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.
