Difference Between Vapor and Gas

Introduction

Vapour is described as a substance that usually exists in the gaseous phase at a temperature (t) below its critical temperature (𝑇𝑐). That means the vapour can be frozen back to a liquid by or through exceeding pressure (p) on it and without decreasing the temperature. vapour pressure is the pressure at equilibrium that occurs from a liquid (l) or a solid (s) at a certain specific temperature.

And the vapour pressure (p) of a liquid (l) or a solid (s) at equilibrium is not get affected or hampered by the quantity of contact with the liquid (l) or the solid interface. Gas is one of the very important fundamental or universal states of matter. Gas is a mixture of a variety of pure gases. The gaseous state of matter takes place between the liquid (l) and the plasma states. In the gaseous phase, molecules are in random motion, and it has no fixed shape or size, if we supply very high pressure to it, they may become liquid or solid.

What is vapour?

Vapour is nothing but a gaseous substance that exists at a temperature (T) below its critical temperature. It means that vapour (gas) can be frozen or condensed to form a liquid by exceeding the pressure (P) on this and without lowering the temperature. In our atmosphere in the presence of ordinary temperatures (T) gaseous water also called water vapour will be frozen or condensed back to the liquid phase only if its partial pressure (p) gets increased or exceed sufficiently. A vapour may exist or co-exists with a different state which is a solid state. And, when this becomes true, the two phases that are vapour and solid will be in equilibrium and the equilibrium vapour pressure (v.p.) of the liquid (or solid) will be equal to the partial pressure of the gas. One commonly known example is water, which has a critical temperature (𝑇𝑐) of 374 °𝐶. And, this is the highest or maximum temperature at which liquid (l) water can exist.

Characteristics

• Vapours (water vapours) are generally transparent like most of the other constituents of the atmosphere.

• The constituent molecules of the vapour possess different types of motions and those motions are vibrational, rotational, and translational motion.

• Vapours are gases formed at a temperature below the critical temperature.

• The Colour of the vapour formed completely depends on the substances that undergo vaporization.

• The vapour cloud formed is long, thin, and generally found in the shape of a cigar.

• vapours can travel long distances but under certain meteorological conditions.

What is vapour Pressure?

Vapour pressure also called equilibrium (zero external forces) vapour pressure is described as the pressure (P) that is exerted by the vapour in the thermodynamic equilibrium that is present with its frozen phases (liquid or solid) at the given temperature (T) in a closed system.

For any liquid, its evaporation rate can easily be indicated by the equilibrium vapour pressure. It also indicates the tendency or capability of the particles to leave or escape from the liquid (l) or solid. A substance or material present with a higher vapour pressure (v.p.) at normal temperatures (T) is usually termed as volatile. vapour pressure can also be defined as a pressure exerted by the vapour present above the liquid interface or surface.

HellTchi, Vapor pressure, CC BY-SA 3.0

Vapour pressure directly depends on the kinetic energy supplied to it. As the kinetic energy (K.E) of the molecules increases or exceeds, the number of molecules converting into a vapour also exceeds, and hence vapour pressure (v.p.) also increases.

Characteristics

• Vapour pressure is directly proportional to the temperature supplied.

• Vapour pressure does not depend on the quantity of the liquid present inside the container, it will only depend on the temperature.

• For a liquid, when it is boiling its vapour pressure remains equal to the external pressure.

• If the intermolecular forces increase, then the vapour pressure will gradually decrease.

• Vapour pressure is independent or not dependent on the surface area of the container or the material.

What is Gas?

Gas is one of the very important states of the matter. Gases can be of pure form, elemental form, or compound form. Gas is a mixture like the air that contains a variety of gases like nitrogen, argon, neon, etc. Gases are generally colourless, and the molecules of the gases are in constant random motion; they don't have any definite shape or size, they can completely occupy the space of the container and they are highly compressible. And the gaseous state of matter takes place or occurs between the other two states of matter, and they are liquid and plasma states. Particles of the gases are widely separated from each other and hence have very weak intermolecular bonds as compared to solids and liquids. Gases generally have low density and viscosity as compared to other states of matter (mainly solid and liquid). Gas particles diffuse to homogenous distribution to fill the container.

