Colligative properties

What happens to the boiling point of water if we add table salt? Why does water boil faster in high altitude locations? Why is salt added on the roads in winter?

These questions relate to the properties that compare the behavior of solutions with their pure solvent.

At colligative properties Solutions are those that relate directly to the number of solute particles that are dispersed (dissolved) in a given solvent.

They depend on the number of particles dispersed in the solution, regardless of the nature of that particle. These are the changes that solutes cause to the solvent.

During the study of colligative properties, it is always necessary to compare the behavior of the solution with its pure solvent.

Solvent pure water and solute cooking salt. When mixed, they cause changes in the physical properties of the solvent, in this case increasing the melting point (mp).

An example of this comparison is the boiling point of water. Make sure that when boiling pure water at sea level the boiling temperature of the water (pure solvent) is 100 ° C. However, when heating an aqueous NaCl solution, it is apparent that the boiling point of water increases.

Raising the boiling point of water in the solution will always have effects on some physical properties of a solvent, which are:

- decrease in vapor pressure
- Boiling point increase
- freezing point decrease
- increased osmotic pressure

These effects are known as colligative effects, which depend solely on the concentration (amount) of particles that are dispersed in a solvent.

Colligative effects define the four colligative properties, which are as follows:

- tonoscopy
- boiling
- cryoscopy
- osmometry

Dissolved particles

Before starting the study of colligative properties, it is important to know how to calculate the number of particles that are dissolved in solutions. Two types of particles will be found, molecular and ionic.

Molecular Solutions

They are the solutions that have molecules as dispersed particles. The number of particles (dissolved molecules) is equal to the number of particles in solution. Examples of molecular particles:

- glucose - C6H12O6
- sucrose - C12H22O11
- urea - CO (NH2)2

The calculation of molecular solutions is made from the concept of mol, taking into account the Avogadro number.

1 mol of particles = Avogadro's number = 6.02.1023 particles

Example: Calculate the number of sucrose particles contained in 1L of 2mol / L solution: