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Wednesday, 8 February 2017

States of Matter III

  • Gases consist of large number of identical particles (atoms or molecules) that are so small and so far apart on the average that the actual volume of the molecules is negligible in comparison to the empty space between them. They are considered as point masses. This assumption explains the great compressibility of gases.explains the great compressibility of gases.
  • There is no force of attraction between the particles of a gas at ordinary temperature and pressure. The support for this assumption comes from the fact that gases expand and occupy all the space available to them.
  • Particles of a gas are always in constant and random motion. If the particles were at rest and occupied fixed positions, then a gas would have had a fixed shape which is not observed.
  • Particles of a gas move in all possible directions in straight lines. During their random motion, they collide with each other and with the walls of the container. Pressure is exerted by the gas as a result of collision of the particles with the walls of the container Collisions of gas molecules are perfectly elastic. This means that total energy of molecules before and after the collision remains same
2. Behaviour Of Real Gases: Deviation From Ideal Gas:Real gases show deviations from ideal gas law.
    (a) Pressure correction: pressure exerted by the gas is lower than the pressure exerted by the ideal gas.

    (b)Volume Correction: (V–nb) where nb is approximately the total volume occupied by the molecules themselves. Here, b is a constant.

3. Van der Waals equation : Constants a and b are called van der Waals constants

4. Significance of Vander wall parameter: Vander wall parameter a is the measure of intermolecular forces while b is the measure of effective size of gaseous molecules Unit of a = bar L3 mol-2 Unit of b = L mol-1

5. The deviation from  ideal behaviour can be measured in terms of compressibility factor Z, which is the ratio of product pV and nRT. Mathematically Z= nRTpV

6. The temperature at which a real gas obeys ideal gas law over an appreciable range of pressure is called Boyle temperature or Boyle point.

7. Critical temperature (TC) of a gas is highest temperature at which liquifaction of the gas at critical temperature is called critical volume (VC) and pressure at this temperature is called critical pressure (pC). The critical temperature, pressure and volume are called critical constants. Surface tension is defined as the force acting per unit length perpendicular to the line drawn on the surface of liquid. It is denoted by Greek letter 23. γ . It has dimensions of kg s–2 and in SI unit it is expressed as N m–1.

8. Viscosity is a measure of resistance to flow which arises due to the internal friction between layers of fluid as they slip past one another while liquid flows. Strong intermolecular forces between molecules hold them together and resist movement of layers past one another. Greater the viscosity, the more slowly the liquid flows. Viscosity of liquids decreases as the temperature rises because at high temperature molecules have high kinetic energy and can overcome the intermolecular forces to slip past one another between the layers.

9. Viscosity coefficient is the force when velocity gradient is unity and the area of contact is unit area. Thus ‗ η ‘ is measure of viscosity. 
SI unit of viscosity coefficient is 1 newton second per square metre (N s m–2) = pascal second (Pa s = 1kg m–1s–1).

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