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

Elements

• 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

Compounds

• 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