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Colligative Properties are based on the amount of a substance not what it is. Quantity not quality.

Dissociation- when neutral combinations of particles separate into ions while in aqueous solution

Up to this point you have only thought about solution as a particle dissolved in water.

This is true for many covalent compounds. They remain intact.

(1 mole of particles)    C₆H₁₂O_6(s) ==>C₆H₁₂O_6(aq)    (1 mole of particles)

Now we have to go back and remember what happens to Ionic compounds when they dissolve in water.

Ionic compounds dissociate.

 (1 mole of particles)       NaCl_(s) ==> Na⁺_(aq) + Cl^-_(aq)    (2 moles of particles)

(1 mole of particles)       CaCl_2(s) ==> Ca²⁺_(aq) + 2Cl^-_(aq)      (3 moles of particles)

(1 mole of particles)      Al(NO₃)_3(s) ==>Al³⁺_(aq) + 3NO₃^-_(aq)    (4 moles of particles)

Acids Ionize (covalent)

 (1 mole of particles)       HCl_(g) ==> H⁺_(aq) + Cl^-_(aq)    (2 moles of particles)

 SO WE HAVE MORE PARTICLES THEN WE THOUGHT

van't hoff Factor (i)= the number of moles of particles after a substance has dissolved

Boiling Point Elevation-The boiling point of a solvent will increase when another compound is dissolved in it.

In other words- the more solute particles (van't hoff) you add the higher the boiling point.

Advanced Description-When a solute is added to a solvent, the vapor pressure of the solvent (above the resulting solution) is less than the vapor pressure above the pure solvent. The boiling point of a solution, then, will be greater than the boiling point of the pure solvent because the solution (which has a lower vapor pressure) will need to be heated to a higher temperature in order for the vapor pressure to become equal to the external pressure (i.e., the boiling point).

Freezing Point Depression- The freezing point of a solvent will decrease when another compound is dissolved in it.

Regents Questions-follow the link for the answer and explanation

Jan 2004 #13 Compared to pure water, an aqueous solution of calcium chloride has a
(1) higher boiling point and higher freezing point
(2) higher boiling point and lower freezing point
(3) lower boiling point and higher freezing point
(4) lower boiling point and lower freezing point

Aug 2009 #19 How do the boiling point and freezing point of a solution of water and calcium chloride at standard pressure compare to the boiling point and freezing point of water at standard pressure?
(1) Both the freezing point and boiling point of the solution are higher.
(2) Both the freezing point and boiling point of the solution are lower.
(3) The freezing point of the solution is higher and the boiling point of the solution is lower.
(4) The freezing point of the solution is lower and the boiling point of the solution is higher.

Jan 2003 #19 What occurs when NaCl(s) is added to water?
(1) The boiling point of the solution increases, and the freezing point of the solution decreases.
(2) The boiling point of the solution increases, and the freezing point of the solution increases.
(3) The boiling point of the solution decreases, and the freezing point of the solution decreases
(4) The boiling point of the solution decreases, and the freezing point of the solution increases.

Jan. 2007 #19 Compared to a 2.0 M aqueous solution of NaCl at 1 atmosphere, a 3.0 M aqueous solution of NaCl at 1 atmosphere has a
(1) lower boiling point and a higher freezing point
(2) lower boiling point and a lower freezing point
(3) higher boiling point and a higher freezing point
(4) higher boiling point and a lower freezing point

June 2003#23 At standard pressure when NaCl is added to water, the solution will have a
(1) higher freezing point and a lower boiling point than water
(2) higher freezing point and a higher boiling point than water
(3) lower freezing point and a higher boiling point than water
(4) lower freezing point and a lower boiling point than water

Jan 2010#39 Which solution has the highest boiling point at standard pressure?
(1) 0.10 M KCl(aq)
(2) 0.10 M K₂SO₄(aq)
(3) 0.10 M K₃PO₄(aq)
(4) 0.10 M KNO₃(aq)

on to Boiling Point Elevation and Freezing Point Depression Calculations

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