Gas phase

[Odiljon]

Hi, I am not professional engineer yet, but I am trying to be reading some engineering books.
Recently I read a book, where was written " system cannot contain more than one gas phase but can contain any number of liquid and solid phases.
e.g. An alloy of copper and nickel, for example, contains two solid phases"

But how is it possible, we know that each gas has different density, so the heavier gases are in lower part of the system.
How to understand the wording? could you explain me please?

 

[TD2K]

If you mix oil and water, you can see two distinct phases.

When you mix two gases, even though they can have different densities they form a single phase. Air is a mixture of oxygen and nitrogen primarily. Even though the oxygen is heavier than the nitrogen, the two gases exist as a single phase.

http://en.wikipedia.org/wiki/Phase_(matter)

If I'm not understanding your question please rephrase it.

 

[moltenmetal]

I believe the term that is missing is "at equilibrium".

You can, with care, produce a layer of one gas on top of another. One layer will have one set of properties (density, thermal conductivity etc.) and the other will have different properties- i.e. there will be two distinct gas "phases"- for a while. However, all gases are infinitely mutually miscible: if you wait long enough, concentration gradient-driven diffusion and even mere molecular motion will generate a single gas phase with uniform properties throughout. A mixture of gases, even say hydrogen and zenon which vary greatly in density, will NOT separate into a layer of hydrogen rich zenon at the top and a layer of hydrogen-lean zenon at the bottom, no matter how long you wait.

Similarly, you can make a layer of methanol on top of a layer of water. Wait a while, and you will have a uniform methanol/water mixture.

Not so with immiscible liquids. Wait as long as you like, and they will remain in two distinct layers, with different properties from one another at equilibrium. You can have separate phases within a stable single pseudo-phase such as an emulsion of oil in water stabilized by a surfactant, but each phase has distinct bulk properties.

 

[MortenA]

moltnmetal,

I dont think that is a valid example. The two gas phases that you mentions are NOT two destict phases as oil/water or different solid phases, but a concentration gradient that even no agitation over time will even out due to brownian movement etc.

The same goes for the MeOH eaxmple - not two phases but a gradient.

Best regards

Morten

 

[IRstuff]

"PHASE RULE

(Physics) A rule stating that the number of degrees of freedom in a material system at equilibrium is equal to the number of Components minus the number of Phases plus the constant 2. For example, the system of water vapor, liquid water, and solid ice has zero degrees of freedom because the three phases of vapor, liquid, and solid coexist in one component, water. A solution of salt in water, for example, is a chemical system in which the components are salt and water. The chemical components of a system can exist as gas, liquid, or solid phases. The phase rule is applicable only to systems, called heterogeneous systems, in which two or more physically distinct phases are in equilibrium. A system cannot contain more than one gas phase but can contain any number of liquid and solid phases. A water solution of salt contains three phases: Salt comprises the solid phase, water comprises the liquid phase, and water vapor formed by evaporation of the water comprises the gas phase. Water is an example of a heterogeneous chemical system of one component. The liquid and gas phases, water and water vapor, coexist over a wide range of temperatures and pressures. At one temperature and pressure, called the Triple Point, all three phases of water (liquid), water vapor (gas), and ice (solid) coexist at equilibrium. The phase rule is expressed by the equation F = C - P + 2, where F is the number of variables (usually temperature, pressure, and concentration) that can be changed without causing the disappearance of a phase or the appearance of a new one. C represents the number of chemical components of the system and P the number of phases present."

The Wikipedia article seems to be a little more clearer:

http://en.wikipedia.org/wiki/Phase_rule

TTFN
faq731-376

 

[moltenmetal]

"The phase rule is applicable only to systems, called heterogeneous systems, in which two or more physically distinct phases are in equilibrium."

QED