What is Fluid?- Phases, Types, Properties, and Flow

What is Fluid?

A fluid is a substance that is continuously (deformed) under constant shear stress, no matter how small. A fluid is a subset of the phases of matter and consists of liquids, gases, plasmas, and, to some extent, plastic solids. 

In common usage, “fluid ” is often used as a synonym for “liquid”, which has no implication that gas may also be present. In this article, we discuss Types of Fluid And Various Types of Fluid Flow.

Matter around us exists in three phases (excluding plasma)

  • Solid
  • Liquid
  • Gas
Three Phases of Fluid
Three Phases of Fluid


Atoms of solids are in close contact, forces between them that allow atoms to vibrate but not to change positions with neighboring atoms. (These forces can think of as springs that can stretch or compress, but are not easily broken.)

Thus, a solid resist all types of stress. A solid cannot be easily deformed because the atoms that make up the solid are not capable of moving freely. Solids also resist compression, because their atoms are part of a lattice structure in which the atoms relatively fix distance.

Under compression, the atoms will force into each other. Most of the examples we have studied so far include solid objects that are very few when stressed.


In contradiction, liquids easily deform when apply and do not return to their original shape after the force is removed because atoms are free to slide and change about their neighbors, i.e. they flow (hence Are a type of fluid).

Molecules are held together by their mutual attraction. When a liquid is placed in a container with no lid, it remains in the container (provided there is no hole under the surface of the liquid in the container!). Because atoms closely fill, liquids, such as solids, resist compression.


Atoms in gases are separated by distances that are larger than the size of atoms. The forces between the gas atoms are therefore very weak, except when the atoms collide with each other.

Thus gases not only flow (and are therefore consider fluid) but are relatively easy to compress because there are too much space and little force between the atoms. When placed in an open container gas, unlike liquids, will escape.

The major difference is that gases easily compress, while liquids are not. We will usually refer to both gases and liquids only as liquids, and distinguish between them only when they behave differently.

Types of Fluid

The fluids are classified into the following types:

  • Ideal fluid
  • Real fluid
  • Newtonian fluid
  • Non-Newtonian fluid
  • Ideal plastic fluid
  • In-compressible fluid
  • Compressible fluid

On the basis of the shear and velocity gradient relationship, fluids can broadly be classified into the following types of fluid:

types of Fluid
Types of Fluid

Ideal fluid: A fluid, which is incompressible and is having no viscosity, knows as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluid, which exists, has some viscosity.

Real fluid: A fluid, which possesses viscosity, knows as real fluid. All the fluids in actual practice are real fluids.

Newtonian fluid: A real fluid, in which the shear stress is direct, proportional to the rate of shear strain (or velocity gradient), knows as a Newtonian fluid.

Non-Newtonian fluid: A real fluid, in which the shear stress is not proportional to the rate of shear strain (or velocity gradient), knows as Non-Newtonian fluid. In the case of such a fluid, the relationship between the shear stress and the rate of strain is an arbitrary functional relation, either implicit or explicit.

Ideal plastic fluid: A real fluid, in which the shear stress is more than the yield value and the shear stress is proportional to the rate of shear strain (or velocity gradient), knows as ideal plastic fluid.

In-compressible fluid

When the density of the fluid doesn’t change with the application of external force, it knows as an incompressible fluid.

compressible fluid

When the density of the fluid changes with the application of external force, it knows as compressible fluid. Density and Viscosity is different according to the types of fluid as shown below.

The Table Below represents the Density and Viscosity of Different Types of Fluids

Table o property

Different Properties of Fluids

Though each fluid is different from others in terms of composition and specific qualities, there are some properties that every fluid shares.

These properties can be broadly categorized under:

  • Kinematic properties: These properties help in understanding the fluid motion. Velocity and acceleration are the kinematic properties of the fluids.
  • Thermodynamic 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 color and odor.

1. Density

The density of a fluid is defined as the ratio of the mass of the fluid to its volume. And the density of gases is dependent on pressure and temperature, while the density of liquid remains constant.

The density of water is 1000 kg.m-3 while the density of air is 1.225 kg.m-3. There is some basic properties associate with fluids that help us to understand them in a better way.

ρ = mass of fluid / volume of fluid

ρ = Density

kg.m-3 = SI unit of density

2. Viscosity

  • Viscosity is the fluid property that determines the amount of resistance of a fluid to overcome stress. It is the property of a fluid that causes the fluid to provide resistance to the flow of one layer of fluid over another adjacent layer.
  • In a liquid, viscosity decreases with an increase in temperature. In a gas, viscosity increases with an increase in temperature.

