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Elements and Compounds

Pure Substances
  • A pure substance is one which does not contain any impurities
  • It consists of only one kind of thing which may be atoms, molecules or ions
  • Pure substances are either elements or compounds but never mixtures
  • An element is a substance that cannot be separated into anything simpler by chemical means
  • It consists of atoms which must have the same number of protons and electrons even if they have different number of neutrons
  • Except for metallic mercury and non-metallic bromine which are liquids, most elements are solids and a few are gases
  • The gaseous elements include the noble gases which are mono-atomic, fluorine, chlorine, and oxygen gases which are diatomic, and tri-atomic ozone
  • Elements constitute the building units of all other substances
  • A compound is a new substance formed when two or more elements are chemically combined together.
  • A compound is always different in physical and chemical properties from the elements that make it up.
  • Every compound can be represented by a chemical formula which expresses the fixed mass ratio of the elements that make it up, for example, CO2 is a compound in which carbon and oxygen are in the ratio 3:8 by mass
  • The chemical formula of a compound always reflects the elements which make up the compound. For example:
    • Water (H2O) is made up of hydrogen and oxygen
    • Ethanol (C2H5OH) is made up of carbon, hydrogen, and oxygen
    • Limestone (CaCO3) is made up of calcium, carbon, and oxygen
Criteria of Purity
  • Melting point, boiling point, density, refractive are the criteria of purity
  • A pure solid (element or compound) has a definite melting point.
    • Its melting point is sharp
    • An impure substance melts at a lower temperature and over a range of temperatures, hence its melting point is not sharp
  • A pure liquid has a definite boiling point at a given pressure, usually atmospheric pressure
    • An impure liquid has a lower vapor pressure and hence a higher boiling point at atmospheric pressure than the pure liquid
  • Impurities in a substance makes the melting point lower than it should be and the boiling point higher than it should be
  • The presence of dissolved impurities in a liquid causes the vapor pressure of the liquid to decrease
  • A liquid boils only when its vapor pressure equals that of the external pressure which is usually atmospheric pressure
  • A lot of heat is therefore required to push the reduced pressure up to the value of the atmospheric pressure, this causes the boiling point of the impure liquid to increase