Interactions (Colligative properties)

When a solution is prepared, we have seen there is an interaction between the solute particles and the solvent particles to produce the solution. How does this interaction affect the physical properties of the solvent? This question can be answered by considering colligative properties  of a solution. (Click on the preceding link for a definition of colligative properties.  Read from the introduction down to the list of colligative properties.)

Another way to state the definition of colligative properties is: properties of solutions that depend on concentration of solute in solution.

This discussion will consider only vapor pressure, freezing point depression and elevation of boiling point.

This would be a good time to review the Moles and Avogadro’s number unit under the Basics Menu item .

Some information regarding vapor pressure, freezing point depression and elevation of boiling point needs to be introduced at this time.

Vapor pressure:

Click the following link to Wikipedia for a definition and discussion of vapor pressure.

The following series of sketchs provides a visual of vapor pressure and how it is measured.

Visual 1
This is a sketch of an empty flask with a stopper and a pressure gauge.

VP flask gauge

Visual 2
This sketch is of the flask with a liquid solvent added to it.

VP flask liquid gauge

Visual 3
After some time has passed, some of the liquid molecules leave the surface of the liquid solvent and become vapor molecules and the pressure will increase.

VP flask liquid gauge some vapor
Visual 4
At a later point in time, molecules in the vapor state will return to the liquid state. At the same time other liquid molecules are going to the vapor state.

VP flask liquid gauge equil vapor

This will continue and when the number of liquid molecules going to the vapor state equals the number of vapor molecules going the liquid state, this situation is called vapor-liquid EQUILIBRIUM.

This is a state where the rate of liquid molecules going to the vapor state equals the rate of vapor molecules going to the liquid state. At equilibrium, the pressure in the flask stays constant.

Equilibrium is a very important concept in chemistry that will be discussed in other sections later.

Watch the video Introducing Chemical Equilibrium

Vapor Pressure Lowering Table:

Look at the table and observe the change in vapor pressure of water as the concentration of the solutes increases.  In the following table, vapor pressure of water (the solvent) is given on the second row.  The temperature is 25 degrees C.  Changes due to added solute are given in the lower rows.

H2O   760 mm
NaCl 0.50 747.7
NaCl 1.0 734.8
NaCl 2.0 707.8
CaCl2 0.5 743.0
CaCl2 1.0 720.2
CaCl2 2.0 764.7

The table contains data for two solutes with the same molarity. A close examination of the data indicates NaCl does not lower the vapor pressure as much as CaCl2 . The difference is due to the fact that when each NaCl unit dissolves in the solvent, there two ions, Na+ and Cl, produced. When each CaCl2 is dissolved, it produces three ions Ca+2 and 2 Cl . Another way to say this is for each mole of solute dissolved, NaCl produces 2 moles of ions and CaCl2 produces 3 moles of ions. The result is there is a greater number of particles in a CaCl2 solution than there is in a NaCl at the same concentration.

The vapor pressure is lowered because of the attraction between water water molecules and the ions.

Associated information regarding vapor pressure. Liquids that have a higher vapor pressure will evaporate more rapidly than liquids with a lower vapor pressure. Liquids that have a higher vapor pressure are also more volatile.

Freezing point:

Liquids will change to the solid state at a temperature called the freezing point. The temperature of the change depends on the liquid. Water (H2O) freezes at zero degrees Celsius, ethanol freezes at -114.6 degrees Celsius, and acetone freezes at -94.8 degrees Celsius.

Freezing point depression occurs when a solute is placed in a solvent.

Freezing Point Table:

Look at the table below and observe the change in freezing point of water as the concentration of the solutes increases.

SOLUTE MOLARITY (mol/L) Freezing Point (C)
H2O   0
NaCl 0.50 -1.73
NaCl 1.03 -3.57
NaCl 2.03 -7.35
CaCl2 0.52 -2.63
CaCl2 0.98 -5.86
CaCl2 2.0 -16.7
C12H22O11 (Sucrose) 0.50 -110
C12H22O11 0.99 -2.64
C12H22O11 1.46 -4.93

The table contains data for three solutes with approximately the same molarity. Again as the concentration increases the effect of more particles in solution is observed.   As mentioned above, CaCl2 will produce more solute particles than NaCl.  The last solute, sucrose, is a molecular (polar) compound and when it dissolves there is only particle produced for each molecule dissolved,

Associated information regarding freezing point depression. In colder climates where water freezes frequently, a compound called antifreeze is added to the water in the radiator to lower the freezing point so there will be no damage to the engine. Ethylene glycol is the most common chemical used as an antifreeze. Ethylene glycol is toxic if ingested.

Boiling point:

Liquids will change to the vapor state when the vapor pressure of the liquid equals the pressure of the environment above the liquid. Water boils at 100 degrees Celsius when the pressure is 760 mm (one atmosphere).

Boiling point elevation occurs when a non-volatile solute is added to a liquid.

The rational for the increase in boiling point is that extra energy is needed to overcome the attraction between the solute particles and the solvent molecules.

Boiling Point Elevation Table:

 Look at the data and observe the change in boiling point of water as the concentration of the solutes increases.

SOLUTE MOLARITY (mol/L) Boiling Point(C)
H2O   100
NaCl 2.2 101
NaCl 3.0 101.8
NaCl 5.7 102.9
MgCl2 2.2 100.9
MgCl2 3.0 102.2
MgCl2 5.7 103.5

Again the more particles in solution, the greater the affect on the boiling point elevation.

Associated information regarding boiling point elevation. A pressure cooker is an example of elevating the boiling point of water. The major content in a pressure cooker would be water although there could be some dissolved solute. Click the link pressure cooker  for a explanation of how a pressure cooker works.

Altitude has an effect on the boiling point of water. Click the link high altitude cooking  for an explanation.

After this discourse of colligative properties, it is time to review the purpose of the discussion and that is interactions between different chemical species. When a solute dissolves in a solvent to form a solution, there is an attraction between the solute entities and the solvent entities. There is no chemical change. The solvent could be evaporated and collected in a relativity pure form. When the solvent is evaporated, the solute will remain. The two components of a solution, solute and solvent, are not changed. This is an important distinction to know. The next section discusses chemical reactions where chemicals are changed from one compound to another.