Mass transfer can also occur in liquids and solids as well as in gases. For example, a cup of water left in a room will eventually evaporate as a result of water molecules diffusing into the air (liquid-to-gas mass transfer).
A piece of solid CO2(dry ice) will also get smaller and smaller in time as the CO2 molecules diffuse into the air (solid-to-gas mass transfer).
A spoon of sugar in a cup of coffee will eventually move up and sweeten the coffee although the sugar molecules are much heavier than the water molecules, and the molecules of a colored pencil inserted into a glass of water will diffuse into the water as evidenced by the gradual spread of color in the water (solid-to-liquid mass transfer.
Of course, mass transfer can also occur from a gas to a liquid or solid if the concentration of the species is higher in the gas phase. For example, a small fraction of O2 in the air diffuses into the water and meets the oxygen needs of marine animals. The diffusion of carbon into iron during case-hardening, doping of semiconductors for transistors, and the migration of doped molecules in semiconductors at high temperature are examples of solid-to-solid diffusion processes
We have spent a considerable amount of time studying heat transfer, and we could spend just as much time (perhaps more) studying mass transfer. How-ever, the mechanisms of heat and mass transfer are analogous to each other,and thus we can develop an understanding of mass transfer in a short time with little effort by simply drawing parallels between heat and mass transfer.
Establishing those “bridges” between the two seemingly unrelated areas will make it possible to use our heat transfer knowledge to solve mass transfer problems. Alternately, gaining a working knowledge of mass transfer will help us to better understand the heat transfer processes by thinking of heat as a massless substance as they did in the nineteenth century. The short-lived caloric theory of heat is the origin of most heat transfer terminology used today and served its purpose well until it was replaced by the kinetic theory.
Mass is, inessence, energy since mass and energy can be converted to each other according to Einstein’s formula
E = mc^2
, where c is the speed of light.
Therefore, we can look at mass and heat as two different forms of energy and exploit this to advantage without going overboard.
