Physics

Sound

Diffusion in Gases

Diffusion in gases is very fast. This is because the particles in gases move very quickly in all directions.The rate of diffusion of a gas, however, depends on its density. Light gases diffuse faster than heavy gases. In the laboratory, the diffusion of gases is usually demonstrated by setting up a gas jar of bromine vapours and a gas jar of air as shown in Figure 28. We will now give some examples of diffusion in gases from our everyday life.
The smell of food being cooked in the kitchen reaches us even from a considerable distance. This can be explained as follows : When food is cooked, some of the substances in food release gases having the smell of food in them. The particles of these gases move very quickly and mix up with air by diffusion.When the air containing these gases reaches our nose, we get the smell of food being cooked in the kitchen

(even without entering the kitchen). Thus, the smell of food being cooked reaches us even from a considerable distance by the process of diffusion (of food gases into the air). And if fish is being fried in a home, then its peculiar sharp smell can be detected even when we are quite far off, due to the diffusion of gases (produced during the frying of fish) into air. Another point to be noted is that the smell of hot sizzling food reaches us even when we are at a considerable distance but to get the smell of cold food, we have to go close to it. This is due to the fact that the rate of diffusion of hot gases (released by the hot sizzling food) is much faster than the rate of diffusion of cold gases released by the cold food.
When we light an incense stick (agarbatti) in a comer of our room, its fragrance spreads in the whole room very quickly. The fragrance of burning incense stick (or agarbatti) spreads all around due to the diffusion of its smoke into the air. The particles of gases produced by the burning of incense stick move rapidly in all directions. They collide with the particles of air present in the room, mix with air and reach every part of the room very quickly.
When someone opens a bottle of perfume in one comer of a room, its smell spreads in the whole room quickly. The smell of perfume spreads due to the diffusion of perfume vapours into air. When the bottle of perfume is opened, then the liquid perfume quickly changes into vapour (or gas). The perfume vapours move very rapidly in all directions in air, mix with the air particles and hence spread with air in the whole room.The leakage of cooking gas (LPG) in our homes is detected due to the diffusion of a strong smelling substance (ethyl mercaptan) present in the cooking gas, into air.

Notes

Sound - Notes

1. 3. Humidity of Air