Properties of Fluids and Its Classification

Fluids are all around us in the air we breathe, the water we drink, and even in the fuel that runs vehicles. In science, the word “fluid” means any substance that can flow. This includes both liquids and gases. Understanding how fluids and how they are classified how fluids behave is very important in subjects like physics and engineering. It helps us design better machines, vehicles, and systems for everyday use. Whether it’s the oil in an engine or blood flowing in our body, the properties of fluids play a big role. In this article, we’ll look at the basic properties of fluids and how they are classified.

What is a Fluid?

A fluid is anything that can flow and take the shape of its container. This means both liquids and gases are called fluids in science.

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For example:

• Water in a glass takes the shape of the glass
• Air fills up the space in a balloon

Both of them can flow, so they are fluids.

Property	Solids	Fluids (Liquids & Gases)
Shape	Fixed shape	No fixed shape
Flow	Do not flow easily	Flow and spread easily
Compression	Hard to compress	Gases can be compressed easily

Fluids can be grouped based on how they behave or respond to forces. Here are the main types:

Ideal Fluid

A made up or perfect fluid used in theory

It has no viscosity (no resistance to flow) and can’t be compressed

It doesn’t exist in real life but helps us understand basic fluid concepts

Real Fluid 

All fluids around us are real fluids

They have some viscosity and can be slightly compressed

Examples: Water, oil, air

Newtonian Fluid 

These follow Newton’s law of viscosity (flow is steady and predictable)

Examples- Water, alcohol

Non-Newtonian Fluid

These do not follow Newton’s law. Their flow changes with applied force

Example: Toothpaste, ketchup, slime

Compressible Fluid

Fluids that can be compressed (volume can change under pressure)

Example- Air

InCompressible Fluid

Fluids that are hard to compress (volume stays almost the same)

Example- Water (in moist conditions)

Fluid Properties

Fluids possess different properties of different basis which can be used to characterise the fluids. The main properties of the fluids are Viscosity, Density, Specific Weight, Specific Gravity, Bulk Modulus, kinematic Viscosity. We will discuss all of them in a detailed way in this section of the article.

Fluids are generally classified under three types of properties;

Kinematic Properties

These properties help in understanding fluid motion. Velocity and acceleration are the kinematic properties of the fluids.

Thermodynamics Properties

These properties help in understanding the thermodynamic state of the fluid. Temperature, density, pressure, and specific enthalpy are the thermodynamic properties of the fluids

Physical Properties

These properties help in understanding the physical state of the fluid such as colour and odour.

Want to learn about Capacitors, read about it here.

Density

Density is the intensive property of matter. Density is the mass per unit volume for a substance or matter.

For fluids, the density is the mass per unit volume. It is denoted by the rho (ρ).

Volume

\rho=\frac{\text { mass }}{\text { Volume }}

\begin{tabular}{|l|l|}

\hline Substance & Density \\

\hline Air ( $\rho$ air) $\rho_{\text {air }}$ & $\frac{1.2 \mathrm{~kg}}{\mathrm{~m}^3}$ \\

\hline Water ( $\rho w a t e r) \rho_{\text {water }}$ & $\frac{1000 \mathrm{~kg}}{\mathrm{~m}^3}$ \\

\hline Mercury ( $\rho \mathrm{Hg}$ ) $\rho_{\mathrm{Hg}}$ & $\frac{13600 \mathrm{~kg}}{\mathrm{~m}^3}$ \\

\hline

\end{tabular}

Ideal fluids vs Real Fluid Differences

Feature	Ideal Fluid	Real Fluid
Viscosity	Zero (no internal friction)	Has viscosity (offers resistance to flow)
Compressibility	Incompressible	Can be compressible or incompressible
Flow Type	Smooth and steady	May be irregular due to resistance
Energy Loss	No energy loss while flowing	Loses energy due to friction and turbulence
Exists in Reality?	No – only in theory	Yes – all natural fluids are real

Applications of Fluid Properties in Real Life

In Engineering:

• Used in hydraulic systems like brakes and lifts
• Helps design efficient vehicles and airplanes 

In Medicine:

• Understanding blood flow helps treat heart and pressure issues
• Medical tools like IV drips and blood pumps rely on fluid movement

In Industry:

• Oils and lubricants reduce machine friction
• Pumps and pipes transport liquids and gases in factories

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