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Definition of fluid, Comparison of fluid with solid, Difference between liquid and gas

Definition of fluid, Comparison of fluid with solid, Difference between liquid and gas

Based on the strength of intermolecular forces of matter, any matter can be divided into three categories: solid, liquid and gases. Term fluid is used for the matter which can flow. So liquid and gas are considered as fluid. If you take some water in glass it will acquire the shape of glass and if you put that water in a balloon it will change its shape according to the shape of the balloon. In this article we will be studying fluid and its properties. 

Table of Contents

  • Definition of fluid
  • Properties of fluid
  • Comparison of fluid with solids
  • Difference between liquid and gas
  • Mechanical properties of fluid
  • Practice problems
  • FAQs

Definition of Fluid

By definition, everything that can flow is a liquid. The water we drink and the air we breathe are examples of liquids. Basically, all liquids and gases are fluids. A material that cannot withstand tangential or shear forces in a liquid or gas state, or more generally in a stationary state, and whose shape changes continuously under such loads, is called a fluid. The fluid changes shape continuously when force is applied.


Properties of Fluid

Though each fluid is different from others in terms of composition and specific qualities, there are some properties which every fluid shares. These properties can be broadly categorised under:

  • Kinematic properties: These properties help in understanding fluid motion. Velocity and acceleration are the kinematic properties of a liquid.
  • Thermodynamic properties: These properties help to understand the thermodynamic state of a liquid. Temperature, density, pressure, and specific enthalpy are thermodynamic properties of a liquid.
  • Physical properties: These properties help you understand the physical state of the liquid, such as colour and odour.

Comparison of fluid with solids

Consider a solid element that is subjected to a shear stress by a shear force F. The solid will show no deformation or very little deformation if that force F is little in amount. Whereas, when we consider a jello, it shows a noticeable change for the same amount of force F and it returns to its original shape after the removal of the shear force. Thus, upon application of the shear force on the jello, the deformation occurs but it is not continuous with time. Now, consider a fluid element that is subjected to a shear stress by a shear force F. In this case, deformation occurs and it increases continuously with time. Thus, fluid can be defined as a state of matter that cannot withstand or resist a shear stress when it is at rest 


Difference between liquid and gas

Gas molecules have no definite volume and shape. A closed container is required to store gas molecules. Their molecular attractiveness is minimal compared to liquid and solid molecules. These molecules flow in all directions and show a constant, random, and free molecular motion. The table below gives the Difference Between Liquid And Gases.

Comparison basis




It has no definite shape and takes the shape of the container

It has a definite volume but no definite shape




Molecular arrangements

Random but more sparsely arranged

Random but little sparsely arranged

Motion of molecules

Free, random and constant


Molecular attraction



Storage facility

Needs a closed container to store

Needs a container to store


No fixed shape or volume

No fixed shape but has volume



Nearly difficult


Flows in all the directions

Always flows from higher to lower level

Intermolecular space



Speed of sound

Lowest among solids and liquids

Slower than solid but faster than gas

Mechanical properties of fluid

Let’s understand some important mechanical properties of fluid one by one.

Density: The density of an object or a substance is its mass per unit volume. Mathematically, it is denoted as the following: Density(⍴)= image

Some standard values of density are: ρwater = 1000 kg m-3 = 1 g cm-3

ρmercury = 13600 kg m-3 = 13.6 g cm-3 ρair= 1.225 kg m-3 = 0.001225 g cm-3

It is a scalar quantity whose SI unit is kg m-3 and dimension [ML-3].

Density of mixture of two fluids or a mixture of fluids: 

The definition of density remains the same as:

mixture= image

  1. a) When the masses of fluids are given:

Suppose two fluids of mass and density m1, ρ1 and m2, ρ2 respectively are mixed together and the final mass and density of the mixture of the fluids be m and ρ respectively.The density of the mixture will be:

mixture= image = image= image

  1. b) When the volumes of fluids are given:

Suppose two fluids of volume and density V1, ρ1 and V2, ρ2 respectively are mixed together and the final volume and density of the mixture of the fluids be V and ρ respectively. The density of the mixture will be:

mixture= image = image= image

Specific Gravity

Specific gravity, also known as relative density, is the ratio of the density of a substance to that of a standard substance. Here, the standard substance is considered as water at 4 ℃. In short, the specific gravity (SG) is denoted as

SG= image

We know that the density of water at 4 ℃ is 1000 kg m-3 and the density of mercury is 13600 kg m-3 . Thus, the specific gravity or relative density of mercury will be 13.6.