3. Pressure

  • It is the term used in fluids which is analog to the term stress used in solids. Both are the ratio of applied force and area of the surface (on which the force applies) Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio of the 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.

4. Temperature

This is the property that determines the level of warmth or coolness or intensity of heat of a fluid. Temperature measured using temperature scales. 3 are the common-use temperature scales.

  • Celsius (or centigrade) scale
  • Fahrenheit scale
  • Kelvin scale (or absolute temperature scale)

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

5. Specific gravity

  • Specific gravity is the ratio of the specific weight of the given fluid to the specific weight of the standard fluid. It is denoted by the letter ‘S’. It has no unit.
  • Also, specific gravity may also be defined as the ratio between the density of the given fluid to the density of the standard fluid.

6. Specific Weight

  • A specific weight is a weight possessed by the 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 Weight

  • A specific weight is a weight possessed by the unit volume of a fluid. It is denoted by ‘w’. Its unit is N/m3.
  • And specific weight varies from place to place due to the change of acceleration due to gravity (g).

Different Types of Fluid Flow

Fluid flow has all kinds of aspects – static or unstable, compressed or inconsistent, viscous or non-viscous, and rotational or irrational, to name a few. Some of these characteristics reflect the properties of the liquid, and others focus on how the fluid is growing.

1. Steady or Unsteady Flow

Fluid flow can be steady or unsteady, depending on the fluid’s velocity:

Steady: In steady fluid flow, the velocity of the fluid is constant at any point.

Unsteady: When the flow is unsteady, the fluid’s velocity can differ between any two points.

2. Compressible and In-compressible flows

Incompressible flow is a flow in which the variation of density within the flow is assumed to be constant. In general, all liquids are treated as incompressible fluids. On the contrary, flows that are characterized by a varying density are said to be compressible.

Gases are normally used as compressible fluids. However, all fluids, in reality, are compressible because any change in temperature or pressure results in changes in density. In many situations, though, the changes in temperature and pressure are so small that the resulting changes in density are negligible.

3. Laminar and Turbulent Flows

Laminar And Turbulent flow
Laminar And Turbulent flow

A Laminar flow (Streamlines) is one in which each fluid particle has a well-defined path. In such flow, the paths of fluid particulars do not intersect with each other. In turbulent flow, the paths of fluid particles may intersect with each other.

Consider water flowing through a pipe at low speeds, there is a nice smooth condition, which is called laminar flow. The mixing of warm and cold air in the atmosphere by wind causes clear-air turbulence experienced during an airplane flight, as well as poor astronomical seeing.

Examples of fluids

  • Water
  • Air
  • Blood
  • Mercury
  • Honey
  • Gasoline
  • Any other gas or liquid


What is Fluid in Physics?

In physics, a fluid is a liquid, gas, or other material that continuously deforms (flows) under applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances that cannot resist any shear force applied to them.

What is called fluid?

Liquids and gases are called fluids because they can be made to flow or move. In any fluid, the molecules themselves are in constant, random motion, colliding with each other and with the walls of any container.

What is fluid in physics give example?

Substances that can flow are called fluids. e.g., gases (oxygen, hydrogen), liquids (water, petrol, sulphuric acid).

What is fluid and types of fluid?

A Fluid is a substance that continually flows when an external force is applied. Fluids generally include liquids, gases, and plasmas. To some extent, plastic solids are also considered fluids.

What are examples of fluids?

Examples of fluids:

  • Water.
  • Air.
  • Blood.
  • Mercury.
  • Honey.
  • Gasoline.
  • Any other gas or liquid.

How are fluids used?

IV fluids are specially formulated liquids that are injected into a vein to prevent or treat dehydration. They are used in people of all ages who are sick, injured, dehydrated from exercise or heat, or undergoing surgery. Intravenous rehydration is a simple, safe, and common procedure with a low risk of complications.

What are the properties of fluid?

Thermodynamic properties of fluids are density, temperature, internal energy, pressure, specific volume, and specific weight.

What are fluids Class 9?

Fluids are substances that have a tendency to flow. A liquid is a fluid form of matter which occupies the space of the container. Liquids have a well-defined surface.

What are fluid give two examples?

Two examples – Fluids are substances that have the ability to flow. Gases (oxygen, hydrogen), for example, and liquids (water, petrol, sulphuric acid).

Why do fluids flow?

