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An ideal solution follows Raoult’s law, where vapor pressure is proportional to solvent mole fraction. Non-ideal solutions show vapor pressure lowering. Raoult’s law can be used to separate similar compounds by fractional distillation.
In chemistry, an ideal solution is a mixture that follows Raoult’s law, which states that the vapor pressure above a solution is directly proportional to the mole fraction of the solvent. A drop in vapor pressure is commonly observed in mixtures of solvents and nonvolatile solutes, or substances dissolved in a solvent. An ideal solution of similar compounds can use volatility differences to separate the compounds by fractional distillation.
Liquids vaporize and condense continuously. One in an open container placed in a closed system will vaporize until the gas-to-liquid condensation and liquid-to-gas vaporization rates are equal. The liquid is then in equilibrium with its vapor. Vapor pressure is the part of the total pressure due to the vaporized liquid. The vapor pressure of an ideal solution is a measure of the concentrations of the molecules in the solution.
Raoult’s law is followed by the most dilute solutions. If the solute and solvent are dissimilar, significant variations from an ideal are observed as solute concentrations increase. When the solute is non-volatile, the vapor pressure of the solvent is actually reduced as solute concentrations increase. This effect, called vapor pressure lowering, does not occur in ideal solutions.
Car owners take advantage of the lowering of vapor pressure by adding ethylene glycol to the water in the radiator. The lower vapor pressure keeps more of the mixture as a liquid and less as a gas. This helps prevent the car from overheating, as the contents of the radiator must be liquid to effectively cool the engine.
Similar compounds, such as those often found in petroleum products, can be separated according to Raoult’s law. The more volatile component will have a higher partial pressure and will distill faster in a distillation column. In this column, as heat is added to the mixture, the more volatile components evaporate first. The contents of the distillation flask concentrate in the second component.
By arranging the distillation steps, an almost perfect separation scheme is possible. In a fractionating column, a solid packing material serves as a multitude of small condensation/vaporization sites. Every small increase in elevation in the tower becomes an equilibrium point between the solute and solvent vapor and gas concentrations. In practice, the more volatile materials become more concentrated as they go up the tower. The mixtures concentrated in the solvent are withdrawn from the bottom of the column.
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