Colligative Properties
Does solute change the properties
of a solvent?
• Consider aqueous solutions
– Solvent = water
• How do properties of aqueous solution
compare to properties of pure water?
Conductivity
• Pure water does not conduct electricity
–Some aqueous solutions do
• To conduct current, solution must contain
ions
• Conductivity depends on nature of the
particle
– Molecular (covalent) solutes do not
conduct; ionic solutes do conduct
Colligative Properties
• Depend on number particles in
solution not on type particles
–i.e. doesn’t matter what kind of
particles: can be ions or molecules
–Concentration of particles does
matter
What are some colligative
properties?
•
•
•
•
Vapor Pressure (↓)
Freezing Point (↓)
Boiling Point (↑)
change in Osmotic Pressure
• higher the concentration of solute
particles, the larger the effect
Does it matter if solute is ionic or
covalent?
• number of particles will vary
• Ionic solutes
– dissolve to produce (+) & (-) ions  more
particles per mole of solute
• Covalent solutes
– 1-to-1 relationship between moles of solute
and moles of dissolved particles
C6H12O6
• Covalent substances:
–dissolve as molecules:
C6H12O6(s) + H2O(l)  C6H12O6(aq)
1 mole sugar → 1 mole molecules
NaCl
• Ionic substances:
–dissolves (dissociates) as ions:
NaCl(s) + H2O(l)  Na+1(aq) + Cl-1(aq)
1 mole salt  2 moles ions
MgCl2
• Ionic substance:
–dissolves as ions
MgCl2(s) + H2O(l)  Mg2+(aq) + 2Cl-(aq)
1 mole salt  3 moles ions
As solute is added to a solvent, what
happens to freezing point & boiling
point of solution?
1) The freezing point ↓ & the boiling point ↓
2) The freezing point ↓ & the boiling point ↑
3) The freezing point ↑ & the boiling point ↓
4) The freezing point ↑ & the boiling point ↑
Answer:
2
Which solution containing 1 mole of
solute dissolved in 1000 g of water has
the lowest freezing point?
1) C2H5OH(aq)
2) NaCl(aq)
Be careful!
- do you want greatest
or least change?
3) KOH
What if question asked
which solution has
highest freezing point?
4) CaCl2
Answer:
4
Of the following solutions, the one that
will freeze at the lowest temperature
contains 1 mole of nonvolatile solute
dissolved in
1)
2)
3)
4)
250 g of solvent
500 g of solvent
750 g of solvent
1000 g of solvent
Answer:
1
Which solute, when added to 1000 g of
water, will produce a solution with the
highest boiling point?
1) 29 g of NaCl
2) 58 g of NaCl
1 mole of NaCl →
2 moles of ions
3) 31 g of C2H6O2
4) 62 g of C2H6O2
1 mole of C2H6O2 →
1 mole of molecules
Which solution will freeze at the lowest
temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
2) 1 g of NaCl dissolved per 1000 g of H2O
3) 1 g of C12H22O11 dissolved per 100 g of H2O
4) 1 g of C12H22O11 dissolved per 1000 g of H2O
Which solution will freeze at the lowest
temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
__1 g____
58.8 g/mol
= 0.017 mol → 0.034 mol of ions
(NaCl)
(Na+1 and Cl-1)
2) 1 g of C12H22O11 dissolved per 100 g of H2O
__1 g___
232 g mol
= 0.0043 mol of molecules
Effect of Nonvolatile Solute
• Boiling Point Elevation (↑)
• Freezing Point Depression (↓)
• The more particles, the bigger the
effect!
Applications of colligative
properties
Salting roads in the winter time
Using salted ice to make ice cream