What are colligative properties?

German chemist Wilhelm Ostwald was the first to introduce the concept of colligative properties in 1891. This name arose from his work on the properties of solutes, including:

1. Colligative properties: they depend only on the concentration and temperature of the solute and not on the type of solute particles.
2. Constitutive properties: are those that depend on the molecular structure of the solute particles in a solution.
3. Additive properties: which are the sum of all the properties of the particles and depend on the molecular formula of the solute. For example, the mass.

Colligative properties are not related to the size or any other property of the solutes, but only to the number of particles in the solute. These properties are the result of the effect of solute particles under solvent vapor pressure.

Examples of colligative properties

The colligative properties are:

• Osmotic pressure
• ebulliscopic elevation
• cryoscopic descent
• Solvent vapor pressure drop

osmotic pressure

Osmotic pressure is related to the concepts of diffusion and osmosis. It is defined as the inclination to dilute a solution that is separated from the solvent by a semipermeable membrane. The solute exerts osmotic pressure when faced with the solvent if it cannot cross the membrane that separates them.

We can also say that the osmotic pressure of a solution is equivalent to the mechanical pressure needed to prevent the entry of water when it is separated from the solvent by a semipermeable membrane.

The osmotic pressure is measured with the osmometer. This is a container that is closed at the bottom by a semi-permeable membrane. At the top it has a plunger. If a solution is introduced into the container and then submerged in distilled water, it passes through the semipermeable membrane and exerts a pressure that is capable of raising the plunger. In this way, by subjecting the plunger to a suitable mechanical pressure, it is possible to prevent the water from passing into the solution.

Osmotic pressure is one of the most important colligative properties, especially at a biological level because it is present in cell function and other processes in the organism of living beings.

Ebulliscopic elevation

Ebulloscopic elevation is related to the boiling point of a liquid. The boiling temperature is that whose vapor pressure is equal to atmospheric pressure.

If the vapor pressure decreases, an increase in the boiling temperature occurs. This rise is proportional to the mole fraction of the solute. The increase in boiling temperature (abbreviated DTe) is proportional to the molal concentration of the solute. It is expressed with the following equation:

DTe = Kem

The ebulloscopic constant (Ke) is known as the characteristic of each solvent regardless of the type of solute. For water, the value of the boiling constant is 0.52 ºC/mol/Kg. This means that a molal solution of any solute in water presents an ebulloscopic elevation of 0.52ºC.

cryoscopic descent

Cryoscopic descent is related to the freezing point of a liquid. The freezing temperature of solutions is less than the freezing temperature of the solvent. Therefore, freezing occurs when the vapor pressure of the liquid is equal to the vapor pressure of the solid. This is expressed like this:

DTc = Kcm

The cryoscopic decrease is called « Tc» and the molal concentration of the solute « m» .

The cryoscopic constant of the solvent is called “Kc”. In the case of water, the value of the cryoscopic constant is 1.86 ºC/mol/Kg. That is, the molal solutions (m=1) of any solute in water freeze at -1.86 ºC.

Solvent vapor pressure drop

The vapor pressure of a solvent drops when a nonvolatile solute is added to it. This effect occurs because:

• The number of solvent molecules on the free surface decreases.
• Attractive forces appear between the molecules of the solute and those of the solvent, making their transformation into a vapor more difficult.

Put another way, when we add more solute, a lower vapor pressure is observed. Therefore, the drop in vapor pressure of the solvent in a solution is proportional to the mole fraction of the solute.

This can be expressed by the following formula:

ΔP= x _s P ₀

In this case, x _s is the mole fraction of the solute and P ₀ indicates the vapor pressure of the solvent.

How do colligative properties work?

The operation of colligative properties is evident when a solute is added to a solvent to form a solution. There the dissolved particles displace a part of the solvent in the liquid state, decreasing the concentration of solvent per unit volume. In a dilute solution, it doesn’t matter which particles it is but how many there are. For example, by dissolving calcium chloride (CaCL ₂) altogether three particles are produced: one calcium ion and two chloride ions. On the other hand, if we dissolve table salt or sodium chloride (NaCl) we will obtain two particles: a sodium ion and a chloride ion. In this case, calcium chloride would have a greater effect on colligative properties than table salt. Therefore, calcium chloride is a more effective deicing agent at lower temperatures than common salt.

Although colligative properties are generally considered for nonvolatile solutes, the effect also applies to volatile solutes like salt. If we add a pinch of salt to a cup of water, the water will freeze at a lower temperature than normal, boil at a higher temperature, have a lower vapor pressure, and change its osmotic pressure. 

Another simple example is adding alcohol, a volatile liquid, to water. In this way, the freezing point that pure alcohol or water normally has is lowered, which is why alcoholic beverages do not usually freeze in a domestic refrigerator.

Bibliography

• García Bello, D. Everything is a matter of chemistry . (2016). Spain. Paidos Iberica.
• Nguyen-Kim, MT My life is chemistry . (2020). Spain. Editorial Ariel.
• Masterton, WL; Hurley, C.N. Chemistry: Principles and Reactions . (2003, 4th edition). Spain. B&W.