This chapter discusses the physical properties of water, but by necessity, it contains an explanation of both atmospheric and hydrostatic pressure. The water molecule is dipolar—there are negatively and positively charged sites on opposite sides of the molecule. As a result, water molecules form hydrogen bonds with other water molecules. Hydrogen bonds are stronger than van der Waals attractions that occur among molecules, and impart unique physical properties to water. The dipolar molecule and hydrogen bonding greatly influence the density, vapor pressure, freezing point, boiling point, surface tension, phase changes, specific heat, dielectric constant, viscosity, cohesion, adhesion, and capillarity of water. Water has its maximum density at about 4 °C; this results in ice floating and the potential for thermal stratification of natural water bodies. The physical properties of water are of intrinsic interest, but some of these properties influence water quality as will be seen in other chapters.