Homogeneous Mixtures 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: 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:

• 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.

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.

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.

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^–

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.

• 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, as defined by Wikipedia:

"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."

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 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:

• crude oil
• natural gas
• fresh water
• sea water
• air

Minerals are of crucial important to chemists because – ultimately – all chemical substances are obtained from biological or geological sources.

Atomic ions are ions of single atoms:

Na⁺, K⁺, Ca²⁺, Cl^–, S^2–, 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.

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.

• 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.