Chem question! Please help :D!

[Hiba Alkindi]

Ok Hi everyone and good luck with your OCT/Nov exam
okay I have a question which might sound silly but I really couldn't find the perfect answer!
My question is :
Why does the temp. remains constant between 2 points during freezing?!
and oh I have another question :
at 1 atmospheric pressure a substance in -85 degrees Celsius is a liquid and at -65 degrees Celsius is a gas! So what is it boiling and melting points from these four answers:
A) MP:-105 BP:-50
B) MP:-102 BP:-70
C)MP:-84 BP:-51
D)MP:-66 BP:-42


AND THANK YOU VERY VERY MUCH !

 

[Prithvi Krishna]

First question i am not sure................ but for the second one the answer is B as the substance is already a liquid at -85 degrees which means it already melted and should have a melting point less than -85 and it is a gas at -65 degrees which means it has already been boiled and evaporated and the boiling point should be between -85 and -65. option B only meets these. hope i helped

 

[DarkEclipse]

Since the second question is answered, I'll answer the first.
This is called a "phase transition" and until all of the substance is completely frozen, the temperature remains constant.Any change of phase occurs with a change in heat energy.The heat energy exchanges present during a change in phase are changes in potential energy. In none of the changes of phase is heat input used to speed up the molecules, nor is heat output ever occurring as a result of the molecules slowing down. The average speed of molecules is the same before and after a phase change, so the average kinetic energy (temperature) is the same.

This is a question more or less based on Latent Heat, and if it is, it can also be answered as:
The temperature remains constant during the process of freezing. When any liquid is at its freezing point, the further removal (or loss of) LATENT HEAT of Freezing (or Solidification), will cause change from the Liquid State to the Solid State... WITH NO CHANGE IN TEMPERATURE. The heat removed in the transition to the solid state at the same temperature, goes into removing sufficient heat energy as Kinetic Energy to cause the molecular structure of the solid state to form. In the case of freezing water, the molecules form a 'Crystal Lattice Structure' of solid ice.

Either way, both of these answers are correct (at least in my opinion).