[confusedstud]

When we measure the PV isotherm of a real gas, there will be a point where the pressure remains constant and the volume decreases. This is where the gas undergoes liquefaction. So in this case I was taught that the pressure remains constant even though the volume decreases because the number of moles of gas decreases as well. This would mean that we are still measuring the pressure of the gas. Is this true?

But even so, after all the gas had been liquefied to form a liquid, the PV isotherm shows a sharp increase in pressure with a small decrease in volume. So would it mean that after that point we are measuring the pressure of the liquid?

So which pressure are we actually measuring in a PV isotherm?

[mjc123]

You are measuring the pressure of the system. If the gas and liquid are in equilibrium their pressure is the same.

[confusedstud]

You are measuring the pressure of the system. If the gas and liquid are in equilibrium their pressure is the same.

Why would the pressure be the same during liquefaction? I was thinking the pressure remained the same because even thought V decreases n also decrease so the P remains the same according to P=nRT/V. But since its the pressure of both the gas and liquid, with more liquid molecules won't the P also increase?

[mjc123]

No, as long as it doesn't occupy the whole volume and there's still some gas. Reducing the volume will cause some gas to liquefy to restore the equilibrium pressure.