Water (H₂O) is a polar, protic solvent known for its high dielectric constant, hydrogen-bonding ability, and capacity to dissolve a wide range of polar and ionic substances. Often called the "universal solvent," it plays a fundamental role in both natural and laboratory systems.

In the lab, water is essential as a solvent, diluent, and cleaning agent. Deionized and ultrapure water are used in sensitive applications such as HPLC, LC-MS, and biological assays, while purified water supports reagent preparation, rinsing, and sterilization.