Quick Unlimited solubility means that if phase1 and phase2 are mixed, a single phase, phase3, will be produced, regardless of the quantity ratio between phase1 and phase2 (e.g. water and alcohol).
Limited solubility means that, if phase1 and phase2 are mixed, a single phase, phase3, might be produced. But, whether or not a single phase is produced is dependent upon the quantity ratio between phase1 and phase2 (e.g. water and salt)
Details When different materials are combined, as with the addition of alloying elements to a metal, solutions are produced. It may be of interest in how much of each material can be combined without producing an additional phase. In other words, the solubility of one material in another, is of interest.
Unlimited solubility
Limited solubility
No solubility
Unlimited Solubility If a glass of water is one phase, and alcohol is another phase, and the water is poured into the alcohol and stirred, only one phase is produced. The glass contains a solution of water and alcohol that has unique structure, properties, and composition. Water and alcohol are soluble in each other. Furthermore, they display unlimited solubility. Regardless of the ratio of water and alcohol, only one phase is produced by mixing them together.
Similarly, if any amounts of liquid copper and liquid nickel were mixed together, only one liquid phase would be produced. The liquid alloy has the same composition, properties, and structure everywhere, because nickel and copper have unlimited solubility.
If the liquid copper-nickel alloy solidifies and cools to room temperature, only one solid phase is produced. After solidification, the copper and nickel atoms do not separate but, instead, are randomly located at the lattice points of an FCC lattice. Within the solid phase, the structure, properties, and composition are uniform and no interface exists between the copper and nickel atoms. Therefore, copper and nickel also have unlimited solid solubility. The solid phase is a solid solution.
A solid solution is not a mixture. A mixture contains more than one type of phase, whose characteristics are retained when the mixture is formed. But the components of a solid solution completely dissolve in one another and do not retain their individual characteristics.
Limited Solubility If a small quantity of salt represents one phase and a glass of water represents another phase, and the salt is added to the water and stirred, the salt dissolves completely in the water. Only one phase — salty water or brine — is found. However, if too much salt is added to the water, the excess salt sinks to the bottom of the glass. Now there are two phases — water that is saturated with salt plus excess solid salt. It is found that the salt has a limited solubility in water.
If a small amount of liquid zinc is added to liquid copper, a single liquid solution is produced. When that copper-zinc solution cools and solidifies, a single solid solution having an FCC structure results, with copper and zinc atoms randomly located at the normal lattice points. However, if the liquid solution contains more than about 30% Zn, some of the excess zinc atoms combine with some of the copper atoms to form a CuZn compound. Two solid phases now coexist: a solid solution of copper saturated with about 30% Zn plus a CuZn compound. The solubility of zinc in copper is limited. The solubility increases with increasing temperature.
In the extreme case, there may be almost no solubility of one material in another. This is true for oil and water, or for copper-lead alloys.
Conditions for Unlimited Solubility
In order for an alloy system, such as copper-nickel to have unlimited solid solubility, certain conditions must be satisfied. These conditions are the Hume-Rothery rules.
