The thermodynamics needed to treat the behaviour of solutions is explored in Chapter 5.8 of the Physical Chemistry book. The aim of this post is to use real life examples to explain the property of dilute solutions labelled colligative properties.
Colligative properties are an interesting scientific occurrence that can be applied in controlled environments (e.g. laboratory) as well as in day-to-day life. Examples of colligative properties include freezing point, boiling point, vapour pressure and osmotic pressure.
What is a colligative property?
We can divide the properties of solutions into two main groups: colligative and non-colligative properties. A colligative property is a characteristic of a substance (such as its freezing point, boiling point, vapour pressure or osmotic pressure) that can be changed when a solute is added to it.
It is dependent only on the amount of dissolved particles in solution and it does not take into account their identity, structure or mass. In other words, irrespective of whether the substance is salt, sugar or gasoline – it will behave the same way.
Non-colligative properties on the other hand, depend on the identity of the dissolved species and the solvent.
Freezing Point Depression
It is common knowledge that salt applied dry is not efficient at melting ice in the winter until it is dissolved into brine. This process can be explained in terms of colligative properties. Once salt is dissolved in water, the solute (in this case salt brine) will determine its freeze-point lowering potential or freezing point depression.
Any substance that dissolves in water has this effect. In other words, adding another compound or substance to a liquid result in a phenomenon called freezing point depression, whereby the solution has a lower freezing point than that of the pure solvent. Similarly, this occurrence can be applied to explain why seawater has a lower freezing point than that of pure water.
Boiling Point Elevation
A common example of boiling point elevation can be observed when salt is added to water; thus increasing the boiling point of water. Boiling point elevation occurs when the boiling point of a solution becomes higher than the boiling point of a pure solvent. The temperature at which the solvent boils is increased by adding any non-volatile solute.
Vapour Pressure Lowering
Again we can explain vapour pressure lowering as a colligative property of matter because the vapour pressure of a liquid will decrease when another substance is dissolved in it. The decrease will be proportional to the quantity of the added substance.
We can analyse why this occurs. By adding substance to a liquid, the number of liquid molecules at the surface to escape and produce pressure will be fewer, since the surface now contains a mix of liquid and added substance. Therefore the solvent molecules will have a lower probability to escape the solution than the pure solvent. This fact is reflected in the lower vapour pressure for a solution relative to the pure solvent.
Osmotic pressure arises when there is a concentration difference between two solutions separated by a semi-permeable membrane. The (osmotic) pressure is the force of a liquid against this special membrane. The pressure required to stop osmosis is called the osmotic pressure (illustrated above in Figure 5.16).
For instance, when a cell is added to an aqueous solution with a much higher concentration of ions than the liquid within the cell, water leaves the cell by flowing through the cell membrane until the cell shrinks and ultimately breaks the membrane. Alternatively, when a cell is placed in a solution that has a much lower ionic strength, water pours into the cell, and the cell expands until the cell membrane bursts.
These are examples of colligative properties of matter which means that it depends on the number of particles in a solution (its concentration) and not on the structure of the particles or their mass. These properties ideally depend on changes in the entropy of the solution on dissolving the solute. The rationale for these colligative properties is the increase in entropy on mixing solutes with the water.