Why Is Vapor Pressure Considered a Colligative Property?

Vapor Pressure

• Any standing liquid undergoes some evaporation. This occurs when molecules at the upper surface get enough kinetic energy through random collisions with each other to leave the bulk liquid and enter the air as unattached gas-phase molecules. The motion of evaporating molecules above the liquid creates a pressure, known as the vapor pressure of the liquid. As a liquid is heated, more molecules leave and its vapor pressure increases until it reaches its boiling point.

Colligative Properties

• A colligative property is a characteristic of a substance, such as its boiling point, that can be changed by adding another chemical to it. The extent of the change depends on the quantity of atoms or molecules of the added chemical and not, for example, on the molecular size or weight of the added compound. Chemists can demonstrate colligative properties by adding increasing quantities of a chemical (the "solute") into a substance and measuring the associated change in that substance's physical properties.

Types of Colligative Properties

• There are four characteristic properties of substances which are colligative. The boiling point of a liquid is one, since adding a solute to a liquid increases its boiling point. The melting point (or freezing point) of a substance is also a colligative property. Adding a solute to a chemical lowers its melting point. When salt is added to snow, the melting point of the mixture is lowered and the combination melts at temperatures which would otherwise freeze it into ice. Osmotic pressure, the force of a liquid against a special membrane, is colligative and so is vapor pressure.

Vapor Pressure as a Colligative Property

• Vapor pressure is a colligative property since the vapor pressure of a liquid decreases as another substance is dissolved in it and the decrease is proportional to the quantity of the added substance. Chemists can demonstrate this effect experimentally or theoretically by using a relationship between amount of solute and pressure known as Raoult's Law. You can also understand this effect simply by considering that in a liquid with added chemical there are fewer liquid molecules at the surface to escape and produce pressure, since the surface contains a mix of liquid and added chemical.