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What is Viscosity?

Tricia Christensen
Updated May 21, 2024
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Viscosity is a scientific term that describes the resistance to flow of a fluid. The fluid can be a liquid or a gas, but the term is more commonly associated with liquids. As a simple example, syrup has a much higher viscosity than water: more force is required to move a spoon through a jar of syrup than in a jar of water because the syrup is more resistant to flowing around the spoon. This resistance is due to the friction produced by the fluid’s molecules and affects both the extent to which a fluid will oppose the movement of an object through it and the pressure required to make a fluid move through a tube or pipe. Viscosity is affected by a number of factors, including the size and shape of the molecules, the interactions between them, and temperature.


The viscosity of a liquid can be measured in a number of ways by devices called viscometers. These can either measure the time it takes for a fluid to move a particular distance through a tube or the time taken for an object with a given size and density to fall through the liquid of interest. The SI unit of measure for this is the pascal-second, with the pascal being the unit of pressure. This quality is therefore measured in terms of pressure and time, so that, under a given pressure, a viscous liquid will take more time to move a given distance than a less viscous one.

Factors Affecting Viscosity

As a rule, fluids with larger, more complex, molecules will have higher viscosities. This is particularly true for the long, chain-like molecules that are found in polymers and the heavier hydrocarbon compounds. These molecules tend to become entangled with one another, impeding their movement.

Another important factor is the way the molecules interact with one another. Polar compounds can form hydrogen bonds that link separate molecules together, increasing the overall resistance to flow and movement. Although water is a polar molecule, it has a low viscosity due to the fact that its molecules are small. The most viscous liquids tend to be those with long molecules that have noticeable polarity, such as glycerin and propylene glycol.

Temperature has a major effect on viscosity — so much so that measurements of this quality for fluids are always given with temperatures. In liquids, it decreases with temperature, as can be seen if syrup or honey is heated. This is because the molecules are moving about more, and therefore spend less time in contact with one another. In contrast, resistance to movement in gases increases with temperature. This is because, as the molecules move faster, there are more collisions between them, which reduces the ability to flow.

Importance for Industry

Crude oil is often piped long distances across regions with varying temperatures, and the rate of flow in response to pressure varies accordingly. Oil flowing through Alaska is more viscous than oil in pipelines at the Persian Gulf, due to differing ground temperatures, and consequently more pressure needs to be applied to keep it flowing. To address the issue of force needed to deliver oil through piping, sensors in some pipes measure the viscosity of the fluid and determine if greater or lesser pressure must be added to keep the flow of oil constant and steady.

Naturally, motor oil is also subject to changing viscosity when heated by an engine. Oil that becomes too thin from the engine’s heat will not work properly. To solve this problem, polymers are added to the oil to keep friction rates constant under higher temperatures.

Relevance to Volcanism

The viscosity of magma, or hot, molten rock under the Earth’s surface, is an important factor in the study of volcanoes. Runny lava tends to result in more frequent but less violent eruptions, as it flows easily up from magma chambers and out of the volcano. It also allows dissolved gas to bubble out more easily. Thicker magma tends to trap this gas at high pressure, and more force is required to eject the lava from the volcano, allowing great pressure to build up over time. When this type of volcano does erupt, it does so explosively, often with catastrophic consequences.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen , Writer
With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.

Discussion Comments

By anon926206 — On Jan 16, 2014

It's because the molecules are separated when heat is applied, so it is less viscous and runs smoother.

By anon295715 — On Oct 08, 2012

Does oil have a point where it will become a solid or not?

By anon293174 — On Sep 24, 2012

Why do different liquids flow more easily than others?

By anon272021 — On May 29, 2012

@talalwa1mtm: Please help me derive the analog of Hagen-Poiseuille equation for a power law fluid.

By anon171776 — On May 01, 2011

gasoline is a fluid.

By anon131521 — On Dec 02, 2010

what are some high viscosity and low viscosity fluids?

By anon79334 — On Apr 22, 2010

Thank you ever so much wisegeek!

By anon73889 — On Mar 29, 2010

Wow, whew! Thanks wise geek! This is what i need for my science fair!

By anon55951 — On Dec 10, 2009

you could look at the reynolds number and how it describes a fluid.

By anon48762 — On Oct 14, 2009

how come some liquids have more viscosity then other liquids? gerardo lopez

By zyros — On Jul 27, 2009

good explanation.

By anon38508 — On Jul 26, 2009

why does the viscosity of liquid decrease when temperature increases, whereas the viscosity of gas increases with temperature?

By talalwa1mtm — On Dec 26, 2008

derive the analog of Hagen-Poiseuille equation for ostwalde-de-waele model (power law)

By anon9819 — On Mar 14, 2008

What is the criterion of deciding whether the liquid has low, medium or high viscosity?

By anon3625 — On Sep 08, 2007

but why are some liquids more viscous than others is it because they have different solidifying points? or is it their atoms being closer. Or something completely different

Tricia Christensen

Tricia Christensen


With a Literature degree from Sonoma State University and years of experience as a All The Science contributor, Tricia...
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