Sep 252012

Properties of fluids determine how fluids can be used in engineering and technology.Properties of Fluids They also determine the behaviour of fluids in fluid mechanics. The following are some of the important basic properties of fluids:

  1. Density
  2. Viscosity
  3. Temperature
  4. Pressure
  5. Specific Volume
  6. Specific Weight
  7. Specific Gravity

1. Density:

Density is the mass per unit volume of a fluid. In other words, it is the ratio between mass (m) and volume (V) of a fluid.

Density is denoted by the symbol ‘ρ’. Its unit is kg/m3.


In general, density of a fluid decreases with increase in temperature. It increases with increase in pressure.

The ideal gas equation is given by:

PV~=~mRT~~~ lbrace~Where~R right Universal~Gas~Constant~rbrace


P~=~ rho RT~~[Since,~rho~=~m/V]

The above equation is used to find the density of any fluid, if the pressure (P) and temperature (T) are known.

Note: The density of standard liquid (water) is 1000 kg/m3.

2. Viscosity

Viscosity is the fluid property that determines the amount of resistance of the fluid to shear stress. It is the property of the fluid due to which the fluid offers resistance to flow of one layer of the fluid over another adjacent layer.

In a liquid, viscosity decreases with increase in temperature. In a gas, viscosity increases with increase in temperature.

Viscosity – Animation Video

3. Temperature:

It is the property that determines the degree of hotness or coldness or the level of heat intensity of a fluid. Temperature is measured by using temperature scales.There are 3 commonly used temperature scales. They are

  1. Celsius (or centigrade) scale
  2. Fahrenheit scale
  3. Kelvin scale (or absolute temperature scale)

Kelvin scale is widely used in engineering. This is because, this scale is independent of properties of a substance.

4. Pressure:

Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio of force on a fluid to the area of the fluid held perpendicular to the direction of the force.

Pressure is denoted by the letter ‘P’. Its unit is N/m2.

5. Specific Volume:

Specific volume is the volume of a fluid (V) occupied per unit mass (m). It is the reciprocal of density.

Specific volume is denoted by the symbol ‘v’. Its unit is m3/kg.


6. Specific Weight:

Specific weight is the weight possessed by unit volume of a fluid. It is denoted by ‘w’. Its unit is N/m3.

Specific weight varies from place to place due to the change of acceleration due to gravity (g).


7. Specific Gravity:

Specific gravity is the ratio of specific weight of the given fluid to the specific weight of standard fluid. It is denoted by the letter ‘S’. It has no unit.


Specific gravity may also be defined as the ratio between density of the given fluid to the density of standard fluid.

S~=~{ rho_{given~fluid}}/{ rho_{standard~fluid}}

Introductory image source

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I am a mechanical engineer with a passion for technical stuff. I am the founder and former editor-in-chief of

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  • আশীফ ইকবাল

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