Question

Below is a cooling curve for an unknown substance. What phase is present between A and B? What is happening between point B and C on the graph? What is the name of the process and describe using a particle model. What phase is present between C and D? What is happening between point D and E on the graph? What is the name of the process and describe using a particle model. What phase is present between E and F? Is heat being added or subtracted from this system?

Answer 1

1. Since only three phases are known in which matter exists and the temperature profile has three regions where we have a temperature gradient with time signifies that these three temperature gradients represent the three different states. So the first phase (A-B) with the highest temperature should be the gaseous phase.

2. Between point B & C the unknown substance is moving from gaseous phase to liquid phase and the process is called condensation. During this process the particle of the unknown substance come close to each and form a phase which has strong intermolecular forces to bind the particles together but weak enough to allow it have fluidic properties.

3. After gaseous liquid phase is present between C and D.

4. Between point D and E again phase change from liquid to solid takes place and the process is called freezing. During this process the particles of the substance come close to each and form tightly packed structure with strong intermolecular forces and high lattice structure energy.

5. Between E & F solid phase is present.

6. As the temperature of the system is falling down with time and the $\Delta S$ of the system is also decreasing, so a net heat is being taken out of the system or heat is being substracted from the system.