Colligative properties in solution chemistry depend on the number of solute molecules, not their identity. Vapor pressure, boiling point elevation, freezing point depression, and osmotic pressure are colligative properties. They are defined only for ideal solutions and can be understood through common examples, such as adding salt to water to cook pasta faster or using sugar and salt to help freeze ice cream.
A colligative property is a descriptive feature used in solution chemistry. More simply, colligative properties are those solution properties that depend on the number of solute molecules in a given solution, but not on the identity of those solute molecules. There are only a few solution properties that are colligative: vapor pressure, boiling point elevation, freezing point depression, and osmotic pressure. Colligative properties are defined only for ideal solutions.
In chemistry, solutions are defined as consisting of a solute, or dissolved substance, and a solvent, or dissolving substance. For example, if some table salt is dissolved in water, the salt is the solute and the water is the solvent. The colligative properties of this solution are properties that depend only on the number of salt molecules or on the ratio of the number of salt molecules to the number of solvent molecules. The colligative properties of the solution do not depend on whether the solute is a salt, or on any of the characteristics of the salt. A colligative property is a property that for any solution will behave the same way, whether the solution contains salt, sugar, or any other possible solute.
Of the four colligative properties of solution, vapor pressure, boiling point elevation, and freezing point depression are closely related. Vapor pressure as a colligative property is described by Raoult’s law. Raoult’s law basically states that for an ideal solution, the vapor pressure of the total solution depends on the vapor pressure of each of the chemical components, as well as the mole fraction of each of the chemical components in the solution. More practically, this relationship means that when a solute is added to a solution, the change in vapor pressure depends only on the ratio of solute to solvent molecules. Again, since it is a colligative property, the change in vapor pressure does not depend on the identity of the added solute.
Boiling point elevation and freezing point depression are colligative properties that each change in tandem with changes in vapor pressure. When a solute is added to the solution, the solutes lower the vapor pressure of the solvent. The pressure change causes a corresponding increase in the boiling point and a decrease in the freezing point of the solution. In other words, when a solute is added to a solution, the solution will now boil at a higher temperature and freeze at a lower temperature.
Osmotic pressure is the fourth colligative property of the solution. Osmosis is defined as the movement of solvent molecules across a semipermeable membrane into an area that contains a larger number of solute molecules. Osmotic pressure is the amount of pressure that must be applied to one side of the semipermeable membrane to prevent the passage of the solvent. The osmotic pressure of an ideal solution at constant temperature is proportional to the solute concentration or, in other words, depends only on the number of solute molecules.
The colligative properties of the solution may seem complex to define, however they can be understood intuitively through some common examples. Many cooks add salt to a pot filled with water when cooking pasta, which causes the pasta to cook faster. This action takes advantage of a colligative property. Adding the salt raises the boiling point of the water, which allows the pasta to cook faster at a higher water temperature.
Freezing point depression is also commonly exploited by chefs. Both sugar and salt as solutes help ice cream freeze. The sugar in ice cream affects the temperature at which the ice cream freezes, and the salt water around the ice cream container creates a colder environment where the ice cream will freeze more quickly.
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