Andrew's experiment on liquefaction of gases

[Kaushik]

Look at the region from C to B.

1. Why is the pressure constant here while the volume of the gas is decreasing?
2. Is it the phase where gas to liquid transition is going on?

What I know

In the region from A to B, as the pressure increases the volume of the gas reduces.
At C, the state of matter completely changes to liquid and how much ever the pressure increases it will remain in the liquid state.
When the temperature is beyond the critical temperature the change in states of matter does not occur.
The pressure and volume corresponding to the critical temperature is critical pressure and critical volume respectively.I don't know exactly what is happening in between point C and B?

 

[phyzguy]

Look at the region from C to B.
1. Why is the pressure constant here while the volume of the gas is decreasing?

PV = nRT. This applies to the gas component only. As we move from C to B, V and n are both increasing in such a way that P stays constant.

2.Is it the phase where gas to liquid transition is going on?

Yes. What is happening is that the amount of material in the gas and liquid phases is changing. As we go from C to B there is more and more gas and less and less liquid. At C it is all liquid, and at B it is all gas.

 

[Kaushik]

PV = nRT. This applies to the gas component only. As we move from C to B, V and n are both increasing in such a way that P stays constant.

Yes. What is happening is that the amount of material in the gas and liquid phases is changing. As we go from C to B there is more and more gas and less and less liquid. At C it is all liquid, and at B it is all gas.

As you said , C to B is the transition phase and there must be some amount of liquid and some amount of gas. At C it is completely liquid, whereas at B it is completely gas.
In between B and C, is the volume(represented by the graph) that of the gas or the mixture of gas and liquid?

My interpretation

1. If the graph represents the volume of the gas, then doesn't the graph indicate that in the liquid phase i.e, before C there should be some volume of gas present. (If and only if the graph represents the volume of the gas and not the mixture. This seems impossible, so I feel like the volume is that of the mixture)
2. If the graph represents the volume of the mixture of gas and liquid , then $\frac{PV}{N} = k$ (at constant $T$) where $k$ is some constant. As you said, $V$ and $n$ of gas decreases in such a way that $\frac{PV}{N} = k$. Same amount of $n$ is converted into liquid with $V_{liquid} < V_{gas}$. Hence, the volume of the mixture reduces as the pressure is kept constant.

So graph representing the volume of the mixture seems correct to me.
Is it true? If yes, is my understanding of why that must be the case right?

 

[mjc123]

Yes, between B and C the volume is the volume of the gas/liquid mixture. If we have a point E on the line between B and C, the volume V_E is the volume of a mixture of gas with molar volume V_B and liquid with molar volume V_C (assuming, for simplicity, that we started with 1 mole of gas).
V_E = n_gasV_B + n_liqV_C
The relative amounts of gas and liquid are given by the lever rule:
n_gas/n_liq = EC/BE