The **key difference between critical constant and Van der Waals constant** is that critical constant refers to the temperature, pressure, and density values for a substance at critical point, whereas Van der Waals constants give the temperature, pressure, and density values for a substance at any point.

The only difference between critical constant and Van der Waals constant values is the point at which the values are calculated. Therefore, we can derive critical constant values using Van der Waals constant values through using the critical point values in the Van der Waals equation.

### CONTENTS

1. Overview and Key Difference

2. What is Critical Constant

3. What is Van der Waals Constant

4. Critical Constant vs Van der Waals Constant in Tabular Form

5. Summary – Critical Constant vs Van der Waals Constant

## What is Critical Constant?

Critical constant is the critical temperature, critical pressure, or critical density of any substance. This term is usually taken as a plural noun because it can refer to three conditions (temperature, pressure or density) at the same point. We can abbreviate critical temperature as Tc, critical pressure as Pc, and critical density as Vc. Moreover, we can calculate critical constants using the values of the Van der Waals constants.

Typically, critical constant values are given for the critical point of a substance. The critical point of a substance is the endpoint of the phase equilibrium curve of that substance. A phase equilibrium curve or a phase diagram is the graph of pressure versus temperature in which the phase changes of the substance are shown. This shows the temperatures and pressure at which the substance exists as a solid, liquid, or gas. The critical point is the temperature and pressure at which the liquid and vapour phases coexist.

## What is Van Der Waals Constant?

Van der Waals constants are the constant values we use in the Van der Waals equation. Van der Waal equation is the modified version of the ideal gas law. This equation can be used for ideal gases as well as for real gases. The ideal gas law cannot be used for real gases because the volume of gas molecules is considerable when compared to the volume of the real gas, and there are attraction forces between real gas molecules (ideal gas molecules have a negligible volume compared to the total volume, and there are no attraction forces between gas molecules).

Here, “a” is a constant that depends on the type of gas and “b” is also a constant that gives the volume per mole of gas (occupied by the molecules of gas). These are used as corrections of the ideal law equation.

The volume of a real gas molecule is not negligible (unlike in ideal gases). Therefore, the volume correction is done. (V-b) is the volume correction. This gives the actual volume that is available for the gas molecule to move (actual volume = total volume – effective volume).

The pressure of a gas is the pressure exerted by a gas molecule on the wall of the container. Since there are attraction forces between real gas molecules, the pressure is different from that of ideal behaviour. Then a pressure correction should be done. (P + a{n/V}2) is the pressure correction. (Ideal pressure = observed pressure + pressure correction).

## What is the Difference Between Critical Constant and Van Der Waals Constant?

Critical constant and Van der Waal constant are terms that are slightly different from each other. The key difference between critical constant and Van der Waals constant is that the critical constant refers to the temperature, pressure, and density values for a substance at the critical point, whereas the Van der Waals constants give the temperature, pressure, and density values for a substance at any point.

The following table summarizes the difference between critical constant and Van der Waals constant.

## Summary – Critical Constant vs Van Der Waals Constant

Critical constant and Van der Waal constant are terms that are slightly different from each other. The key difference between critical constant and Van der Waals constant is that the term critical constant refers to the temperature, pressure, and density values for a substance at the critical point, whereas the Van der Waals constants give the temperature, pressure, and density values for a substance at any point.

##### Reference:

1. Baratuci, William. “Determining the Van Der Waal Constants.” *Ch2, Lesson F, Page 6 – Determining the Van Der Waal Constants*.

##### Image Courtesy:

1. “Phase-diag2” By Matthieumarechal (CC BY-SA 3.0) via Commons Wikimedia

## Leave a Reply