Automotive antifreeze, kidney dialysis and using rock salt to make ice cream don’t seem like they would have anything in common. But they all depend upon the colligative properties of solutions. These properties are the physical properties of solutions that depend only on the ratio of the number of particles of solute and solvent (e.g., salt in water) in solution and not on the identity of the solute.

The human body’s cells, plant cells and solutions such as antifreeze and ice cream depend on colligative properties.

A solvent (such as water) has a vapor pressure denoted by p1. This is equal to one atmosphere of pressure.

At equilibrium, the gas phase (such as water vapor) above the solvent has a partial pressure equal to p1. Adding a solute (like table salt, NaCl), decreases the partial pressure of the solvent in the gas phase. The decrease in vapor pressure is caused by the solvent molecules on the surface of the solution being replaced by solute molecules. The solvent molecules “crowd out” vaporization. Because there are less solvent molecules on the surface, the vapor pressure decreases.

Bringing a solvent to a boil essentially vaporizes the solvent. Boiling point elevation, or increasing the temperature at which the solvent boils, occurs for a similar reason as vapor pressure depression. The increased amount of the solute on the surface inhibits vaporization of the solvent, so it requires more energy input to achieve the boiling point.

This presumes the solute is non-volatile, that is, it has a low vapor pressure at room temperature. A volatile solute with a lower boiling point than the solvent may actually depress the boiling point. Benzene is an example of a volatile organic compound (VOC).

The freezing point of a solution will be lower than that of the pure solvent. Freezing point is the temperature at which a liquid becomes solid at 1 atmosphere. Freezing point depression means the freezing temperature lowers. This means the liquid must be colder to achieve freezing. The reason this occurs is because the presence of a solute introduces more disorder to the system than was present with just the solvent molecules. Therefore, the mixture must be colder to overcome the effects of the more disordered system.

A practical application of this colligative property is automotive antifreeze. The freezing point of a 50/50 solution of ethylene glycol (CH₂(OH)CH₂(OH)) is -33 degrees Celsius (-27.4 degrees Fahrenheit), compared with 0 degrees Celsius (32 degrees Fahrenheit). Antifreeze is added to a car’s radiator so that the car must be exposed to much lower temperatures before the water in the car’s system freezes.

Osmosis occurs when solvent molecules move through a semipermeable membrane. One side of the membrane could contain solvent, and the other side of the membrane would contain solute. Movement of solvent occurs from an area of higher concentration to an area of lower concentration, or from higher chemical potential to lower chemical potential until an equilibrium is reached. This flow naturally occurs, so some input of pressure on the solute side must be applied to stop the flow.

The osmotic pressure is the pressure that would stop that flow. Osmotic pressure generally increases for solutions. The more solute molecules there are, the more the solvent molecules are pressed together. The presence of solute molecules on one side of the membrane means that fewer solvent molecules can cross into the solution side. The osmotic pressure is directly related to the concentration of solute: more solute translates to a higher osmotic pressure.

Colligative properties are all dependent upon the molality (m) of a solution. Molality is defined as moles of solute/kg of solvent. The more, or less, of a solute that is present in ratio with the solvent will affect the calculations of the four colligative properties outlined above.