When is a color change a physical property? When is it a chemical property?

When the following equation is balanced using the smallest possible integers, what is the coefficent of oxygen gas? c7h16o(g) + o2(g) → co2(g) + h2o(g) -1 -5 -8 -16 -21

The answer for #3 is c but i don't know why

Which of the following pairs of elements belong to the same groupa. h and he b. li and bec. c and pb d. ga and ge