This document discusses diffusion in solids. It defines diffusion as the tendency of molecules, atoms, or ions to distribute evenly in space due to random kinetic motion. Diffusion occurs due to concentration gradients and stops when concentrations equalize. In solids, diffusion results from thermally-activated random atomic motion. There are two mechanisms - interstitial, where small atoms fit between others, and vacancy, where atoms jump into vacant sites. Examples given are carbon diffusion in steel via the interstitial mechanism, salt dissolving in water as a solid-liquid diffusion example, and dust particles suspended in air as a solid-gas diffusion example.

1. NAME : Muhammad Hamad Qureshi
ROLL NO. : D-19-CH-61
SUBJECT : Mass transfer
TOPIC : Diffusions in solids
DEPARTMENT : Chemical engineering

2. What is Diffusion :
Tendency of molecules, atom or ions of solids, liquids and
gases to evenly distribute themselves in the available space due to
the random kinetic motion.
Whenever there is concentration difference, diffusion occurs
because of this random movement of molecules. Diffusion occurs
as a result of the 2nd law of thermodynamic which states that the
entropy or disorder of any closed system which is not in
equilibrium must always increases with time (until it reaches
equilibrium). According to Graham's law of diffusion the rate of
diffusion is directly proportional to temperature, but inversely
proportional to the square root of the molar mass of the gas.
Diffusion is rapid in liquid and gaseous states as compared to the
solid state diffusion due to lower inter atomic bonds. The rate of
diffusion is affected by the concentration gradient, temperature,
and pressure. Diffusion continue as long as there is a difference
between the concentrations of a substance across a barrier.
However, diffusion stops, when the concentrations of the
substance on either side of the barrier become equal.

3. Diffusion in solids :
As diffusion can take place in gases, liquids, and solids. In solids,
diffusion occurs due to thermally-activated random motion of atoms
- unless the material is at zero Kelvin or zero absolute(-273.16C).
Mechanisms of diffusions in solids :
The presence of different types of defects gives rise to different
mechanisms of diffusion.
1. Interstitial mechanism:
If an atom on an interstitial site moves to one of the neighboring
interstitial sites, then diffusion is occurs by an interstitial
mechanism. It is suitable for species that have enough small atom
to fit into the interstitial sites e.g. Hydrogen.

4. 2. Vacancy Mechanism :
The diffusion is said to take place by the vacancy mechanism if an
atom on a normal site jumps into an adjacent unoccupied lattice site
(vacancy). It should be noted that the atoms move in the direction
opposite the vacancies. The Vacancy mechanism of diffusion is slower
than the interstitial mechanism because bonding of interstitials to the
surrounding atoms is normally weaker and there are many more
interstitial site than vacancy sites to jump to.

5. Examples of diffusion in solids :
1. Solid-solid :
Steel is an example of a solid–solid solution. It is an iron
solvent with a carbon solute. The carbon atoms do not replace the iron
atoms, but fit in the spaces between them. It happens in accordance
with interstitial mechanism and is often called an interstitial alloy.
Other examples are metals alloys pearls and opals.
2. Solid-liquid :
Salt in water is an example of solid-liquid solution. When solids comes
in contact with the liquid it dissolves forming a solution. When shaking
salt into water, the salt dissolves and the ions move until they are
evenly distributed. The process through which formation of a solution
of the salt in the water takes place is called dissolution.

6. 3. Solid-gas :
Dust particles in smoke are an example of solid-gas solution.
Aerosols are the fine solid particles suspended in the air. Examples
of natural aerosols are fog, mist, dust, forest exudates and geyser
steam. The direct conversion of solid into gas by the process called
sublimation (naphthalene, benzoic acid, ammonium chloride and
benzene are the substances that follow sublimation process).

7. Thank You