Lesson 1 - Static Matter Gases

Written and Composed 

by Ms. Nabeela

Lecturer (Assistant Director)

 at GIK College  

 STATE OF MATTER O1: GASES

           GAS: The state of matter in which the particles/molecules are free to move and
           are separated by large distances exerting negligible force of attraction upon
           one another is called Gas.

    CHARACTERISTICS OF GAS:-

    a.    Colour: Majority of gases are colourless. Only a few gases are coloured. e.g NO2
            is reddish brown, iodine vapours are violet. O2, N2, H2 and He are colourless gases.
    b.    Odour: Majority of gases are odourless. Some gases have characteristic odour. O2, N2, H2 and
           He  are odourless gases.
    c.    Indefinite Volume: Gases have indefinite volume. As gas molecules move freely in all the
           available space, so can accommodate in all the space. Therefore the volume of a gas is equal
           to the volume of the container in which it is confined, and changed with the volume of the container.
    d.    Indefinite Shape: As the molecules of a gas moves freely in all available space, so it assumes
           the shape of a vessel/container in which it is kept. Therefore gases have no definite shape.
    e.    Miscibility: Molecules of gases can freely mix due to the diffusion and effusion provided that
           they do not react chemically.
    f.    Motion: The molecules of a gas move freely in all the directions because they exert negligible
          force of attraction upon one another. Their movement is in straight line until and unless
          they strike/collide with one another.
    g.    Kinetic Energy: As the molecules of a gas move freely, it shows that they have certain kinetic energy.
    h.    Pressure: Gas molecules during their motion collide with one another thus exerts pressure upon
           one another and the walls of the container.
    i.    Expansion: Increase in the volume of a gas by increasing temperature or reducing pressure is
          called expansion. Gases have the ability to expand on reducing the external pressure or
          increasing temperature, due to which they move far away from one another thus increasing volume.
    j.    Contraction: Decrease in the volume of a gas by decreasing temperature is called contraction.
          Gases also have the ability to contract as the temperature decreases, because their kinetic energy
          decreases due to which they come close to one another thus attracting one another due to which
         empty spaces and the volume decreases.
    k.   Low Density: As the gas molecules are far apart from each others, so have large empty spaces,
          that’s why they have very low densities.
    l.    Diffusion: The spontaneous emission of gas molecules from region of higher concentration to
          a region of lower concentration in bulk is called diffusion. As gas molecules move freely in all the
         directions so can diffuse into one another.
    n.   Effusion: The spontaneous emission of the gas molecules from a region of higher concentration to
          lower concentration one by one through a small pore/orifice. So gases show the property of effusion.
    o.   Compressibility: Decrease in the volume of a gas by applying pressure is called compression.
         Gases have the ability of compression. As the external pressure upon the gas is increased, the
         molecules come closer and the empty spaces decrease, in turn volume also decreases.

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