Classification of Matter
all material things are made of matter, and chemists are profoundly interested
in the nature of material stuff.
The Four Aristotelian
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, here.
- Read more about
Elements in Carmen Giunta's Elements and Atoms: Case Studies
in the Development of Chemistry, here.
- Fathi Habashi of
Laval University discusses Zoroastra and The Theory of Four Elements,
here, and Cambodia's Four Elements,
The Year 1800: Organic &
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.
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,
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.
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.
understood that atom types could be classified and grouped into a
periodic table of chemical elements, and by 1900 the only non-radioactive
s, p or d-block element that remained to be discovered was rhenium,
- 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.
- 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.
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:
- gaseous, such
- liquid, such
- solid, such as
However, for classification
purposes it is useful to divide materials into:
- pure substances:
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
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
can all be regarded as solutions, and they can form in various
- A mixture of
two or more gases: air
- A gas dissolved
in a liquid: soda water
- A mixture of
two or more miscible liquids: water and methanol
- A solid fully
dissolved in a liquid: 1.0 molar NaCl (aq)
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.
In the natural world, nearly all matter is heterogeneous,
apart from air, fresh clear water and various minerals such as quartz, rock salt, sulfur etc.
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.
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
A farmer may
want to know the boron levels because boron 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, both under a microscope and over the area of the farm: boron levels will vary from field to field.
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 can be assumed to be be
to great lengths to transform heterogeneous matter into homogeneous matter. They grind
and sort, but the favoured methods are fractional distillation, dissolution, selective precipitation and filtration.
Colloids are defined
is a heterogeneous mixture composed of tiny particles suspended
in another material. The particles are larger than molecules but
less than 1 Ám 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)."
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
Many real world solid
materials are composites:
- Many inorganic
materials like rock are composite. Granite is a mixture of of
feldspar (65-90%) , quartz (10 to 60%) and biotite or mica (10
- 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
- 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,
|Pure Substances, as noted by John Dalton, have a fixed stoichiometric composition with simple ratios of atoms: 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.
It is often said that there are 92 naturally occurring 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
Note that this definition includes methane, CH4,
as a binary because it has only C-H bonds, 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
|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. Examples include: diamond, C, boron, B, silica, SiO2, gemstones, etc.
are (in the Drude model) a lattice of cations
immersed in a sea of mobile valence electrons that are 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. All the elements on the left hand side of the periodic table are metals. Indeed, as the pure elemental substance the majority of the elements present as metals.
|Alloys are "a partial or complete solid solution of one or more elements in a metallic matrix", from here. If
can only be determined if an alloy is heterogeneous, homogeneous or
stoichiometric by microscopic, physical and chemical examination. Read more a alloys elsewhere in the Chemogenesis web book, here.
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:
- 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 spontaneous-dipole/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 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 contribution.
(Many thanks to members of the ChemEd
list for the above points.)
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, as discussed here.
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
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:
ammonium nitrate [NH4]+[NO3]
hexaaquacopper(II) chloride [Cu(H2O)6]2+
are between ionic, molecular and network. Examples include metal oxides,
such as magnesium oxide and calcium oxide, as well as metal sulfides
and phosphides. This topic is discussed in detail here.
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.
consisting of long chains, such as low density polyethylene, are
essentially molecular and are often thermoplastic and melt on
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, as defined
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."
An interesting video of melting glass in a microwave oven. The secret is to make a spot red hot first:
Melt A Beer Bottle
Minerals As defined
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
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.
definition of mineral would/should encompass:
- crude oil
- fresh water
- sea water
Minerals are of crucial
important to chemists because ultimately all
chemical substances are obtained from biological or geological sources.
|Transient & Hypothesised
Atomic ions are
ions of single atoms:
K+, Ca2+, Cl, S2,
All ions require
a counter ion to maintain electrical neutrality.
Molecular and complex
ions are ionic compound entities:
All ions require
a counter ion to maintain electrical neutrality.
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
- 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.
The web based Chemical Thesaurus
reaction chemistry database, here,
holds information about "chemical entities", and their interactions
The database does not just
include "real" matter, as discussed above, but also generic
entities that are idealised objects that are representative of real
matter. This is because chemists
often discuss structure and reactivity in terms of hypothetical generic
entities and the idea is used throughout The
Chemical Thesaurus reaction chemistry database. Examples of generic
- 2° alcohol
- carboxylic acid
- p-block element
- electrofugal leaving
- oxidising agent
- spectator ion
The term "entity"
is used because it is inclusive and so can be used to group together "all
objects of chemical interest" including:
- Atoms, isotopes,
molecular substances & discrete molecules, photons, metals, alloys,
ionic salts, network materials,electrons, ions, radicals, reactive intermediates,
generic species such as nucleophile, and even specialist apparatus like
the Dean & Stark trap.
No other term is so
- The sodium ion,
Na+, is a chemical species but not a substance or a material.
- Diamond is a material
and it is a substance, but not a species.
- Aldehydes and nucleophiles
are hypothetical, generic objects.
- The Dean & Stark
trap is glassware.
Formally [ie built
into the database schema] The Chemical Thesaurus sub-classifies chemical
- Real, neutral, >99% pure substance or homogeneous mixture of substances of known
composition: sodium chloride, methane, iron, aluminium, 1.00 mol/L NaCl(aq),
- Real charged ion:
sodium ion Na+, chloride ion Cl, carbenium
- Real transient
species: methyl radical H3C, benzyne
- Real mineral of
approximate composition: apatite, calcite, gypsum
- Real biological
entity that may be of approximate composition: coconut oil, fumarate
- Real commercial
material of approximate composition: dynamite, glass
- Generic neutral
entity: aldehyde (generic), cyclic ester (generic), metallic alloy (generic)
- Generic charged
entity: acyl cation (generic), carbenium ion (generic), vinyl anion
- Reaction generic:
nucleophile (generic), [free] radical (generic)
What is Chemistry?
There is a "What
is Chemistry?" page that developed out of a 2005 workshop discussion,
Overview of the Chemogenesis Analysis
© Mark R. Leach 1999-
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