Thermodynamics water/water vapor question

[paul_harris77]

I am having problems with a Thermodynamics question:

A tube has its axis vertical with internal diameter 0.005m and length 0.2m. The tube is evacuated and a mass of water is added and then the tube sealed. Thus the tube contains both liquid water and water vapor, with a meniscus between the two.

The tube is then heated slowly to a temperature above 647.3K and the meniscus is observed to move upwards or downwards, depending on whether the total mass of liquid and vapor is above or below a critical value. If it is above, the meniscus rises towards the top of the tube before disappearing. If it is below, the meniscus falls towards the bottom of the tube before disappearing.

What is the value of the critical mass?

I am struggling to work out why the meniscus would move up at all.

Surely its maximum height is the height of the meniscus at the very beginning when all of the water is liquid?

Once the water is added, some evaporates as the pressure in the tube is initially 0. Then it would reach an equilibrium of water vapor and liquid water. Wouldn't this cause the meniscus to drop as the volume of water liquid decreases?

The question then states that the temperature is increased above 647.3K. Surely this would cause the saturation vapor pressure to increase and the meniscus to fall further?

Any help would be greatly appreciated.

Many thanks

Paul

 

[Ygggdrasil]

For a given volume of gas, the gas will exert a higher pressure at higher temperatures. You need to take this fact into account as well as the increased vapor pressure of water at higher temperatures.