If you poured some water down a slope, it would flow freely and quickly, but what about if you repeated the process with honey? Can you imagine how honey would flow down a slope? Sticky oozy honey would probably move quite a bit slower than the water. This is because honey has a different viscosity1 to water.
Viscosity is the measure of resistance2 of a fluid to flow. A fluid that is highly viscous3 has a high resistance (like having more friction) and flows slower than a low-viscosity fluid. To think of viscosity in everyday terms, the easier a fluid moves, the lower the viscosity. Using our earlier example, which fluid has a greater viscosity? Honey would move slower than water, so honey would have a greater viscosity.
Pressure4 and temperature5 are also considered when talking about the viscosity of a liquid. If the viscosity does not change with pressure, we describe something as being a Newtonian fluid6 (named after Sir Isaac Newton). However, if the viscosity does change as stress7 or temperature changes, we describe something as being a non-Newtonian fluid.
Viscosity is also related to a fluid’s density8. Density describes the space (volume9) that is occupied by a substance’s particles and is measured in kilograms10 per cubic metre11 (kg/m3). Note that this does not describe a substance’s weight12.
Viscosity is usually measured in pascal-seconds (Pa-s), a measure of force13 equalling 1 kilogram14 per metre per second (kg/(m·s)).
While viscosity is most commonly associated with fluids, gases15 also have viscosities.
Related content
The Racing marbles activity compares the viscosity of common household substances by measuring how fast marbles move through the liquids.
The PLD webinar Chemistry made simple – properties of matter explores other properties of matter16.
- viscosity: The measure of resistance of a fluid to flow. Thick slow-flowing liquids have a high viscosity, thin fast-flowing liquids have a low viscosity.
- resistance: 1. The opposition to the flow of electric current through a circuit. 2. The ability to withstand harmful or unfavourable conditions, such as an infectious disease.
- viscous : Having a thick, sticky consistency between solid and liquid.
- pressure: The force per unit area that acts on the surface of an object.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- Newtonian fluid: A fluid that maintains constant flow rate regardless of the amount of stress applied.
- stress: In mechanics, a force applied to a body.
In ecology and physiology, a response to a stimulus, e.g. an environmental factor, that disturbs or interferes with the normal equilibrium. - density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- volume: 1. The quantity of space occupied by a liquid, solid or gas. Common units used to display volume include cubic metres, litres, millilitres, tablespoons and teaspoons. (Volume can refer to a large quanity of something). 2. The degree of sound intensity or audibility; loudness.
- kilogram: The base unit of mass in the International System of Units (SI).
- metre: The base unit of length in the International System of Units (SI).
- weight: Force due to gravity acting on an object, measured in newtons.
- force: A push or a pull that causes an object to change its shape, direction and/or motion.
- kilogram: The base unit of mass in the International System of Units (SI).
- gases: The state of matter distinguished from the solid and liquid states. Gases have the ability to diffuse readily and to become distributed uniformly throughout any container.
- matter: The basic structural component of all things that have mass and volume.