When a soluble substance is dissolved in water, the solution is termed as "aqueous" and is represented by (aq) while writing a chemical equation. For eg- When an acid is diluted, its state is "aqueous" whereas if we take a concentrated acid, its state is liquid.
Water is denoted by (l) because is a liquid.
Another example, potassium permanganate is a dark purple soluble solid. For a chemical reaction, it is first dissolved in water and then used as this solution. While using it as a solution (i.e. in the dissolved state), its physical state is AQUEOUS.
