For questions relating to compounds dissolved in water and resulting reactions and acid-base equilibria.
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 water.
In spite of simple formula, water displays highly complex behavior on molecular level - hydrogen bonding and autodissociation create various ionic clusters. Adding both ionic and molecular solutes further complicates attempts to characterise solutions, as water is a strongly solvating most of them, using a wide range of interactions.