Colligative Properties of Electrolyte Solutions

Vapor Pressure of Electrolyte Solutions

The vapor pressure of an electrolytic solution is dependent on the ratio of solute to solvent molecules in a solution.

Learning Objectives

Compare the relative vapor pressures of a pure solvent and an electrolyte solution composed of the same solvent

Key Takeaways

Key Points

  • Vapor pressure is a colligative property, so the vapor pressure of solutions is directly proportional to the amount of solute present in a solution.
  • When a solute is present in a solvent, the vapor pressure is lowered because fewer solvent molecules are present at the top of the solution.
  • Raoult’s law details the calculations for acquiring the vapor pressure of an ideal solution.

Key Terms

  • partial pressure: The pressure that one component of a mixture of gases contributes to the total pressure.
  • ideal solution: A solution with thermodynamic properties analogous to those of a mixture of ideal gases.
  • electrolyte: A substance that, when in solution or when molten, ionizes and conducts electricity.

Electrolyte Solutions

A simple example of an electrolyte solution is sodium chloride in water. In the presence of water, solid sodium chloride dissociates as it is dissolved, forming an electrolyte solution:

[latex]\text{NaCl}_{(\text{s})} \rightarrow \text{Na}^+_{(\text{aq})} + \text{Cl}^-_{(\text{aq})}[/latex]

Nonelectrolyte solutions are those in which the solute does not dissociate into ions when dissolved; sugar does not dissociate, for example. The number of moles of dissolved particles is greater for electrolyte solutions, so there will be a greater impact on colligative properties.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phase, either liquid or solid, at a particular temperature. Basically, it is a measure of how much the solvent molecules tend to escape from a liquid or solid phase into the atmosphere. Vapor pressure of a liquid is a colligative property.


Vapor pressure: The picture shows the transition, as a result of the vapor pressure, from particles in liquid phase to gas phase and vice-versa.

To better visualize the effect of solute on the vapor pressure of a solution, consider a pure solvent. This pure solvent has a certain vapor pressure associated with it. Subjected to temperatures below the solvent’s boiling point, the molecules going into the gaseous phase are mostly situated on the top layer of the solution. Now consider a solution composed of both solvent and solute. Some solute molecules will occupy space near the surface of the liquid, decreasing the number of solvent molecules that can be there. Therefore, fewer molecules are changing from the liquid phase into the gas phase, and the vapor pressure of the solvent decreases.

In an electrolyte solution, the number of dissolved particles is larger because the solute breaks apart into ions. The greater the number of ions, the larger the impact on colligative properties will be.


Which would have the lowest vapor pressure at 25 oC?

a) 0.1 M solution of NaCl

b) 0.1 M solution of C6H12O6 (glucose)

c) 0.1 M solution of Al(NO3)3

The correct answer is c. All of the solutions have the same concentration, but when NaCl dissolves, it breaks into 2 particles. Glucose is a non-electrolyte and does not break apart. When Al(NO3)3 dissolves, it produces 4 particles in solution (1 Al3+ and 3 NO3), and will have the greatest impact on the vapor pressure.

Medical Solutions: Colligative Properties

Medical solutions must be tailored to restore and maintain a proper homeostatic environment.

Learning Objectives

Discuss the importance of colloidal properties in the use of medical solutions

Key Takeaways

Key Points

  • A saline solution ‘s main use is to rehydrate and carry ions needed for cellular activity into the body.
  • Often used intravenously, saline solutions deliver ions and water into the body when a patient is dehydrated or unable to maintain a proper internal electrolytic environment.
  • Delivery of the solution at proper concentration levels is vital to ensure that the cells do not burst from too much water intake or shrivel from excess water loss.
  • Plasma osmolality is a measure of the electrolyte -water balance.

Key Terms

  • hypertonic: Having a greater osmotic pressure than another.
  • osmolality: The molality of an ideal solution that would exert the same osmotic pressure as the solution being considered.
  • hypotonic: Having a lower osmotic pressure than another.

The Human Solution

At normal physiological conditions, organisms regulate their internal environments and maintain stable, constant conditions despite influences from the outside environment. Internally, many mechanisms allow for a constant environment, but often, when a person becomes sick or incapacitated, the homeostatic environment becomes disrupted. To assist in the treatment and recovery process, medical scientists and doctors often introduce electrolytic solutions into the body. The solutions must be of proper osmolality and concentrations, otherwise irreversible damage can be caused. These electrolytic solutions share the same colligative properties as chemical solutions.

Saline Solutions

Medical solutions are important for treating dehydration and for cleaning and treating wounds. One class of medical solutions is known as saline solutions. These solutions are composed of water and sodium chloride. Saline solutions are typically used for rinsing contact lenses, nasal irrigation, and cleaning new piercings. Saline solutions can vary in their concentrations.

Typically, saline is found at a 0.90% w/v of NaCl in water. Referred to as “normal saline,” this type of electrolytic solution is used frequently in intravenous drips for patients who have lost a lot of water and are at risk for dehydration. Normal saline is also used to treat decreased blood volume. The saline solution is expected to restore the salinity levels in the blood. Most commonly, saline is used in intravenous (IV) therapy, which provides water and electrolytes to a patient. Normal saline has low osmolality, which can introduce problems, so IV solutions generally have glucose added to maintain a safe osmolality.


Saline solution: A bag of saline. Saline can be used to increase blood volume when a blood transfusion is not possible.

Electrolytes in Solutions

As mentioned before, humans maintain a homeostatic internal environment. The electrolyte-water ratio that regulates many of the body’s functions is part of this. Plasma osmolarity is the measure of the body’s electrolyte-water balance. Its name is derived from osmosis, which is the net movement of solvent molecules through a partially permeable membrane; the molecules travel from a region of higher solute concentration to a region with lower solute concentration. There are two common measurements used to determine the amount of electrolyte in a solution.

Osmolality is affected by changes in water content, whereas osmolarity is affected by temperature and pressure. These two values are slightly different; osmolarity is slightly less than osmolality because it does not take into account the weight of the solutes. The normal range of osmolality in human blood plasma is 270-310 milli-osmoles/kg.

Cell membranes are permeable to water, so the osmolality of the extracellular fluid (ECF) is approximately equal to that of the intracellular fluid (ICF). Any shift in the osmolality of the ECF will directly impact the ICF and can cause problems with normal cell functions and water volume. Therefore, the introduction of saline that is too hypotonic will cause water to fill the cells too rapidly, potentially causing the cells to burst. Conversely, the introduction of saline that is too hypertonic will cause water to leave the cells, making them shrivel. There are medical solutions with a range of concentrations to ensure the cell maintains an isotonic environment.