In solids, these particles hold on to one another tightly. For liquids and gases, these particles can flow over or alongside one another. That is why liquids and gases are also called fluids: because they can flow. This happens because of the tiny particles that make up a fluid press against everything around them.

What are the two main types of fluids?

Real Fluid: These are your regular fluids that show some viscosity. Newtonian Fluid: These are fluids that adhere to Newton’s laws of fluid motion. Non-Newtonian Fluid: These are fluids that do not follow Newton’s laws of fluid motion.

What are the 3 types of fluids?

Types of Fluids:

  • Ideal fluid. A fluid is said to be ideal when it cannot be compressed and the viscosity doesn’t fall in the category of an ideal fluid
  • Real fluid.
  • Newtonian fluid.
  • Non-Newtonian fluid
  • Ideal plastic fluid.
  • Incompressible fluid.
  • Compressible fluid.
  • Steady or Unsteady Flow.

What is difference between liquid and fluid?

Fluid is a common state of certain substances or a type of matter. The liquid is one of the three phases or states of matter. Fluid’s flow and has some viscosity (thickness). Liquids also flow and it has volume, but no definite shape.

What are two characteristics of fluid?

Fluids have common properties that they share, such as compressibility, density, pressure, buoyancy, and viscosity.

What are the 5 basic properties of fluid?

There are five properties of fluids and those are:

  • Density or Mass Density.
  • Weight Density or Specific Weight.
  • Specific Volume.
  • Specific Gravity.
  • Viscosity.

What is viscosity of fluid?

Viscosity is the resistance of a fluid (liquid or gas) to a change in shape or movement of neighboring portions relative to one another. Viscosity denotes opposition to flow.

Which type of fluid is water?

A fluid, which possesses viscosity, is known as real fluid. All the fluids, in actual practice, are real fluids. Example: Water, Air, etc.

How does fluid flow?

Fluid flow is generally the motion of a fluid that is subjected to different unbalanced forces. It is mainly a part of fluid mechanics and fluid flow generally deals with the dynamics of the fluid. The motion of the fluid continues till different unbalanced forces are applied to the fluid.

What are fluids in chemistry?

A fluid is any substance that flows or deforms under applied shear stress. Fluids comprise a subset of the states of matter and include liquids, gases, and plasma.

What are the 3 properties of gases?

The Properties of Gases. Gases have three characteristic properties: (1) they are easy to compress, (2) they expand to fill their containers, and (3) they occupy far more space than the liquids or solids from which they form.

Why is air a fluid?

A fluid is any substance that flows. Air is made of stuff, air particles, that are loosely held together in a gas form. Although liquids are the most commonly recognized fluids, gasses are also fluids. Since air is a gas, it flows and takes the form of its container.

Why is fluid important?

From our blood system carrying essential glucose, oxygen, and nutrients to cells, to the kidneys getting rid of waste products we no longer want, fluid in the body is vital to allow these to occur. It also lubricates our joints and eyes, helps our digestive system function, and keeps our skin healthy.

How does fluid move into the cell?

Fluid is moved by a combination of osmotic and hydrostatic pressures. The osmotic pressure results from differences in solute concentrations across cell membranes. Hydrostatic pressure results from the pressure of blood as it enters a capillary system, forcing some fluid out of the vessel into the surrounding tissues.

What is intravascular fluid?

That portion of the total body fluid is contained within the blood and lymphatic vessels.

Why is a solid not a fluid?

The word fluid means- a substance that flows, changes shape, is non-rigid and has no distinctive shape. As the solid materials do not do any of the mentioned above, they are not called fluids.

Are all gasses fluids?

Air, and all gasses in fact, are considered fluids. Gasses and liquids are all fluids because the molecules that make them up are in constant motion, they flow.

Why all fluids are not gases?

Since it is known that in liquids the molecules are held together by less strong intermolecular forces of attraction as compared to solids, and in gases, the molecules are held by weak Vander Waal forces. Therefore, the molecules of both liquid and gases can move freely as a result, they are also called fluids.

What is the physical state of a fluid?

There are three physical properties of fluids that are particularly important: density, viscosity, and surface tension. Each of these will be defined and viewed briefly in terms of molecular concepts, and their dimensions will be examined in terms of mass, length, and time (M, L, and T).

What are the four properties of liquid?

All liquids show the following characteristics:

  • Liquids are almost incompressible. In liquids molecules are pretty close to each other.
  • Liquids have fixed volume but no fixed shape.
  • Liquids flow from higher to lower level.
  • Liquids have their boiling points above room temperature, under normal conditions.