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Old December 20th, 2012, 16:01   #14
Rommen
 
Join Date: Jul 2011
Location: Mississauga, Ontario.
You are confusing several concepts, albeit in a fairly logical way. Let's see if we can sort this out.

Quote:
Originally Posted by Ricochet View Post
That doesn't even make sense. Something that compresses into a liquid/gas or whatever can be compressed further depending on its properties. If you compress LPG to the point it becomes a liquid, do you know what happens when you compress it a little further? It becomes a more dense liquid.
First, for most purposes in fluid dynamics, liquids are treated as incompressible. Yes, in reality they may compress slightly at high pressures, but so little that the effect is considered negligible. Therefore for the purposes of answering this question we will assume that once gas-phase propane has been compressed to a liquid it will compress no further.

Quote:
Originally Posted by Ricochet View Post
The temperature drops slightly, the molecules come closer together, and if more is stored in a container; the pressure increases.
Following from the first point comes the concept of vapor pressure. The vapor pressure is the pressure that, in this case, the gas-phase propane exerts on the container walls. The gas will exert the same pressure as long as: there is liquid in the container to maintain equilibrium, the volume of the container remains constant, and the temperature remains constant.

Also, when considering this question temperature must be held constant for both containers. When measuring pressure one must always make note of the temperature as well, as pressure varies directly according to the temperature. Lowering the temperature decreases the vapor pressure - meaning that if Green Gas and propane are the same, their respective vapor pressures will change by the same amount. The only time temperature may be considered a variable was if the container was filled right to the top. If the liquid was put in at a lower temperature, the container sealed and allowed to warm, the liquid would exert a higher pressure due to physical expansion, not a higher vapor pressure. But seeing as propane cans are always left with gas in them, this is irrelevant.

Quote:
Originally Posted by Ricochet View Post
Water is a great example, and shares many similar properties with LPG. Water can be a dense vapor, or a light vapor. It's not like it thaws into a liquid at 1*C, and then boils at 2*C.
Actually, water isn't a very good example from a chemical standpoint. A 'dense' or 'light' vapor is actually just the amount of saturation present; ie a kettle inside a dry house will emit a 'light' vapor as the humidity in the house is low enough to accommodate more airborne water molecules. Conversely, fog is a result of having reached the airborne water saturation point and the excess water molecules float at low level. This is dissimilar from propane as it is not in a closed system and is already present in the atmosphere in significant amounts.

Another reason water isn't the best example is that it exhibits hydrogen bonding, giving it very strange properties in the behavior of its density at low temperatures (notably from 5C to -1C). Propane does not exhibit this and will behave much differently.

Quote:
Originally Posted by Ricochet View Post
It's a liquid until it boils or freezes, and has a different temperature and density at each point between while maintaining its liquid state. Propane is no different, and maintains its liquid form throughout a range of temperature and density.
This statement is technically correct but needs cause and effect to clarify. Propane can be found as a liquid through a variety of temperatures, yes. AT these temperatures, the liquid-phase propane will be in equilibrium with gas-phase propane, which will EXERT a certain pressure. However at certain temperatures or pressures it will stop being a liquid and turn entirely into either a gas or a solid. There is a finite combinations of pressure (changed through container size) and temperature that will result in liquid-phase propane. Going back to what was said earlier about temperature, we will find that the only way to change the pressure while holding everything else constant is to change the temperature, which will result in the same variance for both Green Gas and propane.

And the most important thing of all...

Quote:
Originally Posted by Ricochet View Post
It's physics.
No it's not, it's chemistry! : P
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