Which balanced equation represents a chemical change?
(1) h2o(l) + energy → h2o(g)
(2) 2h2o(l) + energy → 2h2(g) + o2(g)
(3) h2o(l) → h2o(s) + energy
(4) h2o(g) → h2o(l) + energy

The answer is B because energy was added to 2H2O in order to produce a chemical change of 2H2 and 02 seperately.

  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-