# Fluid means Liquid & Gas. Fluid is a matter in which molecules are free to move. As molecules are free to move, they can translate & if molecules of fluid can translate they can have motion so they can have collision among themselves.

Majority property of the fluid are because of that behavior of molecules due to which molecules are free relatively to move.

The main properties of fluids are listed below:-

1. Density or Mass Density
2. Specific Weight
3. Specific Volume
4. Specific Gravity

1. Density :-
Density is the mass per unit volume of a fluid. In other words,

The density of a substance is not necessarily constant throughout the volume of a substance.

In general, the density of fluid can not be uniform throughout the fluid.

2. Viscosity:-
Viscosity is the fluid property that determines the amount of resistance of fluid to shear stress.
A fluid which has no resistance to shear stress is known as an inviscid fluid.
Due to this property fluid offers resistance to flow of one layer of fluid over another adjacent layer.

3. Temperature:
Temperature is the fluid property that gives the measure of hotness or coolness of a fluid or measure of heat intensity of a fluid.
It is generally measured using a device called thermometer.
Thermometers are calibrated in various temperature scales.

The most common scales of temperatures are :-
i) Celcius Scale (Denoted by °C )
ii) Fahrenheit Scale ( Denoted by °F )
iii) Kelvin Scale ( Denote by K )

Kelvin scale is predominantly used for scientific purposes by convections of the International System of Units.
Kelvin scale is used for scientific purposes as it does not depend on the properties of a substance.

4. Pressure :
Fluid Pressure is most often the compressive stress at some point within a fluid.
Fluid pressure occurs in two situations:-
i) Open Condition (also called open channel flow) e.g the ocean, a swimming pool, atmosphere (for gas ), etc
ii) Closed Condition e.g a water line or gas line

Pressure in open condition usually can be approximated as the presence in static or non-moving conditions as motion creates negligible change in pressure.
The pressure at any given point of a non-moving (static) fluid is called the hydrostatic pressure.

5. Specific Volume-
Specific Volume is defined a the ratio of material’s volume to its mass. It is the reciprocal of the density of material.
Specific Volume is inversely proportional to density.

The standard unit of specific volume is cubic meters per kilogram ( m

3 /kg ).

It is an intrinsic property of matter. The intrinsic property does not depend on the sample size.
Also, it is an intensive property that means it is unaffected by how much of a substance exists or where it was sampled.

6. Specific Weight:-
Specific Weight of a fluid is defined as the weight per unit volume. It is denoted by γ (the Greek letter Gamma). Its SI unit is N/m3.

Specific weight is related to density,
γ = ρg
where,

γ = Specific Weight
ρ = Density
g = acceleration due to gravity.

The specific weight of fluid varies with temperature.

7. Specific Gravity:-

Specific gravity is defined as the ratio of the specific weight of the given fluid to the specific weight of standard fluid. It is also known as relative density. It is denoted by letter ‘S’. It has no unit.

8 Surface Tension:-
Surface Tension is the tendency of liquid surfaces to shrink in minimum surface area. Surface tension is caused by cohesive forces between the liquid molecules.
At liquid-air interfaces, surface tension is caused due to greater attraction of liquid molecules to each other as compared to the attraction between the air and liquid molecules.
Due to surface tension, many insects like water sliders can float and slide on the water surface despite being denser than water.

9. Vapour Pressure:-
The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid .
The vapor pressure of a liquid varies with temperature. As the temperature increases,the vapor pressure also increases.
Vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer.

10. Capillarity:-
Capillarity is the ability of a liquid to flow in small passages such as a tube of small cross-sectional area. A liquid can flow in narrow passages without the assistance of external force like gravity. It can flow in narrow passages even in opposition to gravity.

11 Cavitation:-
Cavitaion is the process of formation of vapor bubbles or vapor filled cavities of a flowing liquid in a region where the pressure of the liquid falls below the vapour pressure and collapsing of these vapour bubbles in a region of high pressure.