(GASES)

            The physical state of matter in which, the molecules move randomly in other wise empty     space, filling any size or shape of the container into which it is put.

            Characteristics:-

1.;Colour & Odour:-Most of the gases are colourless and odourless like O₂, H₂, N₂ and CO₂.  Some gases are coloured like NO₂ is reddish and chlorine is yellowish.  H₂S has rotten egg like smell. 

2.         Volume and Shape:-Gases have no definite volume and definite shape.  Gases occupy all the space provided to it and adopt the shape of the container.

3.         Compressibility:-        As the particles of the gases are at large distances from each other.  Therefore they can be compressed. by applying pressure.

4.         Expansion:-     A decrease in pressure increases  the volume of the gases, called expansion of the gases. Sudden expansion of the gases produce cooling called Joule’s Thomson effect.  This phenomenon is applied for the liquefaction of gases.

5.         Diffusion:-       Gases are completely miscible with each other.  This is due to the higher kinetic energy and large empty spaces between the molecules.

6.         Effusion:-        The escaping out of the gas molecules through a pin hole one by one without collision with each other is called effusion.

7.         Pressure:-       Gas molecules colloid with each other & with the wall of the container.  During this collision with the wall of the container, gases exert pressure. 

8.         Inter Molecular Forces of Attraction:-         Gases have weak IMF of attraction.

9.         Inter Molecular Spaces:-        Gases has high inter molecular spaces.

10.       Gases can be liquefied on cooling.  Different gases liquefy at different temperature.

UNITS OF PRESSURE, VOLUME & TEMPERATURE

1.         Pressure:-       The force exerted per unit area is called pressure.  In case of gases the           number of collision exerted per unit area is called pressure of the gases.  As

            Pressure = force / area

            S.I unit of Pressure =  N/m2                                                                         

            1 N/m₂ = 1 Pascal

            Other units are atmospheric pressure, torr, psi e.t.c.

            1 atmospheric pressure            = 760mm Hg

            1 atm Pressure                                     = 760 Torr

            1 atm P = 101325 N/m²  = 14.7PSI (pound per sqare inch)      = 1.01325 bar

2.         Volume:-         As volume = Length x Width x Height

            Therefore unit of volume = Meter x Meter x Meter = Meter³

            It is a big unit. Other smaller units are 1m³ = 1000 dm³

                                                                                    1dm³ = 1000cm³         

            1 dm³ is also called one liter or (1dm³ = 1l)

            liter is also define as “The space occupied by one Kg of water at 4^OC. 

            1 liter = 1000ml.

            Or 1000cm³ = 1000ml          or         1cm³ = 1 ml

3.         Temperature:-            The average kinetic energy of the molecules of a gas is called temperature.  Its unit in SI is Kelvin..  Other units are Celsius scale & Fahrenheit scale.   These different scales are interconverted by the following formulae.

            K = C^O + 273

            F^O = (C^O x 1.8) + 32

            C^O = F - 32/1.8                           0K = - 273.16C⁰ = -459F⁰

Q 1      SI unit for measurement of pressure               

            (1) Pascal         (2) mm of Hg                           (3) atm             (4) torr

Q; 2     Density of gas is usually expressed in

            (1)Kg/m³                             (2)Kg/dm³                       (3) g/dm³                            (4)g/cm³                 

Q3       The temperature and pressure at STP and RTP is

            (1) the same                             (2) stp T and P are high than rtp

            (3) At rtp temperature is high while pressure is the same as stp

            (4) rtp temperature and pressure are low than stp.