Which of these is a pure substance: sodium metal, solid sodium chloride, sodium chloride dissolved in water?

Pure substance is

a) sodium metal : it an element and it is pure substance

b) NaCl : it is an ionic compound and it is also a pure substance

c) NaCl dissolved in water : it is a homogeneous mixture of NaCl in water and hence it is not a pure substance

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-