Intermediate Topic 259 of 1102 States of Matter & Solutions
Viscosity
A liquid's resistance to flow, generally decreasing with rising temperature.
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In-depth explanation of Viscosity
Viscosity is a measure of a fluid's resistance to flow, arising from friction between its layers as they slide past one another. In liquids it is governed mainly by intermolecular forces and molecular size, so it is high for molecules with strong attractions, long chains or extensive hydrogen bonding. The viscosity of a liquid decreases with rising temperature because thermal energy helps molecules slip past their neighbours. In gases, by contrast, viscosity increases slightly with temperature because faster molecular collisions transfer more momentum between layers. Viscosity is measured in pascal seconds and is important in engineering oils and lubricants.
Going deeper: Viscosity
Key ideas: Units: pascal second, with 1 centipoise = 1 mPa s. Water is about 0.89 mPa s at 25 C, glycerol about 950, and air about 0.018. Liquid viscosity falls roughly exponentially with temperature, as in the Arrhenius form eta = A exp(Ea/RT). Poiseuille flow rate scales with the fourth power of tube radius.
Common mistakes and connections: Students believe viscosity is the same as density; oil is less dense than water but much more viscous. Another error is expecting gas viscosity to fall with temperature. It rises. Larger molecules and hydrogen bonding raise viscosity. Links to intermolecular forces, lubricant ratings such as SAE grades and flow in biological systems.
Visualization of Viscosity
Viscosity measures resistance to flow and reflects intermolecular forces; hydrogen-bonded glycerol is over 1000 times thicker than water. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Honey is far more viscous than water at room temperature, and warming it makes it flow noticeably more easily.
Frequently asked questions about Viscosity
What is Viscosity?
A liquid's resistance to flow, generally decreasing with rising temperature.
Is Viscosity a beginner, intermediate or advanced topic?
Viscosity is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Viscosity?
Study these earlier topics in States of Matter & Solutions first: Boiling Point, Normal Boiling Point, Clausius-Clapeyron Equation.
What comes after Viscosity?
The next topics in States of Matter & Solutions are Surface Tension and Capillary Action.
Can I practise Viscosity with quizzes?
Yes. The interactive lesson includes a self-check question on Viscosity, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Viscosity fits in States of Matter & Solutions
Viscosity is topic 40 of 109 in States of Matter & Solutions.