If the density of any substance is less than that of the standard substance, i.e., if the specific 


gravity of any substance is less than one(SG < 1), then the substance will float on that standard 

substance and on the other hand, if the specific gravity of any substance is greater than one (SG > 1), then the substance will sink in that standard substance.


This is the force applied perpendicular to the surface of the object per unit area to which the force is distributed.

P = image

It is a scalar quantity and Its SI unit is Nm-2 or pascal (Pa), dimension [ML-1T-2]

Atmospheric pressure: Whenever a liquid is exposed to the atmosphere, the pressure at the

The free surface of the liquid is the atmospheric pressure (Patm). Approximate value of Patm is 

105 Pa.

Pressure in a fluid

Have you ever seen a human pyramid? Imagine the condition of the guy in the bottommost row. He is obviously carrying a lot more load than the guy on the 1st row. The same is the case with fluid pressure in a column. In a fluid column, as the depth increases, the pressure increases as well. And why is this pressure rising? As it gets deeper, the shallower depth fluid also needs to support the fluid above. Therefore, by definition, fluid pressure is the pressure at some point in the fluid that is generated by the weight of the fluid.


Pfluid= P + ρgh


P = Pressure at the reference point

Pfluid is the pressure at a point in a fluid

ρ is the density of the fluid

g is the acceleration due to gravity (considering earth g=9.8 m/s2)

h is the height from the reference point

The density of the fluid may be calculated with the aid of dividing the mass of the fluid in attention with the extent of fluid taken.

ρ = m/v


m is the mass of the fluid

v is the volume of fluid considered

If the fluid is subjected to atmospheric pressure than the total pressure on the system is given by

Pfluid = Po + ρgh


Po is the atmospheric pressure.

Factors That Affect Fluid Pressure

Fluid pressure is influenced by two factors. These two criteria are the fluid's depth and density.

The fluid's depth: The pressure imposed by the fluid increases as the depth increases.

The fluid's density: Denser fluids, like water, exert greater pressure than lighter fluids, like air. Because the molecules in a denser fluid are closer together, there are more collisions in a given region. More pressure is exerted as a result of this.


Why does pressure exert force perpendicularly?

From formulae if you see the  force exerted by a pressure is directly proportional to the area acted upon as well as the pressure itself. The force acting on the tank's end is perpendicular to the tank's internal surface.

Understand this with an example: 

When a particle collides with a wall, the force exerted by the wall causes the particle's momentum to change (only the direction changes, not the magnitude).

The particle exerts an equal and opposite force back on the wall.So, in the direction perpendicular to the the line of contact the momentum change takes place and we know with Newton's 2nd law that force is equals to rate of change of momentum that’s why gas molecules exert pressure perpendicularly

Practice problems

Q1. Calculate the pressure exerted on a scuba driver when she is 12 metres below the surface of the ocean. Assume the standard atmospheric condition.

Answer: The density of water is 103 Kgm-3 and the the atmospheric pressure is 1.01 105 N/m2:

Pfluid = ⍴gh = (103 )(9.8)(12) =1.17 105 N/M2

 Total pressure= (1.01  image 105 N/M2)+(1.17 image 105 N/M2 ) = 2.18 image  102 N/M2.

Q2. Calculate the specific gravity of iron.

Answer: The density of iron is 7850 Kgm-3. The specific gravity of iron-related to water is calculated as follows:

SG iron =image= 7.85

Hence, the specific gravity of iron is 7.85.

Q3. Two fluids of mass and density 1 Kg, 2 Kgm-3 and 3 Kg, 4 Kgm-3 respectively are mixed together then the density of the mixture will be?

Answer: image


Q4. Two fluids of volume and density 1 m3, 2 Kgm-3 and 3 m3, 4 Kgm-3 respectively are mixed together then the density of the mixture will be?




Question 1. What is the relation between specific volume and density?
Specific volume is the reciprocal of density.

Question 2. Give an example for fluids?
Answer: Examples of fluids are Water, Oxygen, Molten lava, etc

Question 3. Arrange the following terms in the ascending order of their density. Water, Carbon dioxide, Air, Seawater?
Answer: Carbon dioxide< Air< Water< Seawater

Question 4. What is a specific volume?
Answer: Specific volume is expressed as the volume that a fluid occupies per unit mass.

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