What Is Coefficient Of Viscosity Definition Formula SI Unit And Examples
Coefficient of Viscosity is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. This property plays a vital role in fluid mechanics, laboratory experiments, and real-life examples like water movement, lubricant design, and biological transport. Understanding the coefficient of viscosity helps students solve both simple and advanced questions in their exams.
What is Coefficient of Viscosity in Chemistry?
A coefficient of viscosity refers to a constant that measures a fluid’s resistance to flow due to internal friction between its layers. In chemistry, this concept describes how difficult it is for a liquid or gas to move, and is crucial in chapters covering fluid dynamics, mechanical properties of fluids, and surface properties. It is a foundational part of physical chemistry and physics syllabi, especially for competitive exams.
| Parameter | Value/Description |
|---|---|
| Definition | A measure of a fluid’s internal resistance to flow |
| Symbol | η (Greek letter eta) |
| SI Unit | Pascal second (Pa·s) or kg·m-1·s-1 |
| CGS Unit | Poise (P), 1 P = 0.1 Pa·s |
| Dimension | [M1 L-1 T-1] |
| Example Value (Water, 20°C) | 1.002 × 10-3 Pa·s |
Molecular Formula and Composition
The coefficient of viscosity is not a molecule but a physical property: its value is assigned to a specific fluid (like water or glycerine), depending on composition, temperature, and intermolecular forces under study in physical chemistry.
Preparation and Synthesis Methods
The coefficient of viscosity for a fluid is not synthesized, but measured. In laboratories, viscosity is determined using viscometers or by applying Newton's law of viscosity and Poiseuille’s equation. For example, water is passed through a capillary or between parallel plates, and the resistance to flow is measured to find η.
Physical Properties of Coefficient of Viscosity
Physical properties related to the coefficient of viscosity for common fluids are summarized below:
| Fluid | Coefficient of Viscosity (Pa·s) | Nature |
|---|---|---|
| Water (20°C) | ~1.002 × 10-3 | Low viscosity (flows easily) |
| Glycerine (20°C) | ~1.49 | High viscosity (flows slowly) |
| Air (20°C) | ~1.81 × 10-5 | Very low (almost negligible) |
Chemical Properties and Reactions
The coefficient of viscosity itself is not a chemical substance and does not undergo reactions; rather, it depends on physical interactions within fluids. Internal friction arises due to intermolecular attractive forces (in liquids) or collision and momentum transfer (in gases). Changes in solute, temperature, or phase alter this property.
Frequent Related Errors
- Confusing coefficient of viscosity (η) with dynamic or kinematic viscosity.
- Not writing the correct SI or CGS unit in exam answers.
- Confusing the formula η = F·d/(A·v) (where F is force, d is distance, A is area, v is relative velocity).
- Mixing up symbols: μ (mu) or η (eta).
- Ignoring effect of temperature on viscosity, especially in water and oil numericals.
Uses of Coefficient of Viscosity in Real Life
The coefficient of viscosity is widely used in real-life applications and industries, such as:
- Design of lubricants for engines and machinery
- Blood flow analysis in medical science
- Piping and transportation of fluids in chemical industries
- Formulation of cosmetics, paints, and food products
- Studying atmospheric phenomena (rain, air resistance)
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads should be familiar with coefficient of viscosity, as it often features in concept-testing numericals and MCQs. Key exam topics include its definition, dimensional formula, temperature dependence, and practical problem-solving involving viscometric calculations.
Relation with Other Chemistry Concepts
Coefficient of viscosity is closely related to topics such as viscosity and surface tension, physical properties of liquids, dimensional analysis, and cohesion/adhesion. Understanding these links builds strong conceptual bridges across physics and chemistry chapters.
Step-by-Step Reaction Example
1. To determine η experimentally, set up a viscometer or capillary tube, using water as the sample.2. Measure the flow rate (Q), pressure difference (P), length of tube (l), and radius (r).
3. Apply Poiseuille’s Law: η = (π·P·r⁴) / (8·Q·l).
4. Substitute measured values to calculate η for water or other liquids.
Lab or Experimental Tips
Remember coefficient of viscosity by the rule: “higher η means slower flow.” In Vedantu live classes, educators use analogies like pouring honey versus pouring water to visualize viscosity differences. Always record temperature carefully, as even a few degrees can change results.
Try This Yourself
- Write the formula for coefficient of viscosity and define each term.
- Compare the viscosity of honey and water using the experimental setup described.
- Calculate η for a fluid if F = 0.4 N, d = 0.1 m, A = 0.2 m2, and v = 2 m/s.
