Light fittings and electrical plugs are normally made
from such polymers.
Glass,
"A uniform amorphous solid material, usually produced when a suitably viscous molten
material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form."
Minerals As defined by Wikipedia:
"Minerals are natural compounds formed through geological processes. The term
mineral encompasses not only the material's chemical composition but also the mineral
structures. Minerals range in composition from pure elements and simple salts to very
complex silicates with thousands of known forms. The study of minerals is called
mineralogy. "
A slightly wider definition could/should read:
"Minerals are natural materials formed through geological processes."
Not all minerals are chemical compounds, as a chemist understands the term. Brimstone
is very pure elemental sulfur, S8. Very few minerals are able to pass the chemist's pure
substance of uniform composition test as most are mixtures and/or vary in composition
between geographic location.
This wider definition of mineral would/should encompass:
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crude oil
natural gas
fresh water
sea water
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air
Minerals are of crucial important to chemists because – ultimately – all chemical
substances are obtained from biological or geological sources.
Transient & Hypothesised Entity Types
Atomic ions are ions of single atoms:
Na+, K+, Ca2+, Cl–, S2–, etc.
All ions require a counter ion to maintain electrical neutrality.
Molecular and complex ions are ionic compound entities:
CH3COO–
[NH4]+
[Cu(H2O)6]2+
All ions require a counter ion to maintain electrical neutrality.
Free radicals, or simply radicals, are neutral molecular species with a single unpaired
electron in their valence shell. Radicals are discussed in more detail here.
Excited state species are transient atomic or molecular entities formed by moving a
ground state electron to a higher energy orbital. The behaviour of the excited state
species will be very different to the ground state species.
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Excited state sodium atoms emit light of precise wavelength, here.
Ground state singlet oxygen has a different spectrum of reactivity compared with
excited state triplet oxygen, here.