U.S. Geological Survey Gas Hydrates, CC0 1.0

Characteristics

• Gases are highly compressible and less dense or have lower density as compared to solids and liquids.

• Gaseous molecules apply equal pressure in all directions of the container.

• Gaseous molecules have higher kinetic energy this is due to the larger spaces present between them.

• The intermolecular forces are very low or almost negligible in the gas.

• Gases can easily take the shape and volume of the containers.

Macroscopic Gas View

As we know physically it is difficult to observe the properties or characteristics of a particular gas. A larger length scale that usually corresponds to or is linked to a macroscopic or global (world) point of view of the gas. The region (called volume) must be satisfactory or sufficient in size so that it can contain or have large components or sampling of the gas particles. At the macroscopic level, the gas characteristics which are calculated are present either in terms or basis of the gas particles those are- velocity (v), pressure (p), temperature (t), or their extra spaces or surroundings (volume).

Microscopic Gas View

The microscopic (very small) view of the gas is well-defined under statistical mechanics; further, it can be explained by various distinct theories. One of those theories is a kinetic gas theory that assumes that the gas is composed of large numbers of extremely minor or small particles which are in constant, random, and straight-line motion. Also, the gaseous molecules occupy or hold very little or small volume as most of the container or boxes is free space. This theory also assumes that the collisions with one another as well as with the walls of containers are elastic and that there are no forces between the molecules. The theory gives information about the speed of the molecules and the frequency of the collision of the molecules.

Difference between vapour and Gas

Vapour	Gas
Vapour is a mixture or combination of multiple (two or more) distinct phases generally liquid and gaseous phases at room temperature.	The gas itself is a unique and single thermodynamic state at room temperature.
Vapour is a collection of different particles without any fixed shape when we observe it under a microscope.	Gas does not have any fixed shape or size when it is observed under a microscope.
Vapour is not considered a state of matter.	Gas is one of the well-known states of matter.
Vapours formed at a temperature below their critical temperature.	Gases usually formed above their critical temperature but below critical pressure.
Vapour at its original state is a solid(s) or liquid(l) at room temperature.	Gas in its original state is gas only at room temperature.

Conclusion

Vapour is a gaseous substance that exists at a temperature (T) below its critical temperature (𝑇𝑐). It means that vapour (gas) can be frozen to form a liquid by exceeding the pressure (P) on this and without lowering the temperature. vapour pressure is defined as the pressure exerted by the vapour present above the liquid interface or surface. vapour pressure directly depends on the kinetic energy supplied to it. Gas is one of the very important states of matter. Gases can be found in the pure form $\mathrm{(Ne)}$, elemental form $\mathrm{(O_{2})}$, or compound form $\mathrm{(CO_{2})}$. Gas is a mixture like the air that contains a variety of gases like nitrogen, argon, neon, etc. Gases are generally colourless, and the molecules of the gases are in constant random motion. There are two gaseous views, and they are the macroscopic view and the microscopic view.

FAQs

1. What do you mean by vapour?

Vapour is the form of gas that forms at a temperature below its critical temperature. Vapour can be frozen to form a liquid by exceeding the pressure(p) on it and without lowering the temperature.

2. What is the S.I. unit of vapour pressure?

unit of vapour pressure is the pascal (Pa) which can also be written as Newton per metre square.

3. How does vapour pressure vary with temperature?

Vapour is directly proportional to the temperature. On increasing temperature vapour pressure increases and vice-versa.

4. What are the two views of the gases?

The two views of the gases are the macroscopic view and the microscopic view.

The microscopic approach uses statistical techniques to consider each molecule's behaviour. From the macroscopic perspective, we are interested in the overall or average consequences of several molecular transgressions.

5. What do you mean by the kinetic theory of gases?

The kinetic theory of gases describes the macroscopic (larger sized) properties of a gas in terms or with the help of the microscopic (smaller sized) nature of the atoms and the molecules used to make the gas which works on various assumptions.