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Viscosity of Water

We are familiar with the concept of elements, compounds and mixture. Elements are pure substances which combine to form compounds. Mixtures have more than one component which can separate out with the help of different physical methods. In our everyday life, we use many chemical compounds. For example table salt is sodium chloride which is an ionic compounds formed by the combination of sodium ion and chloride ion. The chemical formula for table salt is NaCl. 

Similarly sugar is also a chemical compound which is composed of C, H and O. This is one of the covalent compound in which C, H and O are bonded through covalent bonds. Another most common chemical compound is water. The chemical formula for water is $H_{2}O$ as it is composed of two hydrogen atoms and one oxygen atom. 
Here hydrogen and oxygen are bonded through covalent bonds so water is an example of covalent compound. 

Water

It is a polar covalent molecule in which H and O are bonded through covalent bond. Here O is more electronegative element and H is less electronegative element.  That makes the bond polar and induces partial negative charge on O and partial positive charge on H. Because of electronegative oxygen atom, each water molecule can form maximum 4 hydrogen bonds with same or different molecules. 
Polar Covalent Bond

We are familiar with uses of water. It acts as universal solvent for almost all known substances. This is due to its unique capability of formation of hydrogen bonds and also due to surface tension, viscosity, high heat capacity and heat of vaporization and polarity. Due to polar nature, these molecules attracted towards other polar molecules or ions. The substances which can dissolve in water are called as hydrophilic substances whereas substances which do not interact with water are called as hydrophobic substances. Non-polar molecules like fat and oils are good examples of hydrophobic substances. 

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Viscosity of Water Definition

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Water can exist in all the three possible states of matter; solid, liquid and gases.  At standard ambient temperature and pressure water exists in liquid state. No doubt solid state; ice and gaseous state water vapor are also very common. It covers 71% of the surface of the Earth as t is vital for all living organisms. Some common properties of water are listed below.
  • High-specific heat - 1 calorie/gram °C = 4.186 joule/gram °C
  • High heat of vaporization = 40.65 kJ/mol
  • The lower density of ice = 0.9167 g/cm3
  • High polarity = 1.8546 d
Do you know what viscosity is? You must have seen honey. Take some honey in a Styrofoam cup and pour honey. Repeat same with little amount of water also. Compare to water; honey drains very slowly.  It can be explained in terms of viscosity. The viscosity of honey is large than water so it drains slowly.

Viscosity can be defined as the measurement of fluid's resistance to flow. It explains the internal friction between layers of a moving fluid. In case of fluid with large viscosity resists motion due to internal friction between molecules of fluid layers.

Viscosity of Water Experiment

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Let’s discuss one experiment to check the viscosity of water compare to other liquids.  We need; 
  • Glass jars = 4 
  • Marbles = 4 pieces
  • Liquids = Water, Corn syrup, Cooking oil, Honey 
Take four glass jars and pour one liquid in one jar. Label with the name of liquid you filled. Now take a piece of marble and drop is carefully in one jar. Repeat same experiment with all the four jars. Observe what happens to the marble as it enters the liquid. Record the time taken by each marble to sink at the bottle of the jar slowly.

In a more viscous liquid, the marble piece will take longer time to move through the liquid. The marbles take more time in the jars filled with corn syrup and honey compare to water and cooking oil. So we can say that corn syrup and honey are more viscous liquids. In this manner we can compare the viscosity of two liquids. 

Absolute Viscosity of Water

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We have discussed the term viscosity with the comparison of thickness of liquids. Fluids with different densities cannot compare on same basis so we usually prefer to work on molecular level. Certain properties of fluids like surface tension, viscosity can be explained with the help of molecular level.  The laminar flow of molecular layers is used to explain the viscosity of fluids. 

Today many types of viscometers are available which can be used to determine the viscosity value of different fluids. As the temperature increases, the kinetic energy of molecules increases that enhance the flow of molecules and viscosity decreases.  Viscosity of fluids can be measured as dynamic or absolute viscosity and kinematic viscosity. Absolute viscosity is expressed as coefficient of absolute viscosity. It is measurement of internal resistance of fluids. It can be defined as the tangential force per unit area which is needed to move one horizontal plane with respect to another at unit velocity for unit distance.

The dynamic or absolute of viscosity of water at different temperature values are listed below.

 Temperature [°C]   Dynamic or Absolute Viscosity of water  [mPa.s] 
 20  1.0016
 25  0.89
 30  0.7972
 35  0.7191
 40  0.6527
 45  0.5958
 50  0.5465
 55  0.5036
 60  0.466
 65  0.4329
 70  0.4035
 75  0.3774
 80  0.354

Kinematic Viscosity of Water

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Kinematic viscosity is equal to the ratio of absolute viscosity to density of fluid at given temperature. No force involved in Kinematic viscosity as it can be determined by dividing the absolute viscosity with the mass density of given fluid. The mathematical expression for Kinematic viscosity is as given below. 

ν = $\frac{\mu}{\rho}$

Here
•    ν = kinematic viscosity (m2/s)
•    μ = absolute viscosity (N s/m2)
•    ρ = density (kg/m3)

The SI-system unit for kinematic viscosity is Stoke (St). 
1 Stokes =   10-4 m2/s   = 1    cm2/s

 Temperature[°C]   Kinematic viscosity [mm²/s]   Density [g/cm³] 
 10  1.3063  0.9997
 15  1.1386  0.9991
 20  1.0034  0.9982
 25  0.8926  0.997
 30  0.8007  0.9956
 35  0.7234  0.994
 40  0.6579  0.9922
 45  0.6017  0.9902
 50  0.5531  0.988
 55  0.5109  0.9857
 60  0.474  0.9832
 65  0.4415  0.9806
 70  0.4127  0.9778
 75  0.3872  0.9748
 80  0.3643  0.9718

Coefficient of Viscosity of Water

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Viscosity is determined in terms of coefficient of viscosity. It is a constant value for a fluid and mainly depends on the nature of fluid. For more viscous or thicker fluids, the coefficient of viscosity is greater. It can be determined with the help of Poiseuille’s method in which fluid flows through the capillary tube at different pressures. Dimensional formula for η is [M1L-1T-1] and SI unit is Pascal-second.

Coefficient of viscosity $(\eta)$ = ${F*r}{A* v}$

Here

F = Tangential Force
A =  Area
r = Distance between the layers
v = Velocity

The Poiseuille’s equation for coefficient of viscosity is; 

$\eta$ = $\frac{\pi P r^{4}}{8lV}$

Here:
  • P= Pressure in capillary 
  • l =  length
  • r = radius 
  • V= Volume of liquid 
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