Colligative Properties

Daisha Hopkins & Hannah Nixon

Colligative properties are consistent characteristics that change to mark that a solute has been added to a solvent, not reliant on what the solute is but on how much of the solute is present and its molar relation to the solution. These can be broken down into four specific characteristics: freezing point depression, boiling point elevation, vapor pressure reduction, and osmotic pressure.

Four Specific Properties

Freezing Point Depression:

When a solute is added to a solvent, it lowers the freezing point (the temperature at which liquid becomes solid) of the liquid solvent. The freezing point is when a substance's liquid and solid phases are at equilibrium (a state of balance), or its the temperature at which the molecules of a substance have organized themselves into a precise pattern that will become their solid structure.

The reason the freezing point of a solution is lower than the freezing point of a pure solvent is due to the fact that the solute's dissolved ions create issues when the solvent's molecules try to arrange themselves in their precise pattern.

                                                                                                                                                    Formula

                                                                           ΔT = i(Kb)m

where:
ΔT = Change in temperature in °C
i = van 't Hoff factor
Kb = molal boiling point elevation constant in °C kg/mol
m = molality of the solute in mol solute/kg solvent.


Boiling Point Elevation:

When a non-violate solute is added to a solvent, it takes more energy to heat to the solution due to a combination of a change in vapor pressure and a change in chemical composition. Volatility is the amount of time a chemical agent will stay on the surface of a liquid before it evaporates, so a non-violate solute is a substance that is not likely to evaporate at room temperature.

Since the non-volatile solute takes more energy to make them evaporate, and it is now blocking the path of other molecules of the solvent, more energy is needed overall to cause evaporation.

                                                                                                                                                   Formula

                                                                         ΔT = i(Kf)m

where:
ΔT = Change in temperature in °C
i = van 't Hoff factor
Kf = molal freezing point depression constant  in °C kg/mol
m = molality of the solute in mol solute/kg solvent.


Vapor Pressure Reduction:

When a non-violate solute is added to a solvent, the vapor pressure (pressure at which a substance is at equilibrium) decreases as some of the solvent's surface molecules are replaced by the solute. Vapor pressure is the amount of pressure exerted by a vapor against the sides of a closed container.

So, with the non-volatile solute, and the knowledge from 'Boiling Point Elevation' that these solutions are more difficult to evaporate, this all leads to there being less vapor made by these solvents, thus lowering the pressure.


Osmotic Pressure:

When a solute is added to a solvent, there is a change in osmotic pressure (the pressure needed to counteract osmosis or the flow of water between a membrane to reach equilibrium). Osmosis is the natural, passive flow of water between a membrane. In a container holds two sections of a solvent, divided by a selectively permeable membrane, the section of solvent that has a solute will rise in height with the added mass of the solute, but none of that solute will pass through the separating membrane due to the pressure it adds.

"In order to visualize this effect, imagine a U shaped clear tube with equal amounts of water on each side, separated by a membrane at its base that is impermeable to the sugar molecules (made from dialysis tubing). Sugar has been added to the water on one side. The height of the water on each side will change proportional to the pressure of the solutions." (Princeton University)

Sources:

"Chemistry Explained." Colligative Properties. N.p., n.d.                                                                          Web. 1 Dec. 2014.

"Freezing Point Depression - Boundless Open Textbook."                                                                          Boundless. N.p., n.d. Web. 02 Dec. 2014.

Chemistry Learning Center. University of Illinois at                                                                                 Urbana-Champaign, n.d. Web. 2 Dec. 2014.

“Osmotic Pressure.” Boundless Chemistry. Boundless, 16                                                                       Nov. 2014. Retrieved 02 Dec. 2014

"Osmotic Pressure." Princeton University. N.p., n.d. Web.                                                                         02 Dec. 2014.

"Colloid Osmotic Pressure." , Reverse Osmosis,  Semipermeable                                                             Membrane. N.p., n.d. Web. 02 Dec. 2014.

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