WKS
Colligative Properties of Solutions
(FP depression and BP elevation)
Name Answer Key
Period
Freezing point depression:
It's a FACT: When a solute (a solid, liquid or gas) is dissolved into water, the freezing point of the
solution will be lower than pure water. For example, when NaCl (s) is dissolved into water, the freezing point
of the saltwater solution is lower than 00C. (Thus, it is harder to freeze a solution of water than pure water.)
Boiling Point Elevation:
It's a FACT: When a solute (a solid, liquid or gas) is dissolved into water, the boiling point of the
solution will be higher than that of pure water. For example, if ethylene glycol (antifreeze) is dissolved into
water, the boiling point of the antifreeze solution is greater than 1000C. (Thus, it is harder to boil a solution of
water than pure water. )
How Concentration of Solute affects Boiling points and Freezing points: M↑ Fp↓ and M ↑ Bp ↑
The greater the amount of solute molecules/ions dissolved in the water, the more the boiling point is elevated
and the more the freezing point is lowered.
Ex: Freezing point of a 2 M NaCl solution is lower than a 1M NaCl solution.
Ex: Boiling point of 2 M NaCl solution is higher than a 1 M NaCl solution.
PRACTICE:
1) Arrange the following solutions from lowest to highest boiling point.
2 grams of NaCl / 10 mL of water ; 1 gram of NaCl / 10 mL of water;
Lowest bp
1g
Smallest effect
3 grams of NaCl / 10 mL of water.
2g
3g
Largest effect
highest bp
2) Arrange the following solutions from lowest to highest freezing point.
2 mL alcohol / 20 mL water;
4 mL alcohol / 20 ml water; 3 ml alcohol l/ 20 mL water
Lowest fp
4 mL
Largest effect
3 mL
2 mL
Smallest effect
3) Arrange the following from lowest to highest boiling point.
1 M CO2 (aq) ;
3 M CO2 (aq) ; 4 M CO2 (aq);
Lowest bp
1M
Smallest effect
2M
3M
4) Arrange the following from lowest to highest freezing point
1 M CaCl2 (aq)
3 M CaCl2 (aq)
2 M CaCl2 (aq)
Lowest fp
4M
Largest effect
Now, we know the facts, but WHY????
3M
2 M CO2 (aq);
4M
highest fp
5 M CO2 (aq)
5M
highest bp
Largest effect
4 M CaCl2 (aq)
2M
1M
highest fp
Smallest effect
Boiling point elevation: Why does the boiling point get higher when a solute is added?
5) Describe the interaction between the solvent and solute molecules that increases the boiling point.
Solute molecules increase the IMFs to the solvent. Thus more energy is required to break the IMFs and form
vapor at atmospheric pressure. More energy means higher temperature, so the BP increases.
Freezing point depression: Why does the freezing point get lower when a solute is added?
6) A drop of food coloring was stirred into a glass of water. Then, the glass was put into the freezer overnight.
Describe what the glass of ice looks like now.
The outer edge of the beaker was clear, indicating pure H2O, and the food coloring was segregated to the
center.
7) When put in the freezer, the outside of the glass got colder first.
Thus, what froze first? pure water or colored water pure water
Thus, is it easier to freeze pure water or colored water? pure water
Thus, is it harder to freeze pure water or colored water? colored water
(A lower temp is needed.)
8) Now, explain why a lower temperature is needed to freeze water when a solute is dissolved in it.
(You must discuss the structure of ice in your answer.)
To freeze, the water molecules must form intermolecular attractions (H-bonds) and form the ordered
hexagonal structure of ice. Solute (green food coloring) interferes with (blocks) formation of H-bonds. Since
it is harder to make attractions, a lower temperature is needed so that more energy must be removed for the
endothermic process to occur.
ANALYSIS of Club Soda demo:
Explain as thoroughly as possible. Use concepts discussed above.
9) Unopened bottle of club soda: It is placed in –8°C ice/salt bath. It does not freeze. Why not?
The unopened club soda has a high pressure of CO2, thus the concentration of CO2 in the soda is high, and
the freezing point is reduced to below the temperature of the ice/salt bath. Since the temperature of the club
soda is higher than the freezing point, it remains liquid.
10) Bottle is opened: While the club soda is still at –8°C, the bottle is opened. Now it freezes. Why?
When opened, the CO2 pressure decreases, which (according to Henry’s Law) decreases the concentration
of CO2 dissolved in the soda. As the solute concentration decreases, the FP increases and rises above the
temperature of the soda. Since the temperature is now below the FP, the club soda freezes.