for caco$3($s$)->$ ca $($s$)+$ co$2($g$),c_{in}=3-1-0=2,f=c-p+2=2-3+2=1$ why is it considered to have three phases when there are only soild and gas phase? does every solid substance in any reaction considered to be a different phase? what are other exceptions when calculating f and $c_{in}?$