Difference Between Density and Vapour Density

The key difference between density and vapour density is that the term density measures the mass per unit volume of any substance that can be a solid, liquid or a gas whereas the term vapour density refers to the density of vapour of a substance in relation to the density of the vapour of hydrogen.

The term vapour density is greatly different from the regular term “density” because vapour density specifically describes the density of a vapour as a relative value.

1. Overview and Key Difference
2. What is Density
3. What is Vapour Density
4. Side by Side Comparison – Density vs Vapour Density in Tabular Form
5. Summary

What is Density?

Density is the mass per unit volume of a substance. Density is an important characteristic of matter. It is directly connected to mass. Therefore, getting a clear understanding of it is vital to have a clear understanding of mass. Accordingly, mass is a measurement of the inertia of an object.

For a bulk material with uniform mass distribution, we can easily calculate this parameter by dividing the total mass of the object by the total volume occupied. However, if the mass distribution is not even, we need more complicated methods to measure the density.

Difference Between Density and Vapour Density

Figure 01: A Density Column

Furthermore, we can easily describe the floatation of a substance using its density. Here, floatation means that a fluid or a uniform solid that is denser than a given fluid will drown in the given fluid. Thus, if the density of the fluid or the uniform solid is lesser than that of the given fluid, it will float on the given fluid. Moreover, we can define the term relative density in order to compare the densities of the two fluids. This is the ratio of the two densities and is merely a number.

What is Vapour Density?

Vapour density can be defined as the density of vapour in relation to the density of vapour of hydrogen. On the other hand, we can describe it as the mass of a certain volume of a substance divided by the mass of hydrogen with the same volume. Therefore, there can be several relationship expressions regarding the vapour density as follows:

Vapour density = mass of n molecules of gas/mass of n molecules of hydrogen

Vapour density = molar mass of gas/molar mass of H2

Vapour density = molar mass of gas / 2.016

As an example, the vapour density of a mixture of NO2 and N2O4 is 38.3. Further, vapour density is a unitless quantity.

However, some people tend to define the vapour density with respect to air instead of hydrogen. Here, we can give the air a vapour density of and the molecular weight of air is taken as 28.97 amu. This is an average value. We can divide all other gas and vapour molecular weights by this number to get their vapour density. E.g. vapour density of acetone is 2 in relation to air. In other words, the vapour of acetone is twice as higher as the density of normal air. Therefore, using this definition, we can determine whether a gas is more or less denser than air, which is an important indication for container storage of gases and personnel safety.

What is the Difference Between Density and Vapour Density?

Density is an absolute value, while vapour density is a relative value. The key difference between density and vapour density is that density measures the mass per unit volume of any substance that can be a solid, liquid or a gas whereas vapour density refers to the density of vapour of a substance in relation to the density of the vapour of hydrogen.

The below infographic lists the differences between density and vapour density in tabular form for side by side comparison.

Summary – Density vs Vapour Density

The terms density and vapour density are distinct from each other. The key difference between density and vapour density is that density measures the mass per unit volume of any substance that can be a solid, liquid or a gas whereas the term vapour density refers to the density of vapour of a substance in relation to the density of the vapour of hydrogen.