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Class 9
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Chemistry
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Physical States Of Matter
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-Properties of Gases

Physical States Of Matter

-Properties of Gases

Chemistry

  1. DIFFUSION:

    The spreading of particles from area of high concentration to the area of low concentration, is termed as diffusion.

    OR        

    The spontaneous mixing of molecules by random motion and collision to form homogeneous mixture, is termed as diffusion.

  • The time according to which a gas spreads throughout the container is termed as “rate of diffusion.”

  • Heavier gases diffuse slowly and take more time to spread. Whereas lighter gases diffuse faster and take less time to spread. For example; H2_22​ hydrogen gas having molar mass 2 grams diffuses faster than CO2_22​ carbon di oxide having molar mass 44 grams.

  • Viscosity of the medium in which a gas diffuses impacts rate of diffusion. More viscous medium slowdown the rate of diffusion of a gas. Whereas in less viscous medium a gas diffuses faster.

  • Temperature increases the rate of diffusion of a gas. Increment of temperature, increases the kinetic energy (speed) of gas particles as a result of which gas particles cover larger area of the medium in less time and vice versa.

Examples of diffusion are;

Fragrance of flower, spreading of smoke into the air, delicious pleasant smell of a cooking meal etc.

  1. EFFUSION:

    The escaping of gas molecules through a tiny hole into a space of with low pressure, is termed as effusion.

  • For the process of effusion the diameter of the hole must be less than the gas molecules’ mean free path.

  • Effusion also depends upon the molecular masses of the gases.

  • Heavier gases effuse slowly than lighter gases.

Examples of effusion are;

Leakage of air from the pin hole of the tyre, leakage of helium gas from the pin hole of the balloon etc.

  1. PRESSURE:

    The force exerted by gas particles on the walls of the container is termed as pressure.

    or

    “The force per unit area is called pressure.”

According to kinetic molecular theory, the molecules of gases are in a state of continuous random motion. These molecules collide with one another and with the walls of the container. The pressure of the gas is due to the collision of gas molecules with the walls.

Mathematically;  

Pressure =     or      P = F/A

Units:

Pressure = Force / Area   =      

  Newton /  square meter               
    N/ m2^22     or     Nm-−2^{-2}−2

Because, 

1 N = Kg m s−2^{-2}−2      

 Therefor; Kg m s−2^{-2}−2 m−2^{-2}−2 = Kg m−1^{-1}−1 s−2^{-2}−2

These are the S.I units of pressure,

i-e Nm−2^{-2}−2 = Pa (Pascals) = Kg m−1^{-1}−1 s−2^{-2}−2

Some other units of pressure are;

NORMAL VALUES OF ATMOSPHERIC PRESSURE:

  • 7 psi (Pounds per square inch)

  • 760 mm of Hg (milli meter of hydrargyrum)

  • 760 torr

  • 1 atmospheres (atm)

  • 101325 Pascal (Pa)

  • 1.01325 bar

Instruments used for the measurement of pressure;

Barometer:

The instrument used to measure the atmospheric pressure is termed as barometer.

Manometer:

A manometer is an instrument used to measure and indicate pressure of fluid (gas or liquid)

  1. COMPRESSIBILITY:

  1. The increment of pressure to reduce the volume of gas particles is
    termed as compressibility   

    or

    The capacity of something to be flattened and reduced in size by pressure is called pressure.

According to kinetic molecular theory the molecules of gases are very far from one another, having large spaces present between them. Therefore, by applying pressure, the molecules come closer to each other and the spaces between them are reduced. Hence gases are said to be compressed. The compressibility of gases is higher than liquids and solids

  1. MOBILITY:

    The ability of a substance to move freely is known as mobility.

As the gas molecules are in continuous motion, they can move fast due to high kinetic energy. The molecules move freely in the vacant spaces and this mobility is responsible to produce a homogenous mixture of gases. The gases particles are more mobile than particles of liquids and solids due to weak intermolecular forces present between them.

  1. DENSITY:

    Mass per unit volume is termed as density.

    or

    The density is degree of compactness or closeness of a molecules.

Gases have low density because of light mass and more volume occupied by the gas molecules. Gas density is expressed in grams per cubic decimeter g/dm3^{3}3. Gases are less dense than liquid. The density of gases can be increased by cooling.

Mathematically: Density = mass/ volume         or       D = m /v

Unit:    D = grams / cubic deci meter     or       D = g / dm3^33  or g / L