Experiences

Colligative properties

Elena Margheri

Italy

Vocational School

Chemistry

Teaching in class, Laboratory

Direct

In the final part of the second year of the high secondary school, we usually study the colligative properties of solutions. Facing this topic is not always easy, as students are tired and because this subject is linked to other difficult ones (i.e. stoichiometry, gas laws, etc.).
The sequence of the issues I proposed to the class, is:
boiling point elevation;
freezing point depression;
osmotic pressure.
Variation of state transition temperature in solutions vs pure solvent is well understood owing to life experience of students and it can be stimulated relating it with previous issues, discussing with the class, listening to observations, making answers such as: which is the moment to put salt in water, when you are cooking? Why do you operate in such a way, if the salt solubility is almost independent from temperature? Why do we put salt on the streets when temperature decreases, in winter? What does salt form with water and what will be the difference with respect to pure water?
On the contrary, osmotic pressure, a very important issue in biochemistry, is not usually related to students experience and they are not interested in.
I found useful to link this subject to chemical methods in food conservation, making students recognize that bacteria are cellular systems, so that, like cells, they can be affected by variation of environmental concentration, shrinking or swelling.
Performing the following experience in lab increased the interest and comprehension of students too: we put some hollowed potatoes, filled with distilled water, in a CuSO4•5H2O solution and some other hollowed potatoes, filled with this salt, in distilled water. In each case water passes through the potato walls but the salt, easily revealed by its colour, does not. This is due to potato walls that act as a selective membrane and allow only the water crossing from diluted to concentrated solutions.
After this activity, students were involved in a project in which they act as tutors of younger students (from lower secondary school), repeating and explaining the previous experience: this allowed them to understand deeply what they have done and meanwhile to experience the difficulty of teaching.
This activity was successful and the comprehension of the physical phenomenon was remarkable.
However, concerning the mathematical treatment, the difficulties remained, perhaps because of the lack of an experience in which the quantitative evaluation of osmotic pressure can be performed.