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Some general practical chemical definitions:

  • An ATOM is the smallest part of a specific element. Atoms of the same element (e.g. sodium) are identical in physical and chemical properties and have same atomic number (the number of electrons or protons). An atom consists essentially of a nucleus (containing protons and neutrons) surrounded by electrons. The atomic mass of an atom is the number of protons and number of neutrons. The atomic mass used in calculations and given in the Periodic Table is a weighted average (as found in nature) of the atomic masses of the isotopes of the element.
  • A NEUTRON is an electrically neutral part of the nucleus of an atom.
  • A PROTON is an electrically positive part of the nucleus of an atom.
  • ISOTOPES of an element have same number of protons, but different number of neutrons, hence different atomic masses.
  • An ELEMENT is made of atoms; there are 103 (or more) elements known and these are listed in a chart (known as the Periodic Table), in order of their specific chemical and physical properties. Example: H, He, Li, Be, B,.............
  • The PERIODIC TABLE is a list or chart of all the known elements in numerical order according to their atomic numbers.
  • A MOLECULE is the smallest part of a compound and is formed by the combination of two or more elements. Example: a molecule of water is H2O.
  • A COMPOUND is made of elements in fixed proportions, e.g. NaCl, Na2CO3. There are billions of natural compounds and new synthetic compounds are being made every day.
  • HOMOGENEOUS matter is uniform throughout its composition with respect to its chemical and physical properties. Example: salt, sugar, flour, cement, etc.
  • HETEROGENEOUS matter is NOT uniform in composition but consists of two or more physically distinct properties. Each physically distinct homogeneous portion of a heterogeneous mixture is called a phase. Example: yoghurt containing fruit particles, pure orange juice with pulp, etc.
  • pH is an arbitrary unit of acidity or alkalinity on a scale of 0 to 14; where 7 to 0 is increasing in acidity (decreasing in alkalinity) and 7 to 14 is decreasing in acidity (increasing in alkalinity). By definition pH is the log of the hydrogen ion concentration: Neutral pH = 7.0 = 10-7.0 grams H+ per litre concentration.
  • ACIDS and BASES, two classes of chemical compounds that display generally opposite characteristics. Acids taste sour, turn litmus (a pink dye derived from lichens) red, and will react with metals to produce hydrogen gas. Bases taste bitter, turn litmus blue, and feel slippery to the touch. When aqueous (water) solutions of an acid and a base are combined, a neutralisation reaction occurs. This reaction is characteristically very rapid and generally produces water and a salt. For example, sulphuric acid (H2SO4), and sodium hydroxide (NaOH), yield water (H2O) and sodium sulphate (Na2SO4):
    H2SO4 + 2NaOH = 2H2O + Na2SO4 (A stoichiometric equation)
    1 molecule H2SO4 + 2 molecules NaOH = 2 molecules H2O + 1 molecule Na2SO4
  • LABORATORY - for definition and explanations of a LABORATORY, check out our Home page!
  • REAGENT is something (a chemical liquid or powder) that will react with something else (a chemical liquid, or powder) to produce a product.
  • MENISCUS is the curved surface of a liquid when the liquid is in a narrow tube such as a burette or pipette.
  • DETERMINATE ERRORS in measurements, are those that can be measured or determined, for example: burette error or calibration factor.
  • INDETERMINATE ERRORS in measurements, are those that can NOT be measured or determined, these are usually random errors.
  • VALENCY is the combining power of an atom or radical, equal to the number of hydrogen atoms that the atom could combine with or displace in a chemical compound (where hydrogen has a valency of 1); e.g. sulphuric acid H2SO4 has a valency of 2.
  • NORMALITY, MOLARITY, MOLALITY AND FORMALITY of standardized solutions:

    Concentration terms for laboratory reagent solutions:

    • NORMALITY = N = number of gram-equivalent weight of solute per 1 liter solution.

      Where gram-equivalent weight is the gram molecular weight of solute, divided by number valency of reaction.

    • MOLARITY or MOLAR = M = number of gram molecular weight (or moles) of solute per 1 liter solution.

      Note: M = moles per liter

    • MOLALITY = m = number of gram molecular weight (or moles) of solute per 1 kilogram solvent.
    • FORMALITY = f = number of gram formula weight of solute per 1 liter solution.
  • MOLE (mol) is the SI base unit and si defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
  • STOICHIOMETRY is the relative proportions in which elements form compounds, or ratios in which substances react with one another. Example: H2SO4 + 2NaOH = 2H2O + Na2SO4.
  • ALLOTROPES of an element are the different physical forms (crystalline or molecular) of that element; e.g. carbon has two allotropes, namely diamond and graphite.
  • STANDARD DEVIATION is the square root of the average of the squares of deviations about the mean of a set of data. Thus, Standard deviation (s) is a statistical measure of spread or variability of a set of data.
  • COEFFICIENT OF VARIANCE (of a mean) is a statistical representation of the precision of a test result. The function: standard deviation / mean x 100%.
  • QUALITATIVE ANALYSIS involves determining what elements or compounds are present in a substance.
  • QUANTITATIVE ANALYSIS involves determining the quantities (amount) of each element or compound present in a substance.
  • FUNCTIONAL GROUPS are the identifiable groups of atoms exhibiting characteristic properties or reactions of a compound. For example alcohols have the –OH group.
  • HOMOLOGOUS SERIES is a series (list) of related compounds that have the same functional group but differ in their formula by a fixed group of atoms. For example, the carboxylic acids: formic acid (HCOOH); acetic acid (CH3COOH); propionic acid (C2H5COOH).
  • ELECTROMOTIVE SERIES or ELECTROCHEMICAL SERIES is a series (list) of elements arranged in order of their electrode potentials (or ability to replace metals from one another when in their salt form). The series, in order, is K, Ca, Na, Mg, Al, Zn, Cd, Fe, Ni, Sn, Pb, H2, Cu, Hg, Ag, Pt and Au (where H2 is taken to be zero). Thus, a copper salt will replace a zinc metal.
  • DISTILLED WATER is pure water made by boiling the water and condensing the steam (vapour); this should preferably be done in an all-glass water still, otherwise a stainless steel water still. This water contains no dissolved gases but will absorb carbon dioxide from the atmosphere. However, this water could be high in silicates due to the glass apparatus. Sometimes the water is triple distilled to obtain an electrical conductivity of less than 1 microSiemen per centimetre.
  • DE-IONISED WATER is pure water made by passing water through ion-exchange resins to remove soluble inorganic salts; however it does not remove organic impurities and dissolved gases in the water. This type of water should not be used for microbiological tests. For ultra pure water, the water can be first distilled than passed through a de-ioniser, or alternatively use membrane filtration or reverse osmosis techniques.
  • KILOGRAM (kg) is the SI base unit of mass and is officially defined as being equal to the mass of the international prototype of the kilogram.
  • BRIX DEGREES is the measurement of per cent sucrose by mass in aqueous sugar solutions; sometimes instrument known as a Saccharometer
  • SALINOMETER or SALOMETER DEGREES is the measurement of the per cent salt NaCl in aqueous solutions at 20°C.
  • LACTOMETER DEGREES is the measurement of the density or SG of milk products.


The word CHEMICAL is, unfortunately, used in everyday language (e.g. the media) to describe a hazardous substance. This is scientifically incorrect as all matter is composed of chemicals; there are good chemicals such as fresh food, vitamins, medicine, water, etc., and there are bad chemicals such as cyanides, DDT, toxins, etc.

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