Molecules of water have billions of collisions per second.  These collisions occur in all directions, as seen in Brownian motion, the random lurching of a microscopic particle suspended in a liquid.  Pascal’s principle says that pressure applied to a confined fluid is transferred to every point in the fluid.  The pressure at a depth beneath the surface of a liquid due to its weight is P = density x g x depth.

The buoyant force is the upward force that a fluid exerts on an object either totally or partially immersed in it.  Archimedes’ principle states that the buoyant force on a floating or submerged object is equal to the weight of the fluid it displaces.  If the object’s density differs from that of the fluid, there is a difference in weight between the object and the fluid it displaces, and the object rises or sinks.

Friction retards the speed of the layer of liquid molecules next to a solid surface.  These immobile molecules slow the molecules next to them, and so on.  Liquid flows also slow due to viscosity, internal friction arising between the liquid’s own molecules.  Viscosity decreases as temperature rises because faster molecules slide past one another more easily.  Turbulence occurs because of relative motion between different parts of a stream, initiating small eddies in the liquid.

Liquids have surface tension because the net molecular attractions on surface molecules pull them into the liquid.  Cohesion is the attraction between molecules of the same type, while adhesion is the attraction between molecules of different types.  Strong adhesion can cause capillary action, the rising of a wetting liquid into a vertical space.

Whenever a liquid meets a gas, some of the bombarding gas molecules dissolve into the liquid and some of the liquid molecules evaporate into the gas.  At any given temperature and pressure, there is a level at which a liquid becomes saturated, which means that the maximum quantity of gas is dissolved in that liquid.  Earth’s lakes and oceans serve as storehouses for various gases, including oxygen, which fish and other aquatic life need to survive.