• Mass transfer by diffusion in a binary gas mixture. We know that molecules of elements have mass.
• When they move from one location to another location, we consider mass transferring, and the topic to study the movement is called mass transfer.
• Let’s visualise how this can be understood.
For example, if we take a bunch of marbles (instead of molecules) in a container. With a slight movement of the container, the marbles move from a higher level to a lower level.
Similarly, the molecules move from the position of high concentration to lower concentration in a mixture, instead of the elevation effect that causes the marbles to move.
For better understanding at the molecular level.
Let's take a gas container with a partition. Two inlets are provided to fill the space in the container with gases. For example, let's say ammonia and air or carbon dioxide and air.
When the partition is removed, the spontaneous movement of molecules happens without any external aid. The concentrated portion of each species moves to a new location by spreading, which we call ‘diffusion’. After some time, uniformity attains overall the mixture.
By observing the graph the slope of the smooth curve give the ratio of the change in concentration and distance of movement of the species. It is called the Concentration gradient. The rate of mass transfer is directly proportional to this concentration gradient

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