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:
Gaseous, such as air
Liquid, such as water
Solid, such as rock
However, for classification purposes it is useful to divide materials into:
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.
Chemical Classification of Matter
Mixtures can be subclassified into four types: homogeneous, heterogeneous, colloidal and composite.
This can all be regarded as solutions, and they can form in various ways:
- 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. 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.
"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:
- 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 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 may seem strange to say there are "about 116 elements", but the number does change. Webelements appears to be the definitive information source.
Binary compound substances
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
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
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
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:
- 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.)
- Molecular 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
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
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. This topic is discussed in detail here.
Polymers consist of a large number of identical monomer components linked together in a chain, and there maybe crosslinking between chains. Properties such as melting point and crystallinity are determined more by chain length and the degree of crosslinking than by the nature of the monomer entities or their bonding.
- 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.
"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 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. "
"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. Most are mixtures and/or vary in composition between geographic location.This wider definition of mineral would/should encompass:
- crude oil
- natural gas
- fresh water
- sea water