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Babo’s Law states that the vapor pressure of a liquid is lowered when a non-volatile substance is dissolved in it. The decrease in vapor pressure is directly proportional to the concentration of the dissolved non-volatile substance. This phenomenon is a colligative property, meaning it depends on the number of solute particles in the solution rather than their identity.


  • Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid or solid phase.
  • Non-volatile Substance: A substance with a very low vapor pressure that does not easily evaporate.
  • Proportional Relationship: The reduction in vapor pressure \((\Delta P)\) is proportional to the molar concentration (mole fraction) of the solute \((X_{\text{solute}})\) in the solution.

Mathematical Expression:

\[ \Delta P = P_{\text{pure}} – P_{\text{solution}} = k \cdot X_{\text{solute}} \]


  • \(\Delta P\) is the decrease in vapor pressure.
  • \(P_{\text{pure}}\) is the vapor pressure of the pure solvent.
  • \(P_{\text{solution}}\) is the vapor pressure of the solution.
  • \(k\) is a proportionality constant.
  • \(X_{\text{solute}}\) is the mole fraction of the solute.


  • Boiling Point Elevation: The boiling point of a solution is higher than that of the pure solvent due to the lowered vapor pressure.
  • Freezing Point Depression: The freezing point of a solution is lower than that of the pure solvent for the same reason.
  • Osmotic Pressure: Related to the concentration of solutes in a solution, important in biological and chemical systems.

Babo’s Law is an important principle in physical chemistry and helps in understanding the behavior of solutions and their colligative properties.

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