TOPIC

Chemistry is the study of physical matter, which is classified in many different ways, such as state of matter (gas, liquid or solid), chemical form (element, mixture or compound), chemical structure (atoms or molecules, etc.) and so on.

It is important to understand the terms "element", "mixture" and "compound", because these convey useful information. Another useful term is the word "substance", which can be used to refer to either an element or a compound - but not to a mixture because a "substance" always has a definite composition. The most general term is "matter" which can be used to refer to any of "substances", "elements", "mixtures" or "compounds".

Elements, Compounds and Mixtures

Elements

Consist of only one type of atom - which may, or may not join together to form molecules or large structures, therefore:
Can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen)
Cannot be broken down into a simpler type of matter by either physical or chemical techniques - though some larger elements break-down spontaneously due to being radioactive

Many elements are found in nature and so may be called "naturally occurring elements". Other elements have not been found in nature but can be produced in the laboratory. A few more elements are thought to exist but are very rare and even if produced would only exist for a very short time because they are radioactive and would quickly decompose into other elements whose atoms are smaller.

Mixtures

Consist of two or more different elements and/or compounds - physically intermingled,
Can be separated into their constituent parts by physical means (e.g. distilation of liquids or seperating magnetic and non-magnetic solids using a magnet), and
Have many of the properties of their constituent parts (e.g. the element "oxygen" is part of the mixture "air" and some of the properties of air are due to the oxygen, albeit somewhat reduced compared with pure oxygen due to the presence of the other constituents of the mixture called "air").

There are many different types of mixtures, some of which have special names. These include:

Homogeneous Mixtures - in which the two or more substances that form the mixture are evenly distributed throughout the mixture, e.g. vinegar is a homogeneous mixture of ethanoic acid and water.
Heterogeneous Mixtures - in which the two or more substances that form the mixture are not evenly distributed throughout the mixture, e.g. oil and water.
Solutions - a special type of homogeneous mixture in which one substance (called the "solute") is dissolved in another substance (called the "solvent"), e.g. salt water is salt dissolved in water in such a way that the salt no-longer exists as solid particles within the water.
Suspensions - heterogeneous fluid mixtures containing solid particles large enough for sedimentation, which means that the particles (compare with the "solute" part of a solution) will eventually settle to the bottom of the container (unlike in the case of colloids, below), e.g. particles of sand in water.
Colloids - heterogeneous mixtures in which one substance is microscopically dispersed evenly throughout another substance (for comparison, the size of the particles of "solute" are greater in the case of a solution, but much smaller in the case of a suspension). There are many naturally occurring colloids, e.g. milk. They are also very important in biology and medicine.
Alloys - mixtures in which the main element (or elements) are metal(s). A more technical definition of an alloy is that it is a partial or complete solid solution of one or more elements in a metallic matrix. Familiar examples of alloys include bronze, brass and steel.

These different types of mixtures are interesting but may not be essential knowledge for GCSE Chemistry. The descriptions are non-technical (omitting details, e.g. particle sizes), but sufficient for comparison of these different types of mixtures.

Compounds

Consist of two or more atoms of different elements bound together chemically,
Can be broken down into a simpler type of matter (elements)
by chemical means; but not by physical means
Always contains the same ratio of component atoms.
Have properties different from their component elements (e.g. the compound water (H2O) is a liquid at room temperature and pressure and has different chemical properties from those of the two elements, hydrogen (H2) and oxygen (O2), from which it is formed).

A compound is a pure substance that consists of two or more elements chemically combined in a fixed proportion, that can be further subdivided into simpler substances by chemical (not physical) means only.
So, if a quantity of a material consists of atoms of two or more elements joined together and has the same ratio component atoms, then that material is a compound.

Solutions, Suspension and Colloids

Solution

A solution is a homogeneous mixture of two or more substances. A solution may exist in any phase.

A solution consists of a solute and a solvent. The solute is the substance that is dissolved in the solvent. The amount of solute that can be dissolved in solvent is called its solubility. For example, in a saline solution, salt is the solute dissolved in water as the solvent. For solutions with components in the same phase, the substances present in lower concentration are solutes, while the substance present in highest abundance is the solvent.

Using air as an example, oxygen and carbon dioxide gases are solutes, while nitrogen gas is the solvent.

Suspensions

A suspension is a heterogeneous mixture in which some of the particles settle out of the mixture upon standing. The particles in a suspension are far larger than those of a solution, so gravity is able to pull them down out of the dispersion medium (water). The diameter for the dispersed particles in a suspension, such as the sand in the suspension described above, is typically at least 1000 times greater than those in a solution. Unlike a solution, the dispersed particles can be separated from the dispersion medium by filtering. Suspensions are still considered heterogeneous because the different substances in the mixture will not remain uniformly dispersed if they are not actively being mixed.

In chemistry, a suspension is a heterogeneous mixture containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than one micrometer. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but get suspended throughout the bulk of the medium. Particles of suspension are visible to the naked eye. It is when particles are left floating around freely in a solvent. The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the suspended particles are smaller and do not settle.Colloids and suspensions are different from solutions, in which the dissolved substance (solute) does not exist as a solid, and solvent and solute are homogeneously mixed.

Colloids

A colloid is a heterogeneous mixture in which the dispersed particles are intermediate in size between those of a solution and a suspension. The particles are spread evenly throughout the dispersion medium, which can be a solid, liquid, or gas. Because the dispersed particles of a colloid are not as large as those of a suspension, they do not settle out upon standing.

In chemistry, a colloid is a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid; he term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word suspension is distinguished from colloids by larger particle size). Unlike a solution, whose solute and solvent constitute only one phase, a colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). To qualify as a colloid, the mixture must be one that does not settle or would take a very long time to settle appreciably.

The dispersed-phase particles have a diameter between approximately 1 and 1000 nanometers. Such particles are normally easily visible in an optical microscope, although at the smaller size range (r < 250 nm), an ultramicroscope or an electron microscope may be required. Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid.

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

Colloidal suspensions are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.