How determine when to use s, l, g, aq in chemical equations

Use them depending on what state of matter the element is in. E.g.

C + O2 -> CO2

Carbon is in a solid amd reacts with oxygen, a gas, and the product of the equation, which in this case is carbon dioxide is also a gas, therefore the equation would be

C(s) + O2(g) -> CO2(g)

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.