How do chemical equations work?

There is an old saying in banking: #"for every credit there must be"#
#"a corresponding debit."# What does this mean?

You do a job for someone, and he pays you the fee by cheque. You pay this into your account, as a credit item. For this credit, the bank makes a DEBIT item on your customer's account. Debit and credits match up, i.e their sum is ZERO and your account is balanced.

So how does this relate to chemistry? Well, most of the time (in fact all of the time) we start with a limited quantity of reactant. Take methane, which warms out homes, and for which we could write the balanced equation:

#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g) + Delta#

Now charge and mass are balanced in this equation. Are they? For every reactant particle, is there a corresponding product particle? If there are not you know it is not balanced and therefore NOT a valid representation of reality. And of course, these atoms represent massive quantities, i.e. #16 *g# of methane gas, and #64*g# dioxygen gas reactant, and INEVITABLY #80*g# of water and carbon dioxide product. Why #"inevitably?"#

You will note that I included the #Delta# symbol to represent the heat of the reaction. This #"latent heat of reaction"# is also conserved. When #C=O# bonds form, a given and measurable amount of heat is produced. This also follows strict laws of conservation.