Properties of pure substances

Pure substance :

A substance that has a fixed chemical composition ; could be a single chemical element or compound of few elements.

Eg : nitogen , oxygen , water

A mixture of various chemical elements or compounds also qualifies as long as the mixture is homogeneous.

Eg : Atmospheric air-mixture of gases and water vapour , but composition remain same everywhere , homogeneous.

A mixture of two or more phases of a pure substance is still a pure substance.

A mixture of oil and water is , however , not a pure substance as the composition at difference places would be different.

Phases of a pure substance: 

Substances exist in different phases , for example , at a room temperature and pressure , steel is in solid form , mercury is in liquid form and nitrogen in gaseous form.

principal phases-solid , liquid and gas with distinct molecular structures. 

However , a substance may have several phases within a principal phase , each with different molecular structure.

Eg : Carbon in solid phase may exist as graphite or diamond.

       Iron has three solid phases (alpha , gamma , delta).

       Helium has two liquid phases(Helium 3 , Helium 4).

       Ice may exist at seven different phases at high pressure.

A phase is identified as having a distinct molecular arrangement that is homogeneous throughout and separated from other phases by easily identifiable boundary surface.

Eg. Two faces of H₂O iced water , where the boundaries are easily identifiable.

Molecular bonds are the strongest in solids , and the weakest in gases.

The molecules in a solid are arranged in three dimensional pattern (lattice) and repeated throughout the solid. Molecules in a solid do not move relative to each other , but continuously oscillate about their equilibrium positions ; velocity of oscillation depends on temperature.

At sufficiently high temperatures , the velocity and momentum of molecules partially overcome inter-molecular forces , and molecules break away , starting of melting process-liquid phase. In liquid phase , the molecular spaces may not be much different to those of solid phase , but they do not have fixed positions relative to each other.The distances between molecules generally increase slightly as a solid turns to liquid , but water is rare exception.

In the gas phase , the molecules are far apart from each other , and there is no order of arrangement. Molecules in gas phase are at a considerably higher level of energy than they are in liquid or solid phases. Therefore a gas must release a large amount of its energy before it can condense or freezes.    

Phase change process of pure substances:

There are many practical situations where two phases of a pure substance coexist in equilibrium.

Eg. Liquid and vapour coexist in the boiler and the condenser of a steam power plant. 

      Liquid and vapour coexist in the freezer (evaporator) of a refrigerator.

Since all pure substances exhibit same general behavior during phase change , water is used to demonstrate the basic principals involved.

Compressed liquid and saturated liquid: 

Consider water at 20^°C and 1 atm pressure ; under these conditions water exist in the liquid phase , and it is called sub-cooled liquid or compressed liquid ; meaning that it is not close to vaporize .         ( figure 9 , state1)

If heat is added and the temperature of water increased up to 100^°C  , when the water is still in liquid form and is about to vaporize , it is called saturated liquid. (figure 9 , state 2) 

Figure 9 : Phase change process of water.

Saturated vapour and superheated vapour:

Further addition of heat to saturated liquid initiates boiling , temperature rising will stop until all the water is vaporized( increase of volume ). The resulting vapour when all the liquid is boiled out will be at the same pressure as of liquid and at a temperature of 100^°C , which is identified as saturated vapour or in other word vapour which is about to condense is called saturated vapour (figure 9 , state 4). Even a small loss of heat causes this vapor to condense.

Once the phase change is completed further addition of heat increase both the temperature and specific volume of vapour (steam). Slight reduction of temperature will not cause condensation. This vapour is called super-heated vapour.