In chemistry, the colligative properties are characteristics of chemical solutions that depend on the number of particles in the solution. solute compared to the particles of solventnot the chemical identity of the solute particles. However, the colligative properties not depend on the nature of the solvent. The four colligative properties are freezing point depression, boiling point elevation, vapour pressure lowering and osmotic pressure.
Colligative properties apply to all solutions, but the equations used to calculate them apply only to ideal solutions or weak solutions of a non-volatile solute dissolved in a volatile solvent. More complicated formulae are needed to calculate the colligative properties of volatile solutes. The magnitude of a colligative property is inversely proportional to the molar mass of the solute.
How colligative properties work
Dissolving a solute in a solvent introduces additional particles between the solvent molecules. This reduces the concentration of the solvent per unit volume, essentially diluting the solvent. The effect depends on how many additional particles there are, not their chemical identity. For example, when dissolving sodium chloride (NaCl), two particles are obtained (a sodium ion and a chloride ion), while the sodium chloride is dissolved in the sodium chloride. calcium chloride (CaCl2) produces three particles (one calcium ion and two chloride ions). Assuming that both salts are completely soluble in a solvent, calcium chloride has a greater effect on the colligative properties of a solution than table salt. So, adding a pinch of calcium chloride to water lowers the freezing point, raises the boiling point, lowers the vapour pressure and changes the osmotic pressure more than adding a pinch of sodium chloride to water. This is why calcium chloride acts as a de-icing agent at lower temperatures than table salt.
The 4 colligative properties
Freezing point decrease
The freezing points of solutions are lower than the freezing points of pure solvents. The freezing point depression is directly proportional to the molality of the solute.
Dissolving sugar, salt, alcohol or any chemical in water lowers the freezing point of water. Examples of freezing point depression include sprinkling salt on ice to melt it and chilling vodka in a freezer without freezing it. The effect works in solvents other than water, but the amount of temperature change varies with the solvent.
The formula for the freezing point is:
ΔT = iK f m
where:
ΔT = Temperature change in °C
i = van ‘t Hoff factor
K f = molal constant of freezing point depression or cryoscopic constant in ° C kg / mol
m = molality of the solute in mole solute / kg solvent
There are tables of molal freezing point depression constants (K f ) for common solvents.
| Solvent | Normal freezing point ( o C) | K f ( o C / m) |
| acetic acid | 16,66 | 3,90 |
| benzene | 5.53 | 5.12 |
| camphor | 178,75 | 37,7 |
| tetrachloride carbon tetrachloride | -22,95 | 29,8 |
| cyclohexane | 6.54 | 20,0 |
| naphthalene | 80,29 | 6,94 |
| water | 0 | 1.853 |
| p -xylene | 13.26 | 4.3 |
Freezing point depression constants
Boiling point elevation
The boiling point of a solution is higher than the boiling point of the pure solvent. As with freezing point depression, the effect is directly proportional to the molality of the solute. For example, adding salt to water increases the temperature at which it boils (although not by much).
The boiling point elevation can be calculated from the equation:
ΔT = Kb m
where:
K b = ebulloscopic constant (0.52 ° C kg / mole for water)
m = molality of the solute in mole solute / kg solvent
There are tables of ebulloscopic constants or boiling point elevation constants (Kb ) for common solvents.
| Solvent | Normal boiling point (oC) | K b (oC/m) |
| Benzene | 80,10 | 2,53 |
| Camphor | 207,42 | 5.611 |
| Carbon disulphide | 46,23 | 2,35 |
| Carbon tetrachloride | 76,75 | 4.48 |
| Ethyl ether | 34,55 | 1.824 |
| Water | 100 | 0.515 |
Vapour pressure reduction
The vapour pressure of a liquid is the pressure exerted by its vapour phase when the condensation and vaporisation occur at the same rate (they are in equilibrium). The vapour pressure of a solution is always lower than the vapour pressure of the pure solvent.
The way this works is that the solute ions or molecules reduce the surface area of the solvent molecules exposed to the environment. Then, the vaporisation rate of the solvent decreases. The condensation rate is not affected by the solute, so the new equilibrium has fewer solvent molecules in the vapour phase. Entropy also plays a role. The solute particles stabilise the solvent molecules, making them less likely to vapourise.
Raoult’s law describes the relationship between vapour pressure and the concentrations of the components of a solution:
P A = X A P A *
where: ‘
P A is the partial pressure exerted by the component A of the solution
P A * is the vapour pressure of pure A
X A is the mole fraction of A
For a non-volatile substance, the vapour pressure is only due to the solvent. The equation becomes:
P solution = X solvent P solvent *
Osmotic pressure
Osmotic pressure is the pressure necessary to prevent a solvent from flowing through a semi-permeable membrane. The osmotic pressure of a solution is proportional to the molar concentration of the solute. Thus, the more solute is dissolved in the solvent, the higher the osmotic pressure of the solution.
The van’t Hoff equation describes the relationship between osmotic pressure and solute concentration:
Π = icRT
where
Π is the osmotic pressure
i is the van’t Hoff index
c is the molar concentration of the solute
R is the ideal gas constant
T is the temperature in Kelvin
Ostwalt and the history of colligative properties.
The chemist and philosopher Friedrich Wilhelm Ostwald introduced the concept of colligative properties in 1891. The word “colligative” is derived from the Latin word colligatus (“bound together”), referring to the way in which the properties of solvents are linked to the concentration of solutes in a solution. Ostwald actually proposed three categories of solute properties:
- Colligative properties are properties that depend only on the concentration and temperature of the solute. They are independent of the nature of the solute particles.
- Additive properties are the sum of the properties of the constituent particles and depend on the chemical composition of the solute. Mass is an example of an additive property.
- Constitutive properties depend on the molecular structure of a solute.
Comparative Table of Colligative Properties and Examples
| Colligative Property | Definition | Example |
|---|---|---|
| Steam Pressure | The addition of a non-volatile solute reduces the vapour pressure of the solvent. | Dissolving salt in water reduces the vapour pressure of the water. |
| Boiling Point | The presence of a solute raises the boiling point of the solvent. | A salt water solution boils at a higher temperature than pure water. |
| Freezing Point | The presence of a solute lowers the freezing point of the solvent. | A salt water solution freezes at a lower temperature than pure water. |
| Osmotic Pressure | The pressure required to stop the flow of solvent through a semi-permeable membrane. | The pressure exerted by a solution of sugar in water separated by a membrane of pure water. |