Final Wrap-Up
We explored the coefficient of viscosity—its meaning, formula, units, significance in everyday life, and connections to other essential chemistry topics. To strengthen your understanding and exam preparation, check out live classes, revision notes, and more detailed explanations from Vedantu’s expert chemistry educators.
Related topics for deeper learning on Vedantu:
- Viscosity and Surface Tension (Applications, differences & real-life uses)
- Physical Properties of Liquids (Overview of viscosity with other liquid properties)
- Cohesion and Adhesion (Role in fluid mechanics and surface tension)
FAQs on Coefficient Of Viscosity In Fluid Mechanics
1. What is coefficient of viscosity in chemistry?
Coefficient of viscosity (η) is the measure of a fluid’s internal resistance to flow and is defined as the tangential force per unit area required to maintain a unit velocity gradient between two parallel layers of the fluid.
- It describes how strongly molecules resist relative motion.
- Higher η means the liquid flows more slowly (e.g., glycerol).
- Lower η means the liquid flows easily (e.g., water).
- Mathematically expressed using Newton’s law of viscosity: F = ηA(dv/dx).
2. What is the formula for coefficient of viscosity?
The formula for the coefficient of viscosity is η = F / [A(dv/dx)], based on Newton’s law of viscosity.
- F = tangential force applied
- A = area of the layer
- dv/dx = velocity gradient between layers
- In simplified form: F = ηA(dv/dx)
For flow through a capillary tube (Poiseuille’s equation): η = (πr4ΔP) / (8Vl), where r = radius, ΔP = pressure difference, V = volume flow rate, and l = length of tube.
3. What are the SI units of coefficient of viscosity?
The SI unit of the coefficient of viscosity is pascal-second (Pa·s).
- 1 Pa·s = 1 N·s·m-2
- Dimensional formula: M1L-1T-1
- In CGS system, the unit is poise (P)
- 1 Pa·s = 10 P
Water at 20°C has a viscosity of about 1.0 × 10-3 Pa·s.
4. How does temperature affect the coefficient of viscosity of liquids?
The coefficient of viscosity of liquids decreases with increase in temperature.
- Heating reduces intermolecular attractions.
- Molecules move more freely, reducing resistance to flow.
- Example: Water flows faster when heated.
For gases, viscosity increases with temperature because molecular collisions become more frequent.
5. What is the difference between viscosity and coefficient of viscosity?
Viscosity is the general property describing resistance to flow, while the coefficient of viscosity (η) is the numerical measure of that resistance.
- Viscosity: qualitative concept (thickness of fluid).
- Coefficient of viscosity: quantitative value with unit (Pa·s).
- Example: Honey has higher η than water.
6. What is Newton’s law of viscosity?
Newton’s law of viscosity states that the shear stress between adjacent fluid layers is directly proportional to the velocity gradient between them.
- Mathematical form: F = ηA(dv/dx)
- Valid for Newtonian fluids like water and air.
- Shear stress ∝ rate of shear strain.
Fluids that do not follow this law are called non-Newtonian fluids.
7. What are Newtonian and non-Newtonian fluids?
Newtonian fluids obey Newton’s law of viscosity, while non-Newtonian fluids do not show a constant viscosity.
- Newtonian fluids: constant η at constant temperature (e.g., water, ethanol).
- Non-Newtonian fluids: η changes with applied stress (e.g., ketchup, blood, starch paste).
- In non-Newtonian fluids, shear stress is not directly proportional to velocity gradient.
8. How is coefficient of viscosity measured experimentally?
The coefficient of viscosity is commonly measured using an Ostwald viscometer or by applying Poiseuille’s equation for capillary flow.
- Measure time taken for a fixed volume of liquid to flow through a capillary.
- Compare with a liquid of known viscosity (usually water).
- Use relation: η1/η2 = (ρ1t1)/(ρ2t2)
Here, ρ = density and t = flow time of the liquid.
9. What is the relationship between viscosity and intermolecular forces?
The coefficient of viscosity increases with stronger intermolecular forces in a liquid.
- Strong hydrogen bonding → high viscosity (e.g., glycerol).
- Weak van der Waals forces → low viscosity (e.g., hexane).
- Larger molecular size also increases η.
Thus, polarity and molecular structure directly influence viscosity in chemistry.
10. What is the importance of coefficient of viscosity in chemistry?
The coefficient of viscosity is important because it helps understand fluid flow, molecular interactions, and industrial processes.
- Used in studying reaction rates in solutions.
- Important in polymer chemistry and colloids.
- Applied in lubrication, pharmaceuticals, and petrochemical industries.
- Helps determine molecular weight of polymers using viscometry.
It is a key physical property in physical chemistry and chemical engineering.
