How to get critical temperature from experiment?

In summary, there are two main methods for determining the critical temperature of a substance. One method is to plot P-V graphs and observe the graph shape. Another method is to use a constant pressure calorimeter and track temperature changes at different pressures to identify the critical point. Additionally, an estimate can be made using surface tension and Eotvos rule, but it may be difficult to measure accurately. It is important to ensure that there is enough liquid initially in the container to accurately determine the critical temperature. Another method is to start with vapor in a container at a fixed temperature and increase the pressure isothermally until liquid starts to form, repeating at higher temperatures until liquid no longer forms. Understanding VLE (vapor-liquid equilibrium) and
  • #1
Outrageous
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Besides plotting P-V graphs,then see the graph shape.
Do we have any other way to get the value of critical temperature of a substance?

Thank you.
 
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  • #2
Outrageous said:
Besides plotting P-V graphs,then see the graph shape.
Do we have any other way to get the value of critical temperature of a substance?

Thank you.

What is your understanding of the definition of the critical temperature? This should help you identify a method.
 
  • #3
Critical temperature of a substance is the temperature which the vapour of the substance cannot be liquefied no matter how high pressure applied to the vapour.
So I should keep the volume constant and then heat the liquid inside the fixed volume until at a temperature ,there will no more liquid? Then at that pressure will be the critical pressure.
 
  • #4
There is nothing really special about critical temperature at any given pressure.* So I don't think there is a direct experiment to look for critical temperature. You can, however, look for critical point which will give you both critical temperature and critical pressure. You do this by following along the transition point until the latent heat of transition goes to zero.

For example, you can use a constant pressure calorimeter, supply heat at steady rate and track temperature change. Look for the place where temperature temporarily levels out - that's your transition temperature. Increase pressure and repeat. The plateau will now have narrower range in energy. Keep increasing pressure until that plateau disappears. That's your critical point.* Edit: Actually, there is an estimate you can make based on surface tension and Eotvos rule. Surface tension at critical temperature does go to zero, so I guess, I shouldn't say that there is nothing special. I suspect, it's very difficult to measure the surface tension near the critical point, but you should be able to at least extrapolate the approximate critical temperature by taking measurements at different temperatures. If nothing else, this will tell you where to look for critical point in finer detail.
 
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  • #5
Outrageous said:
Critical temperature of a substance is the temperature which the vapour of the substance cannot be liquefied no matter how high pressure applied to the vapour.
So I should keep the volume constant and then heat the liquid inside the fixed volume until at a temperature ,there will no more liquid? Then at that pressure will be the critical pressure.

That could work, but you have to be sure that you have enough liquid fraction initially within the container so that all the liquid doesn't evaporate before you reach the critical temperature. Completely filling the container initially should do the trick, but you should do some calculations for various liquids of known VLE and PVT (vapor) behavior to make sure that that is the case.

You can also use the definition. Start our with vapor in a container at a fixed temperature, and increase the pressure isothermally until liquid (fog) starts to form. If it does, start again at a higher temperature. Keep increasing the temperature until liquid no longer forms no matter how high the pressure.
 
  • #6
Chestermiller said:
but you should do some calculations for various liquids of known VLE and PVT (vapor) behavior to make sure that that is the case..

What are VLE and PVT?
 
  • #7
Outrageous said:
What are VLE and PVT?

VLE stands for vapor-liquid equilibrium (saturation pressure vs saturation temperature). PVT stands for pressure, volume, temperature behavior; in this case, I meant it to mean the PVT behavior of the vapor.
 

1. How can I determine the critical temperature of a substance through experimentation?

The critical temperature of a substance can be determined through various experimental techniques, such as differential scanning calorimetry, pressure-volume-temperature measurements, and thermodynamic analysis. These methods involve subjecting the substance to different temperature and pressure conditions and observing its behavior, such as changes in volume or heat absorption. By analyzing the data obtained from these experiments, the critical temperature can be determined.

2. Is it possible to accurately measure the critical temperature of a substance?

While it is possible to measure the critical temperature of a substance through experimentation, it is important to note that there may be some uncertainty in the results. This is due to factors such as experimental error, variations in the composition and purity of the substance, and the complexity of the substance's behavior near its critical point. Therefore, it is important to carefully design and conduct experiments to minimize these uncertainties and obtain the most accurate measurement possible.

3. Can the critical temperature of a substance be predicted theoretically?

Yes, the critical temperature of a substance can be predicted theoretically using thermodynamic models, such as the Van der Waals equation or the Redlich-Kwong equation. These models take into account the physical properties of the substance and can provide a reasonable estimate of the critical temperature. However, experimental validation is still necessary to confirm the accuracy of these predictions.

4. How does the critical temperature of a substance affect its properties?

The critical temperature of a substance is the temperature above which it cannot exist in liquid form, regardless of the pressure. This temperature also marks the point where the substance's properties, such as density and heat capacity, exhibit significant changes. Beyond the critical temperature, the substance displays more gas-like behavior, and its properties become less dependent on temperature and pressure.

5. Can the critical temperature of a substance be manipulated or controlled?

The critical temperature of a substance is a fundamental property that is determined by its molecular structure and interactions. Therefore, it is not possible to manipulate or control this temperature directly. However, by altering the composition or pressure of a substance, it may be possible to shift its critical temperature to a different value. This is commonly done in industrial processes to optimize the properties of a substance for a specific application.

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