2Al(NO3)3 + 3Na2S = Al2S3 + 6NaNO3
2Al(+3) + 6NO3(-) + 6Na(+) + 3S(-2) = Al2S3 + 6Na(+) + 6NO3(-)
2Al(+3) + 3S(-2) = Al2S3
Cr2(SO4)3 + 3Na2CO3 = Cr2(CO3)3 + 3Na2SO4
2Cr(+3) + 3SO4(-2) + 6Na(+) + 3CO3(-2) = Cr2(CO3)3 + 6Na(+) + 3SO4(-2)
2Cr(+3) + 3CO3(-2) = Cr2(CO3)3