Chemistry Challenge Involving Water

[engineerjoe]

Hello,

 

I have been working on an idea for a class room model that would illustrate 2 topics of education: water cycles and liquid properties. The goal would be to place water in a chamber or series of chambers and apply a vacuum to lower the boiling temperature. A heat source less than 212 degrees F would be applied to boil the water and show the relationship between pressure and temperature and water phases. The water vapor would then condense and return to a liquid form and begin the cycle again demonstrating the water cycle. Research on this subject leaves me with two questions that I have been stuck on for over a week:

 

1. All examples I have found show applying a continuous vacuum to the apparatus in order to keep the pressure low when water vapor is being created. Is there any way to create a vacuum chamber and then ‘cap’ the system so it is self-contained and does not require leaving a vacuum pump running? What if the initial pressure was so low that the water vapor created did not increase the pressure significantly? What about mechanically expanding or compressing the chamber?

 

2. I have read about ‘de-gassing’ a liquid in a vacuum and my understanding of this is when the soluble air particles in the water are pulled out. This creates the appearance of boiling but it is deceptive and only lasts about 60 seconds. Once this stops, if the temperature is raised and the pressure is lowered will the water ‘boil’ or does degassing prevent boiling from happening?

 

I appreciate all of your help on this subject. Chemistry was never my strong point in school and I am attempting to work with what little knowledge I have.

[Mr Skeptic]

1. So long as you use a heat source and have a colder spot to condense, the water should boil/evaporate from the hot part and condense on the cold. You'd have to have a good condenser to do this I would think. Maybe with ice water. So long as you have the water vapor condensing, that should decrease the pressure so that it can boil at a lower temperature.

 

However, it is generally ill-advised to boil things in sealed containers. Are you sure your heat source will not surpass 212 F? Because if it does, you could end up with an explosion and glass shards flying everywhere. Generally when I use a condenser, the tip of it is open to the atmosphere.

 

2. Degassing occurs because gases are less soluble at high temperatures. This is opposite of what people are used to with solids, but to dissolve the gas molecules have to slow down whereas the solid molecules would have to speed up. Anyhow, there is not very much air in the water and it will all be gone soon enough. Either heating, or a vacuum, should degas the water. After all the dissolved gas is removed, it will no longer degas.