Miscibility

• "Miscibility" describes the mixing of two substances together. No physical or chemical transformation takes place between the two substances in question, and they retain both their respective chemical and physical properties. Although there is no change, the substances do form a homogeneous mixture, meaning that they are visually indistinguishable from each other.
  
  

Solubility

• "Solubility" identifies the ability of a substance, called a solute, to completely dissolve in another substance, referred to as a solvent. Unlike miscibility, solubility refers to a physical change as the molecules dissolve and separate in the solution. The degree of solubility of a solute is affected by several factors, including, but not limited to, the temperature, pressure and chemical properties of the solvent.
  
  

Solubility Product Constant

• While miscibility is more of a qualitative observation, solubility can be expressed quantitatively as the solubility product constant, or Ksp. The Ksp of a substance identifies the number of moles of solvent dissolved per liter of solution. A mole is a chemical unit of measurement that identifies the number of molecules of a particular element and is equal to approximately 6.02 x 10^23 molecules. Thus, a high Ksp value identifies a solute that readily dissolves in a particular solvent.
  
  

Ethanol Example

• An excellent example of a miscible mixture is combining ethanol and water. The two substances combine to form a single liquid phase. Therefore, it would be correct to say that ethanol is miscible in water. Despite this miscibility, it should also be noted that ethanol is partially soluble in water. In solution, ethanol may donate its hydrogen ion to a water molecule, identifying the physical change associated with solubility.