Theory:

Pure substances are those that are completely made up of one form of particle. Physical processes cannot distinguish pure substances from other forms of matter.
Example:
Salt, sugar, oxygen, copper, iron etc.,
Based on the kind of atom, we classify pure substances as
  • Elements
  • Compounds
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Compounds:
 
It's a form of matter created by combining two or more elements in a specific mass ratio. Chemical methods may be used to decompose it into its constituent components.

Water\(H_2O\), oxygen \(O_2\), nitrogen dioxide \(NO_2\), salt \(NaCl\), and so on.
 
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Properties of Compounds:
 
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  • Two or more elements are chemically mixed in a compound. 
  • The elements in a compound are present in a fixed mass ratio. The elements hydrogen and oxygen, for example, combine chemically in a fixed ratio to form the compound, water. This proportion will not change.
  • Physical methods cannot isolate the constituents of a compound. For example, filtration, cannot isolate sodium chloride.
  • The constituents of a compound lose their identities, i.e., A compound's properties vary from those of its constituent elements. Water \(H_2O\) is an example of a compound, as we chemically separate these elements; their properties differ from that of water.