Properties of Fluids

Dhristi JEE 2022-24

What is Fluid?

We know that fluid mechanics is the study of forces and flow within the fluid; however, we must also know what fluid is.


Well! The fluid is basically a name given to the substance that flows on being subjected to an external force. Liquids and gases both are fluids. One should also know that fluids follow the law of inertia and they are shapeless, which means that they take the shape of a container in which they are kept. 


So, we will be studying its mechanics, properties, and a lot of other things. 


Properties of Fluid

We can find various properties of fluids because each fluid has its own composition and specific specialties, and much more, so let’s discuss these in detail:

  • Kinematic Properties: 

We know that fluid tends to remain at rest unless it is forced to flow which means according to Newton’s first law of motion or the law of inertia, the fluid can be studied under the scope of kinematics. Wherein velocity and acceleration are the key parameters to describe the fluid in motion.

  • Thermodynamic Properties: 

If we talk about fluid, they do have stored potential energy and when they are forced to flow, their energy transforms to kinetic energy, and the molecules inside the fluid set into motion, which in turn, sets fluid into motion. 


However, fluid-like honey with higher density during winters is placed under the sunlight and gets into a molten state. The molten stage of honey allows its quickening flow. 

It means on increasing the temperature, the density decreases, and the kinetic energy of molecules increases.


So, the factors like temperature, density, pressure, entropy, enthalpy decide the flow of fluid. 

  • Physical Properties: 

In Chemistry, there are certain chemical compounds that we deal with. To determine many attributes like the type of reaction that occurs, molality, and molarity, we need to know what kind of compound (fluid) formation occurs. So, by knowing their color and odor, we can determine the compound so formed. That’s why we say that fluids have physical properties like color and odor. 


Physical Properties of Fluids



In this context, we will discuss the physical properties of fluids with their description and related diagrams:

  • Density: 

If we talk about density, we can think of emulsions and gels. An emulsion is a mixture of two or more liquids that don’t mix such as butter, yolk.  Another one is a gel, a gel is a polymer network formed through the physical aggregation of polymer chains, caused by hydrogen bonds, crystallization, helix formation, complexation. For example, hair gels, face wash, etc.


Now, comparing these two, we find that gel is denser than emulsion as the gel is partially solid, while emulsions are formed by a mixture of liquids.


The density is denoted by the symbol ρ. The formula for density is mass per unit volume and its unit in MKS and CGS forms are as follows:


MKS unit: kgm⁻³


CGS unit: gcm⁻³

  • Specific Weight

A unit volume of fluid possesses some weight and that weight is the specific weight. As we can see the term ‘weight’ so obviously is a gravity-dependent physical quantity. 


The specific weight is denoted by a small letter ‘w’. 


The formula for specific weight is Mass/Volume. The specific weight (specific force of gravity or mg) for water is 9.8 Nm⁻³.

  • Temperature

As we discussed in the above context, fluid is a temperature-dependent quantity. If we raise the temperature of honey during winters, it will come out of the jar with the flow. It means the interatomic force of attraction between the molecules decreases and the molecules gain kinetic energy because of which we get the easy outcome of the solidified honey. 


The graph for the temperature of a gas and liquid (fluid) is as follows:

  • Pressure

We see that in an airfoil (So, here airfoil is nothing but the fluid) there is a pressure difference in the above and the below layer.  because of which the movement occurs. This phenomenon is seen in airplane flight (aerodynamics). 


The pressure is something that occurs by the action of force on a unit area, which is denoted by a symbol ‘P’, its formula is Force/Area. It is measured in Nm-2.

  • Specific Volume

The reciprocal of density is the specific volume. It is given by Volume/Mass. It is denoted by ‘v’ and measured in the following units:


MKS unit: m³kg⁻¹


CGS unit: cm³g⁻¹


Properties of Fluid Mechanics 

Fluid mechanics is the phenomenon and an important branch of Physics that deals with two properties of fluids, i.e., the fluid at rest (hydrostatic) and that of in motion or a flow (hydrodynamics). 


So, basically, fluid mechanics is the branch of physics that is concerned with the mechanics of fluids viz: liquids, gases, and plasmas(the fourth state of matter) and the forces subjected to these.


We find applications of fluid mechanics in a wide range of disciplines including the following:

  • Mechanical engineering

  • Civil engineering

  • Chemical engineering

  • Biochemical engineering

  • Geophysics

  • Oceanography

  • Meteorology

  • Astrophysics

  • Biology

A Short Description on Unique Properties of Fluid

As we all know, matters in nature exist in three states namely solid, liquid and gas. Solids are rigid physical objects with very strong interatomic bonds. They possess a definite shape and require very high temperatures to melt or change into liquid form and gaseous form or vapourize. Contrastingly, liquid and gas do not have a definite shape and are known as fluids. They deform easily because the interatomic forces are relatively weaker than that present in the solids. Because of this condition matters in this state show very special characteristics and are defined in 5 properties as density or specific density, specific weight or weight density, specific volume, specific gravity,  and viscosity. Specific density is the same as the density of solid-state matters. It is defined as the quantity of mass per unit volume of the liquid. It changes with the temperature and pressure of the fluid.  

For example, the density of water is 1000 liter per meter cube volume of the liquid and it is highest at 4 degrees celsius. The density of liquid changes less in relation to the change of density of gases with the change of temperature. The second property is the specific weight or weight density of any fluid. It is the quantity of weight exerted per unit volume of the fluid. The third property is specific gravity. It is the unit density per unit volume of the fluid. It is also known as the relative density. To compare the specific gravity of all liquids the density of water is taken as standard. Likewise to compare the specific gravity of gaseous substances the density of air in the environment is taken as standard. The other property of fluids is viscosity which is experienced when fluid is inflow. The atomic particles are connected to each other by a force of interatomic attraction. So when a fluid flows remaining under the influence of gravity then it differentiates into different layers one over another.  The resistance created by the atoms of the lower layers against the atoms of the upper layer of the fluid in motion is known as viscosity.

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FAQs (Frequently Asked Questions)

1. What is Surface Tension?

The tension is generated when fluid comes into contact with the other surface. 

If we talk about oil, if the drop of oil falls on the surface, it tends to remain congested, take the least area, or simply remain in a spherical shape because of the surface tension. One must note that this shape is possible only when this drop is free of gravity or other external forces.   

So, basically, surface tension is the tension on the free surface of the fluid at the rest position. The formula for the surface tension is Force per unit length (as that of tension) and is measured in N/m.

2. State Any Specialty in Fluid Mechanics.

Fluid mechanics models matter from a microscopic viewpoint. If we talk about fluid dynamics, it is an active field of research, and mathematically complex. 

Many problems are best addressed by numerical methods, typically using computers for which we employ a modern method called computational fluid dynamics or CFD. Particle image velocimetry is an experimental method for visualizing and analyzing fluid flow, and also brings advantage to the highly visual nature of the fluid flow.

3. What is the Elasticity of a Fluid?

When the fluid restores its original volume after an applied external force is removed from it, we say that the fluid is elastic. 

4. What is the reason for the properties of fluids different from the matter in solid-state?

The basic difference between the fluid and solid state of the matter is the difference of the strength in the attraction or bond between the atoms of the matter in different phases. The difference in the state of existence depends on some intricate factors that are yet not identified by the researchers. For example, mercury is a metal that exists in the liquid phase at normal room temperature. The other examples of matters in the liquid state are mostly non-metals. To understand more about different phases of matters students can refer to the articles provided on the Vedantu website. They can register themselves on the portal free of cost and can download the same articles free of cost. 

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