Question

(a) Just like the familiar liquid solutions, two metal ions or atoms combine to form solid solution. What are substitutional solid solutions and interstitial solid solutions? Explain with examples and sketches.
(b) How well the solid atoms combine to form solid solutions depends on how many of the Hume-Rothery rules have been violated or not violated. Describe Hume-Rothery rules?

Answer 1

a)

Substitutional Solid Solution

Subsitutional Solid Solutions means that the atoms of the Solute are substituted at the atomic sites of the Solvent. Basically, the solute atom replaces the atom of the solvent. An example of Substitutional solution would be a Gold Copper alloy (Au-Cu) . Since gold is soft, is added to it. The Gold is the solvent while the copper is the solute. The Copper atom replaces the atom of gold to form the alloy.

Interstitial Solid Solution

Interstitial Solid Solutions means that the atoms of the Solute are substituted at the interstitial sites of the Solvent. The interstitial sites are the spaces between atoms. A good example of Interstitial Solid solution is Carbon in Iron. Carbon is the solute, while Iron is the Solvent. The carbon atoms occupy the interstitial sites of the Iron atoms. This increases the strengh of the solid solution which is primarily known as Steel.

b)

Hume Rothery Rules

The ability of solid atoms to combine to form solid solutions depends on their solubility. One element is the solute (small amount) and the other is the solvent (large Amount). The solubility limit of the solute in the solvent is determined by the Hume Rothery Rules. These are as follows

1. Atomic Size Factor : If the atomic sizes of the Solute and solvent differ by less than 15 percent, it is said to be favourable atomic size factor for formation of solid solution. The Solid Solubility is limited if their difference is greater than 15 percent.
2. Chemical Affinity Factor : Higher the chemical affinity of two elements, more restricted is their solid solubility. This leads to a greater tendency for the formation of a compound. If the elements are separated widely in the Periodic Table, their chemical affinity will be more.
3. Relative Valency Factor : To achieve maximum solubility, it is desirable that the valency of the two elements is same. A metal with higher valency can dissolve only a small amount of a metal with lower valency. At the same time, it may be possible that the metal wth lower valency has excellant solubility for the metal with higher valency.
4. Crystal Structure Factor : Identical Crystal structure means greater solid solubility. The solid solubility gets limited with difference in crystal Structure.