Chemistry Reference
In-Depth Information
Analysis
1. Which three solutes did you choose to test in this experiment? Why
did you select them?
2. Describe the factors that you controlled. Why is it important to have
controls in a science experiment?
3. How did the boiling temperatures of the water samples containing
solutes compare to the boiling temperature of pure water?
4. Explain why you think you got the results that you did.
5. When dissolved in water,
ionic compounds
break up into individual
ions
. List the compounds that were used in your experiment and the
number of particles each will break into.
6. Create a graph plotting the boiling temperature of each solution
versus the number of particles formed in a solution. Connect the
dots with a line. What trend do you see?
7. How is the boiling temperature related to the number of particles in
a solute?
What's Going On?
The boiling point of water changes when solutes are added. The
magnitude of this change depends on the number of particles that are
present after the solute dissolves.
Covalent compounds
such as sugar do
not dissociate into ions; ionic compounds such as sodium chloride (table
salt) do. The boiling point elevation of water is known as a
colligative
property
, one that depends on the number of molcules in solution rather
than the properties of those molecules. Examples of colligative properties
include boiling point, freezing point, and vapor point. The properties of
vapor point
depression and boiling point elevation are very closely related
to each other. The vapor pressure is determined by the number of solvent
molecules existing as a gas directly above a solution (see Figure 3). In
order to evaporate, the solvent particles must be in contact with the
surface of the solution. The presence of solute particles in a solution
decreases the number of solvent molecules that can be at the surface of
the solution, which lowers the vapor pressure and raises the boiling point.
Therefore, water in the solution must be heated to a higher temperature in
order for evaporation to occur.
The change in the boiling point of a solution containing solutes depends
on the number of particles that exist in a solution. Since ionic compounds
