When one liquid dissolves in another, the molecules of the solvent are caused to move apart so as to accommodate the solute molecules. Similarly, the solute molecules must also be separated so that they can take their places in the mixture. In both these processes energy is required. Finally, as the solute and solvent molecules are brought together, energy is released because of the attractive forces between them. When solute and solvent molecules are strongly attracted to each other, more energy is released in the final step. Three cases may arise under these circumstances. The overall dissolution process results either in evolution of heat or absorption of heat, or energy released in the final step is the same as the absorbed in the first two, i.e., net change is zero.

 

 

1.     Benzene and carbon tetrachloride.              No evolution or absorption of Heat.

 

2.     Acetone and water:                                          Evolution of heat.

 

3.     Ethyl alcohol and water:                                  Absorption of heat.

 

A liquid may or may not be soluble in another liquid. Depending upon the relative solubility of a liquid in another, the following three cases are possible.

 

                                                                                      Liquid-Liquid Systems