Classification of Matter

Classification of Matter

Matter is anything that has mass and occupies space i.e everything seen or unseen.

On a broad basis, matter can be subdivided into two general categories: pure or distinct substances and impure substances (mixtures).

Pure substances are distinct substances having the following general properties:

i. All parts are the same throughout (homogeneous).

ii. Composition is definite.

iii. They cannot be easily separated by physical means.

iv. Separation by physical methods is not easy.

v. They have a unique set of physical properties (boiling point, melting point, density, physical state, spectroscopic properties, etc.).

Physical properties are, in fact, used as the criteria for determining the purity level of substances. Pure solid substances have sharp melting point or very narrow melting range (Example: a reasonable pure benzoic acid sample has a melting point of 122°C or a melting range 120°C to 122°C). However, impurities (contaminants) cause a depression in melting points and widening of the melting range of solid substances (Example: an impure sample of benzoic acid has a melting range 110°C to 118°C).

For liquid, the criterion for determining the purity level of samples is the boiling point or relative density. A reasonable pure liquid has a constant boiling point or a low boiling range (Example: 99°C or 98°C to 100°C for distilled water). Impurities cause an elevation of the boiling point and the widening of the boiling range of a hitherto pure liquid (Example: 99°C to 105°C for an impure water sample).

Pure substances are either elements or compounds.

State of Matter and Phase Changes

Chemist usually describe substances as matter. Each form of Matter is made up of aggregates of small particles called molecules. Molecules consist of one or more tiny particles called atoms. A molecules may be monoatomic (e.g. He, Ne, Ar) or diatomic (e.g. H2, O2, N2, Cl2) or polyatomic (e.g. O3, P4, S8). Hence:

Atoms >>> Molecules >>> Matter

Matter exists in three phases commonly referred to as the states of matter. These are gas, liquid and solid.

The particles (molecules) of a gas are very far apart and are held together by very weak attractive forces, those of a liquid are much more closely held together by stronger attractive forces, those of solid are most closely held together by very strong attractive forces.

The phase transfer processes are freezing, melting, boiling, condensation and sublimation.

Solid     >>>     Liquid     (Melting)

Liquid    >>>     Solid      (Freezing)

Liquid    >>>     Vapour    (Boiling)

Vapour   >>>     Liquid     (Condensation)

Solid     >>>     Vapour    (Sublimation).

We are going to learn more about matter in the next article