The phase diagram is a convenient way to indicate the phases of a substance as a function of tempera — Analytical Chemistry Chemistry Question
Phase Diagram
The phase diagram is a convenient way to indicate the phases of a substance as a function of temperature and pressure. Answer the following questions based on the phase diagram of water given below:
[VISUAL]
What phases are present at A, B, and C?
Model Answer
A: solid; B: solid, liquid, and gas states coexist; C: liquid and gas states coexist.
Why does ice not sink in its own liquid?
Model Answer
The negative slope of the solid/liquid line indicates the liquid state of water is denser than its solid state. Therefore, ice may not sink in its own liquid.
When water freezes, it expands. Explain this observation using the Clapeyron equation, which may be expressed by:
dp/dT = ΔH / (T ΔV)
Here, ΔH and ΔV denote the change of molar enthalpy and molar volume of water, respectively.
Model Answer
Clapeyron equation is expressed as dp/dT = ΔH / (T ΔV), where ΔH is molar enthalpy of water and ΔV is volume change. The phase diagram shows that the slope of dp/dT for the liquid–solid coexistence region is negative, indicating the volume expands when water freezes.
A glass container partially filled with water is connected to a vacuum pump. What changes will be observed when the pump is turned on?
Model Answer
As pressure is lowered, liquid phase transforms directly to gas phase at the same temperature. Thus water may vaporize. At the same time, the process of water evaporation is endothermic as to make the surrounding cooled. The left water becomes frozen. The solid state will sublime until none is left, if the pump is left on.
A man is skating on the surface of a sheet of ice where the air pressure is 1 atm and the temperature is 0 °C. What changes will be observed along the skating track on the surface of the ice, assuming the ice can withstand his weight without cracking?
Model Answer
The ice surface, exerted by a pressure more than one atm, turns to liquid state at 0 °C.