THERMODYNAMICS STATE OF A PURE SUBSTANCE

     A pure substance is a substance of constant chemical composition throughout its mass. It is a one-component system. It may exist in one or more phases.

     Let us consider a unit mass of solid water (ice) as a pure substance enclosed in a cylinder at atmospheric pressure of 1.01235 bar. Keeping the pressure as content, heat the substance and the changes in its states.

     When the heat is added, the temperature rises and the ice expands until a temperature of 0*C (273 K) is reached (as indicated by line AB) at one atmospheric pressure. The heat transferred to the substance while the temperature changes are referred to as sensible heat.

Temperature – Specific volume

     Further heating will not raise the temperature of ice but causes the melting of ice to the liquid phase (along with BC). The changes of phase occur at a constant temperature where the specific volume is reduced. The heat required for this phase change is known as latent heat of fusion.

     Further heating of liquid substance, results in a raise in temperature which causes a small contraction in volume until the temperature is about 4*C and subsequent expansion takes place until a temperature of 100*C (373 K) is reached (at E). Further heating (after E) causes second changes to vapor with a large increase in volume. The heat required in this case is known as latent heat of vaporization.

     When the vapourization is completed, further heating causes a rise in temperature where the substance from saturated vapor goes to superheat region.

     When the same heating is carried out at lower pressures, (less than atmospheric pressure) the following changes are observed:-

  1. There is a slight raise in the melting point of the substance.
  2. There is a marked drop in the boiling point.
  3. There is an increase in volume and enthalpy during vapourization.
     When the pressure is reduced to 0.006112 bars, the melting and boiling temperatures become equal. And the changes of phase, Ice-Water-Steam, is represented by a single horizontal line (line BCF). The temperature at which this occurs is known as the triple point. And the line (line BCF) is called the triple point line. For water, the triple point is at 273K. If the pressure is reduced still further, the ice instead of melting, sublimates directly into steam.

     When the heating at a pressure above atmospheric pressure, the following changes are observed:-
  1. There is a considerable drop in melting point.
  2. There is a marked raise in boiling point.
  3. There is a marked reduction in the volume and enthalpy during vapourization.
     At a certain high pressure, the changes of volume fall to zero, and the change of phase from liquid to vapor become a point is known as the critical point. Where the latest hat to vaporization is zero. The properties at this point are known as the 
  1. Critical pressure ( 221.2 bar of water )
  2. Critical temperature ( 647.3 K for water )
  3. Critical volume ( 0.003m^3 / kg for water ) 

Heat and Heat Transfer

    Heat is a form of energy in transfer, which gives us the sensation of warmth of hotness. The degree of hotness or coldness is known as temperature.

     Heat is the form of energy that transfers from one body to another due to a temperature difference. Whenever a temperature gradient (temperature difference) exists, there is an energy transfer as heat transfer.

     The direction of the heat transfer is taken from the high-temperature system to the low-temperature system. Heat flow into a system is taken to be positive, and heat flow out of a system is taken as negative.

MODES OF HEAT TRANSFER

     There are three different modes that heat transfer can occur such as.,
  1. Conduction
  2. Convection
  3. Radiation.

1. Conduction

        Conduction is the form of heat transfer through a body that occurs without any
movement of the body; it is a result of molecular (electron) action. The relative capacity
of a material to conduct heat is known as its conductivity.

2. Convection
         Convection is the form of heat transfer that results from gross movements of 
liquids or gases. convection is the most important mechanism of energy transfer
between a solid surface and a liquid or a gas. 
          According to the mode of motivating flow, convection heat tranfer is classified
into 
        1. Free convection or natural  convection 
        2. Forced convection 
 
3. Radiation 
         Thermal radiation is the form of heat transfer that occurs between two separated
bodies as a result of electromagnetic radiation (sometimes called wave motion). The
heat energy waves travel in straight lines from one body to another without effecting
the temperature of the medium through which they travel. Heat flows from sum to earth by this method.  

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