Colligative properties (Maria M, Kristina P, Victor S and Alvaro Puig)

Colligative properties

Solutions have different properties than either the solutes or the solvent used to make a solution. Those properties can be divided into two main groups: colligative and non-colligative properties. Colligative properties depends only on the number of dissolved particles in solution and not on their identity. Non-colligative properties depend on the identity of the dissolved species and the solvent. 

- Vapor pressure and Raoult's Law

Raoult's law states that the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component in the solution. 

So the addition of a nonvolatile solute to a pure solution decreases the vapor pressure of the solution (related to the relative number of solvent molecules tos solute particles). 

If you have a nonvolatile solute in a solution, the vapor pressure of the solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the solvent in the solution. As we can see explained on the next example: 

* Pure Water

The vapor pressure is caused by some molecules turning from liquid to gas, therefore causing vapor pressure.

* Solution of pure water and salt (that wont turn into a gas)

Because there are now salt molecules taking up the space on the surface, less water molecules can turn to gas. So vapor pressure is lower.

So, the more solute that is added, the lower the vapor pressure.

Français- Marie Raoult discovered that the: 

Vapor pressure of = Vapor pressure of the pure solvent    x    Molar fraction of

       a solution                                                                              Pure solvent

                                                                                         

This is called Raoult's Law. It can only be used when we make a solution with a solute that is non- volatile so it will not turn into a gas.