### 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
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.

Properties of Compounds:

• 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.