Zwolinski and Wilhoit defined, in 1972, "gross" and "net" values for heats of combustion. In the gross definition the products are the most stable compounds, e.g. H 2 O (l), Br 2 (l), I 2 (s) and H 2 SO 4 (l). In the net definition the products are the gases produced when the compound is burned in an open flame, e.g. H 2 O (g), Br 2 (g), I 2 (g) and SO 2 (g). In both definitions the products for C, F, Cl and N are CO 2 (g), HF (g), Cl 2 (g) and N 2 (g), respectively.