Topics Covered :
● Thermal Energy
● Intermolecular Forces vs Thermal Interactions
● The Gaseous State
Thermal Energy :
`text(Definition :)` Thermal energy is the energy of a body arising from motion of its atoms or molecules.
● It is directly proportional to the temperature of the substance.
● It is the measure of average kinetic energy of the particles of the matter and is thus responsible for movement of particles.
● This movement of particles is called thermal motion.
Intermolecular Forces vs Thermal Interactions :
`=>` Intermolecular forces tend to keep the molecules together but thermal energy of the molecules tends to keep them apart.
`=>` Three states of matter are the result of balance between intermolecular forces and the thermal energy of the molecules.
`=>` When molecular interactions are very weak, molecules do not cling together to make liquid or solid unless thermal energy is reduced by lowering the temperature.
`=>` Gases do not liquify on compression only, although molecules come very close to each other and intermolecular forces operate to the maximum.
● However, when thermal energy of molecules is reduced by lowering the temperature; the gases can be very easily liquified.
`=>` Predominance of thermal energy and the molecular interaction energy of a substance in three states is depicted as shown in fig.
The Gaseous State :
`=>` This is the simplest state of matter.
● Air is a mixture of gases.
`=>` We spend our life in the lowermost layer of the atmosphere called troposphere, which is held to the surface of the earth by gravitational force.
● The thin layer of atmosphere is vital to our life.
● It shields us from harmful radiations and contains substances like dioxygen, dinitrogen, carbon dioxide, water vapour, etc.
`=>` In periodic table only eleven elements exist as gases under normal conditions (Fig.4).
`=>` The gaseous state is characterized by the following physical properties :
(i) Gases are highly compressible.
(ii) Gases exert pressure equally in all directions.
(iii) Gases have much lower density than the solids and liquids.
(iv) The volume and the shape of gases are not fixed. These assume volume and shape of the container.
(v) Gases mix evenly and completely in all proportions without any mechanical aid.
`=>` Simplicity of gases is due to the fact that the forces of interaction between their molecules are negligible.
`=>` Their behaviour is governed by same general laws, which were discovered as a result of their experimental studies.
● These laws are relationships between measurable properties of gases.
● Some of these properties like pressure, volume, temperature and mass are very important because relationships between these variables describe state of the gas.
● Interdependence of these variables leads to the formulation of gas laws.