Viscosity, << vihs KOS uh tee, >> is a measure of the resistance of a fluid (liquid or gas) to flow. Fluids with high viscosity, such as molasses, flow more slowly than those with low viscosity, such as water.
For a fluid to flow, the mobile molecules that make up the fluid must move past one another. Two actions oppose this movement, giving rise to viscosity: (1) Molecules collide with one another, and (2) molecules are drawn toward one another due, for example, to attractive forces between opposite electric charges that they carry.
Changes in temperature affect the viscosity of both liquids and gases. The molecules of liquids are close together. As a result, the attractive forces between the molecules are strong. When a liquid is heated, its molecules move apart, reducing the attraction between them. Raising the temperature of a liquid thus decreases its viscosity.
In gases, however, molecules are widely separated. Viscosity therefore results chiefly from collisions. Heating a gas makes its molecules move more rapidly and collide more often. Hot gases are thus more viscous than cold ones.
A liquid may be made more viscous by dissolving polymers (long-chain molecules) in it. The polymers interfere with the movement of the liquid’s molecules past one another. The amount of interference increases with temperature. Polymers are added to motor oils to ensure that the oil will not be too thin when the engine is hot.
