Which of the following can be characterized as a physical change? A. a copper pipe rusts
B. milk goes sour
C. firewood burns
D. sweat evaporates from your skin

D. Sweat evaporates from your skin

Explanation:

A physical change is a reversible process, no new substance is formed. A physical change does not affect the chemical composition of a substance.

Sweat evaporation from the skin is a physical change, because the change that occur is merely a change of state, Liquid to Gaseous state, no new substance is formed.

  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-