A solution of two compounds may be an ideal solution and follow Raoult's Law, or it may show a positive or negative deviation from Raoult's Law. How to predict that whether a solution of two given compounds will show Positive or Negative Deviation from Raoult's law, or if it will be an ideal solution?
For most solutions, it is nearly impossible to predict in advance whether a positive or negative deviation is observed. A deviation from Raoult's law can only be measured experimentally. However, for some solutions, you can predict whether the deviation will be positive or negative by making assumptions about the interactions between solute and solvent molecules.
If the solute-solvent forces of attraction is weaker than solute-solute and solvent-solvent interaction, then there will be a positive deviation, and if the solute-solvent forces of attraction is stronger than solute-solute and solvent-solvent interaction. For example, Acetone and Ethanol show a positive deviation due to weaker interactions due to H-bonding.