What does "conservation" mean?
Well, in terms of mass, if I start with 10*g of reactant FROM ALL SOURCES, at most I can get 10*g of product. In practice, I am not even going to get that, because losses invariably occur on handling, and some mass is lost. And thus for a combustion reaction:
C(s) + O_2(g) rarr CO_2(g)uarr
If I start with 12*g of carbon or coke, and burn it, I can get 44*g of carbon dioxide as a maximum. How is mass conserved here?
Moreover, this combustion reaction, has a certain latent heat, an enthalpy value: when strong C-O or C=O bonds are formed, energy is released to the environment. So I can modify the given equation to reflect this energy transfer:
C(s) + O_2(g) rarr CO_2(g)uarr +Delta
Where Delta represents the energy released; this is certainly quantifiable, and the energy released depends on the mass of carbon dioxide formed.
And likewise for a change of state:
H_2O(g) rarr H_2O(l) + Delta
When steam, H_2O(g) condenses, a certain amount (and certainly quantifiable!) of energy is released. This is the same amount of energy used to vaporize the water to form an equivalent quantity of steam:
H_2O(l) + Deltararr H_2O(s)
When we tabulate these values of "latent heat", of "enthalpy", typically we represent energy as a "reactant" as a positive quantity. Accordingly, energy released by the system, is treated as a negative quantity. All KNOWN chemical reactions conform to the laws of "(i) conservation of energy", and "(ii) conservation of mass".
See here for more of the same with respect to conservation of mass.
