For questions relating to compounds dissolved in water and resulting reactions.
Water is colloquially known as the "universal solvent"; its polar nature and high availability make it central to many key biological processes and "everyday" chemical reactions. As such, many reagents used in the laboratory, especially in elementary chemistry, are typically dissolved in some concentration in water.
This concentration is typically based on molarity; that is, the number of moles of solute dissolved in each liter of water. This makes stoichiometric calculations easier, as a reaction involving 3 atoms of one reactant and two of another can be converted to the number of liters of each aqueous solution with known molarity.