Question

A pure solid sample of Substance X is put into an evacuated flask. The flask is heated at a steady rate and the temperature recorded as time passes. Here is a graph of the results: 190.- TI temperature (°C) 150. 30. heat added (kJ/mol) Use this graph to answer the following questions: What is the melting point of X ? 1°C What phase (physical state) of X would you expect to find in the flask after 19 kJ/mol of heat has been added? (check all that apply) solid liquid gas

Answer 1

A pure sample of substance X is put in a evacuated flask, and heated at constant rate.

Temperature (C) and heat added (KJ /mol) graph was plotted, this graph shows two phase transition state. At phase transition a substance changes phase i.e. solid to liquid or liquid to gas.

At phase transition state temperature remains constant and heat is absorbed as Latent heat of phase change (Heat of Fusion or Heat of vaporisation).Phase transition occurs at a particular specific temperature , which is termed as Melting point (Solid to liquid) and Boiling point (liquid to gas).

Phase change order : Solid $\rightarrow$ Liquid   $\rightarrow$ Gas  

Heat of vaporisation is much more than Heat of Fusion , as larger need of energy for vaporisation.

• Melting point of X = 135^o C ( As it is clear from graph , it is temperature of First phase transition)
• Physical state after addition of 19 KJ/mol of Heat = at this stage phase transition occurs from liquid to gas phase , we can expect presence of both Liquid and Gas states in equilibrium.