Fluid Density Changes Due to Temperature

Density, Volume and Temperature

• Temperature is a measure of how much heat a substance contains. It affects the fluid properties of density as the molecules of a substance absorb or lose heat energy. To calculate the density of a substance, divide its mass by its volume. When a fluid absorbs heat energy, its molecules vibrate faster and move apart, increasing the volume of the mass and decreasing the overall density of the substance.

Viscosity

• A fluid's internal resistance to flow is directly related to its temperature. A fluid generally becomes more dense and gains a thicker viscous consistency when its temperature drops because it loses energy. The molecules move more closely together and the ease of flow is reduced as the molecules become more cohesive. There are two kinds of viscosity: kinematic and dynamic. The numerical parameter, kinematic viscosity, is the ratio of a substance's dynamic viscosity to its density. Dynamic viscosity is the force per unit area required for the fluid to move on a horizontal plane

Pressure

• The density of a substance varies with temperature and pressure. When temperature increases, density decreases in most materials while the volume of the substance increases as the molecules absorb energy and move farther apart. As a measure of force per unit area, fluid pressure can be caused by gravity in a closed container. Fluids take on the shape of their container because the pressure applies in all directions.

States of Matter

• Pressure, density, temperature and the state of matter are all related. A higher temperature decreases the density of a fluid. Increasing the pressure has the opposite effect. The density, or molecular packing, of a substance changes radically as the substance changes from solid to liquid to gas. High temperatures raise the pressure on a fluid. Increased pressure and lowering the temperature make it possible to produce liquid nitrogen, for example. Water is the exception. It contains polar molecules that have a different charge on either end, and the like charges of the individual water molecules repel each other. As temperature increases, water molecules actively bounce around due to the increase in energy and their charge repulsion to make the water expand. The reaction occurs in reverse as thermal energy is reduced and the temperature drops. With most substances, density increases as the substance solidifies, but the charged molecules in water repel each other so that the crowding together of the molecules is not as close as with most substances. As a result, solid ice floats on water.