Q:

Liquid Versus Aqueous States?

I am studying ionic equations and have encountered the statement of the phase of matter in the equation since ionic substances tend to react differently based on their phase.
I know that
(l) state is when the substance itself is a liquid
(aq) state is when we put the substance in an aqueous solutions, most commonly water.
But I don't know
1) Why and when would you use one state over the other. (Answered)
2) How do you define the concentration when you say (aq) state? Is it predefined/can be computed based on the substance or reaction or is it irrelevant?

A:

2) How do you define the concentration when you say (aq) state? Is it predefined/can be computed based on the substance or reaction or is it irrelevant?

When a reaction equation is written down, one usually has a specific process (initial and final states including concentrations) in mind. When this information is not provided, and when it is not say the objective of a (e.g. textbook) question to identify the concentrations, it is usually safe (sometimes even necessary) to assume that the change is between ideal reference states.
Tabulated thermodynamic data such as heats of formation usually refer to ideal standard states, which in the case of a heat of formation of an aqueous solution corresponds to a pressure of 1 bar, and an ideal solution (with the solute having the same properties as in an infinitely dilute solution) at either 1 molal or 1 molar concentration of the solute.
So for instance the process (taken from another answer - thanks @Farooq)
$$\ce{HCl (g) -> H+ (aq) + Cl- (aq)}$$
refers to transfer of a gas molecule into a 1 M or 1 molal solution, if the process refers to standard states (as tabulated data usually does).
During the transfer, the properties of the bulk solvent do not change, and so the solvent does not appear explicitly in the equation. 
It can be hard to understand how a 1 molar or 1 molal solution can simultaneously have the properties of an infinitely dilute solution. That's because we extrapolate from the data for an infinitely dilute state to a finite (1 M or 1 molal) concentration, using Henry's or Raoult's law.

