Some heat is supplied to a mixture of ice and liquid water in a closed container and the contents of the container are thoroughly mixed. If, after the addition of heat, both ice and liquid water remain:
A) the temperature of the system will increase
B) the vapor pressure will decrease
C) the vapor pressure will increase
D) the vapor pressure will remain constant.
Our reasoning is that because there is more liquid water, the vapor pressure will increase. Is this correct reasoning?
The temperature will not change since both phases (ice and liquid) still coexist. The vapor pressure is a function of temperature and that will not change either.
So I would pick answer D, however:
The melting of some ice will decrease total liquid + solid volume. This will open up more slightly more gas volume in the container, for air and water vapor.
If the container is sealed and originaly had some air, the total pressure will decrease slightly as a partial vacuum is created. This will have a very small effect upon the temperature of the two-phase mixture. I don't think they expect you to worry about that.
The equilibrium vapor pressure of ice and water at 273K is slightly different, but not significantly. So I agree D. See http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/graphics/esw_esi.gif
Agreed, D is the only sensible answer, given the information supplied.
The temperature is constant, so the vapour pressure for the mixture is constant.
No, your reasoning is not correct in this case. When heat is supplied to a mixture of ice and liquid water, the temperature of the system remains constant until all the ice has melted. This is known as the phase change from solid to liquid, and during this phase change, the temperature does not change even when heat is added.
Therefore, the correct answer is option D) the vapor pressure will remain constant. The vapor pressure of the system depends on the temperature and does not change during the phase change from ice to water. Once all the ice has melted, then further addition of heat will cause the temperature to increase and the vapor pressure to increase accordingly.