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
